ᗈᓥ኱Ꮫ Ꮫ఩ㄽᩥNature of chemical bonds in double perovskite-typeoxide BaBiO3 and related oxides visualized bysynchrotron-radiation X-ray diffractionᨺᑕග䠴⥺ᅇᢡ䛻䜘䜚ྍど໬䛥䜜䛯஧㔜䝨䝻䝤䝇䜹䜲䝖ᆺ㓟໬≀BaBiO3 䛚䜘䜃㛵㐃㓟໬≀䛾໬Ꮫ⤖ྜ䛾ᛶ㉁2020 ᖺ

ᗈᓥ኱Ꮫ኱Ꮫ㝔⌮Ꮫ◊✲⛉≀⌮⛉Ꮫᑓᨷ

Qing Zhao

┠ ḟ
㸯㸬୺ㄽᩥ
Nature of chemical bonds in double perovskite -type oxide
BaBiO 3 and related oxides visualized by synchrotron-radiation
X-ray diffraction.
Qing Zhao:㸰㸬 බ⾲ㄽᩥ
(1) Charge Order of Bismuth Ions and Nature of Chemical Bonds in
DoublePerovskite-type

Oxide

BaBiO 3Visualized

bySynchrotron Radiation X-ray Diffraction.
Qing Zhao, Tomohiro Abe, Chikako Moriyoshi, Sangwook Kim,
Ayako Taguchi, Hiroki Moriwake, Hong-Tao Sun, and Yoshihiro
Kuroiwa:
Japanese Journal of Applied Physics, accepted on August 17,2020.

㸱㸬ཧ⪃ㄽᩥ
(1) Transformation of Perovskite BaBiO 3 into Layered BaBiO 2.5
Crystals

Featuring

UnusualChemicalBondingandLuminescence.
Hong Li, Qing Zhao, Bo-Mei Liu, Jun-Ying Zhang, Zhi-Yong Li,
Shao-Qiang Guo, Ju-Ping Ma, Yoshihiro Kuroiwa, Chikako
Moriyoshi, Li-Rong Zheng, and Hong-Tao Sun:
Chem. Eur. J. 2018, 24, 8875 – 8882.
(2) Cs 4 PbBr 6 /CsPbBr 3 Perovskite
Luminescence

Quantum

LuminescenceandComposites

Yield:

Formationwith Near-Unity

Large-Scale
Mechanism,

Synthesis,
and

White

Light-Emitting Diode Application.
Ya-Meng Chen, Yang Zhou, Qing Zhao, Jun-Ying Zhang,
Ju-Ping Ma, Tong-Tong Xuan, Shao-Qiang Guo, Zi-Jun Yong,
Jing Wang, Yoshihiro Kuroiwa, Chikako Moriyoshi, andHong-Tao Sun:ACS Appl. Mater. Interfaces 2018, 10, 15905−15912.
(3) Ion-Exchangeable Microporous Polyoxometalate Compounds
with

Off-Center

Dopants

Exhibiting

Unconventional

Luminescence.
Dan-Dan Zhou, Qing Zhao, Fu-Ping Zhu, Zhi-Gang Zhang, Yang

Zhou, Zi-Jun Yong, Ju-Ping Ma, Yoshihiro Kuroiwa, Chikako
Moriyoshi, Yong-Zheng Fang, Jia-Li Gu, Jie Shu, Zhi-Yong Li,
Jian-Mei Chen, Li-Rong Zheng, and Hong-Tao Sun:
Chem. Eur. J. 2018, 24, 9976 – 9982.
(4) X-ray-activated long persistent phosphors featuring strong UVC
afterglow emissions.
Yan-Min Yang,

Zhi-Yong Li, Jun-Ying Zhang, Yue Lu,

Shao-Qiang Guo, Qing Zhao, Xin Wang, Zi-Jun Yong, Hong Li,
Ju-Ping Ma, Yoshihiro Kuroiwa, Chikako Moriyoshi, Li -Li Hu,
Li-Yan Zhang, Li-Rong Zheng and Hong-Tao Sun:
Light: Science & Applications 2018, 7, 88.
(5) Defective [Bi 2 O 2 ]2+ Layers Exhibiting Ultrabroad Near-Infrared
Luminescence.
Hong Li, Xiao-Fang Jia, Qing Zhao, Ju-Ping Ma, Jian-Dang Liu,
Bang-Jiao Ye, Yoshihiro Kuroiwa, Chikako Moriyoshi, Zhi-Yong
Li, Qi Liu, Jun-Ying Zhang, and Hong-Tao Sun:
Chem. Eur. J. 2019, 25, 12842 – 12848.
(6) Defect-Triggered Phase Transition in Cesium Lead Halide
Perovskite Nanocrystals.
Ju-Ping Ma, Jun Yin, Ya-Meng Chen, Qing Zhao, Yang Zhou,
Hong Li, Yoshihiro Kuroiwa, Chikako Moriyoshi, Zhi -Yong Li,
Osman M. Bakr, Omar F. Mohammed, and Hong-Tao Sun:ACS Materials Lett. 2019, 1, 185−191.
(7) Doping Induces Structural Phase Transitions in All-Inorganic
Lead Halide Perovskite Nanocrystals.
Ju-Ping Ma, Jia-Kai Chen, Jun Yin, Bin-Bin Zhang, Qing Zhao,
Yoshihiro Kuroiwa, Chikako Moriyoshi, Lili Hu, Osman M. Bakr,
Omar F. Mohammed, and Hong-Tao Sun:
ACS Materials Lett. 2020, 2, 367−375.

ѫ䄆᮷Abstract
Perovskite-type oxide BaBiO3 has attracted extensive research interests since thediscovery of superconductivity in the doped solid solutions (Ba, K) (Pb, Bi) O3.
Considering the charge balance, the Bi ion normally will take average 4+ valence
state in the compound. However, as the Bi ion is referred to be a “valence skipper”,
hence, the Bi ions are suggested to exhibit charge disproportionation from average
Bi4+ valence state to Bi3+ and Bi5+ valence states with equal ratio. BaBiO3 is supposed
to be not a simple perovskite, but a B-site ordered double perovskite with two
different B-sites in the compound. The charge ordering accounts for the fact thatBaBiO3 is electrically semiconductor with a band gap. However, clear experimental
evidence of Bi charge order is lacking.
Accurate electron charge density distributions of BaBiO3 have been obtained by
the high-resolution synchrotron-radiation powder X-ray diffraction data using the
maximum entropy method/Rietveld method. The charge ordered Ba2Bi3+Bi5+O6
double perovskite adopts four structures, i.e., monoclinic P21/n, monoclinic I2/m,
rhombohedral R3, and cubic Fm3m phases upon heating progress. We investigate
these phase transitions at charge density level, such as the change of chemicalbonding nature and possible modifications in charge density. We found BaBiO3 forms
a two-dimensional covalent layer with P21/n symmetry, a three-dimensional covalent
network with I2/m and R3 symmetry, and an ionic structure consisting of isolated
Bi3+ ions and [Bi5+-O6]7- octahedron upon heating progress.
In order to discover the novel oxides with intriguing properties, engineered local

environments of cations coordinated with oxygen ions in oxides is an effective
pathway. Here we present the successful synthesis of BaBiO2.5 with layered structure
enabled by a non-topotactic phase transformation from the perovskite-type oxide
BaBiO3. Using the maximum entropy method/Rietveld method based on thehigh-resolution powder synchrotron-radiation X-ray diffraction data, we successfully
found that Bi ions form unusual chemical bonds with four coordinated oxygen ionsthat results in an asymmetric coordination geometry. One ionic bond and threecovalent bonds of Bi-O bonds are successfully visualized in this layered structure thatis owing to the hybridization of O 2p and Bi 6p orbitals. Specifically, we find that thisunusual structure shows the near-infrared photoluminescence different from theordinary Bi-containing systems. Experimental and quantum chemistry calculations
motivate us to propose the excitonic nature of near-infrared photoluminescence. Our
work highlights that creating materials with unusual Bi-O bondings and Bi
coordination geometry will provide a pathway to discover the materials with newphysical properties. We believe this work may stimulate interests in exploring a seriesof materials containing heavier p-block elements, thus providing prospects fordiscovering systems with unconventional properties.

Contents
Chapter 1
Introduction…………...............................................................................................................1
1.1 Perovskite-type structure................................................................................................1
1.2 B-site ordered double Perovskite-type structure........................................................4
1.3 Charge order......................................................................................................................6
1.4 Phase transition.................................................................................................................7
1.5 Electron charge density study by synchrotron radiation X-ray diffraction........13
1.6 BaBiO3 and BaBiO2.5....................................................................................................19
1.7 Purposes............................................................................................................................23

Chapter 2
Experimental method........................................................................................................24
2.1 Synchrotron radiation X-ray diffraction....................................................................24
2.2 SPring-8 BL02B2...........................................................................................................27
2.3 Sample preparation........................................................................................................31
2.4 Experiment condition....................................................................................................33

Chapter 3
Analysis method....................................................................................................................37
3.1 Rietveld analysis..........................................................................................................37
3.2 Maximum entropy method (MEM).........................................................................39
3.3 MEM/Rietveld method...............................................................................................45

Chapter 4
Electron charge density study of BaBiO3..............................................................47
4.1 Phase transition of BaBiO3........................................................................................49
4.2 Crystal structure and charge density at 900 K.......................................................51
4.3 Crystal structure and charge density at 600 K.......................................................56

4.4 Crystal structure and charge density at 300 K.......................................................58
4.5 Crystal structure and charge density at 100 K.......................................................60
4.6 Phase transition mechanism.......................................................................................64
4.7 Conclusion.....................................................................................................................66

Chapter 5
Electron charge density study of BaBiO2.5...........................................................67
5.1 Crystal structure of BaBiO2.5.....................................................................................67
5.2 Charge density distribution........................................................................................73
5.3 Photoluminescence mechanism................................................................................75
5.4 Conclusion.....................................................................................................................85

Acknowledgment..................................................................................................................86
References.................................................................................................................................87

Chapter 1
Introduction
1.1 Perovskite-type structure
Perovskite-type structure materials are widely found in nature and firstly get its
name from a mineral with the formula CaTiO3. Generally, the chemical formula ofperovskite can be described with formula ABX3. The A-site is usually occupied by thealkaline or alkaline earth ions with larger ions radii, and the B-site is usually occupiedby the transition metal ions with smaller ions radii, and the X sites are usually oxygenions or halide ions.[1-4] Geometrically, the standard ideal crystal structure ofperovskite is expected to be cubic symmetry with the Pm3m space group as shown in
Fig. 1-1. A-site ions locate at the eight vertices of the cubic cell, and the X site ionslocate at the center of each of the six faces to form an oxygen octahedron, while B siteions occupy the center of the oxygen octahedron. The A-site is surrounded by twelve
oxygen ions. In cubic Pm3m symmetry, the position of the A sites is (0, 0, 0), theposition of the B sites is (1/2, 1/2, 1/2), and the position of the X sites is (1/2, 1/2, 0).

Figure 1-1 Cubic crystal structure of perovskite ABX3 with Pm3m space group.

1Perovskite structures are well known the most frequently involved structures inthe solid-state inorganic chemistry. The cations in perovskite can be used with almost
all the metallic elements in the periodic table. One hand, to form a stable perovskite
structure, the valence balance is necessary. On the other hand, in the perovskite
structure, the ions radii play a very important role in the structural stability. Various
ions can occupy both A- and B- site in a perovskite structure. Here the ionic radius of
A-site, B-site cations and O-anions is recorded as RA, RB and RO, and there is a certain
geometric relationship among these three parameters. For an ideal cubic simple
perovskite structure, there must exist

RA +RO =√2(RB +RO )(1-1)

In the ideal cubic perovskite shown in Fig. 1-1, it can be concluded that two times of
the B-O bond length is exactly equal to the length of its cell parameter, and two timesof the A-O bond length is equal to the length of the plane diagonal. Here, the bondlength is simply equivalent to the sum of the Shannon ion radii of the two ionsconstituting the bond,[5] so the above equation can be obtained. In fact, in many
existing perovskite compounds, the above equation is not absolutely suitable. Anions
and cations are allowed to deviate slightly. So, the actual perovskite structure has arelationship shown below,t=

RA +RO√2(RB +RO )(1-2)

The physical quantity t introduced here is called tolerance factor. It is obvious that theideal tolerance factor t of cubic perovskite is equal to one. However, the tolerance
factor formula ignores the influence of ion coordination number on the effective ion
radius. So, in the actual experiment material, the tolerance factor of most perovskite
materials cannot be equal to one perfectly. Generally, the tolerance factor is between
0.8 and 1. Therefore, when the effective ion radius of the A-site cation is too large or2the ion radius of the B-site is too small, that is, the tolerance factor t value deviatesfrom the allowable range, no stable perovskite structure can be formed. In addition to
the above geometric structure conditions, the anions and cations of perovskite alsoneed to meet the charge balance, so that the overall electric neutrality is maintained,that is, the sum of the valence of cations at A and B positions should be kept at six in aperovskite-type oxide.

Table 1-1 Tolerance factor of A site atom and B site atom of perovskite type oxide atroom temperature. The A site atom is calculated with 12 coordination, the B site atomwith 6 coordination, and the O atom with 2 coordination (1.35 Å).

Materials

Space group

Tolerance factor t

BaTiO3
PbTiO3

P4mm
P4mm

1.071
1.027

KNbO3

Amm2

1.062

KTaO3

Pm3m

1.062

LaAlO3

R3m

1.017

KMnF3

Pm3m

0.980

SrTiO3

Pm3m

1.009

CsPbBr3
PbZrO3

Pnma
Pbam

0.870
0.970

In fact, there are not many perovskite compounds with simple cubic structure, butthere are a lot of distorted derivative structures. Although the ions slightly deviate
from the ideal position of cubic perovskite, the arrangement and distribution of theions are consistent with perovskite, and the space group and lattice constant of thecompound may change, but the overall structure is still perovskite-type as shown in
Table 1-1.[6-8] The calculation of the tolerance factor is performed using nearest
coordinated neighbor oxygen. The ionic radius of the atom at A-site ions with twelve
coordination oxygen is considered, and the ionic radius of the atom at B-site with six
3coordination oxygen is applied, respectively. In the case of the oxygen ions, only the
B-site ions is treated as the first nearest neighbor coordination, and the ion radius withtwo coordination is about 1.35 Å.
For example, the substitution of ions at the B position of perovskite may lead tothe skew and distortion of the octahedron of oxygen at the B position, thus reducing
the symmetry of the crystal, and may changing from a cubic system to a tetragonal,
orthogonal or monoclinic system. In the distorted perovskite-type structure, the cellside length is usually √2 times, n times or n√2 times of the simple cubic cell side
length. Generally speaking, the symmetry of perovskite structure will change with thechange of temperature or pressure. When the temperature or pressure increases, thesymmetry increases, the temperature and pressure decrease, and the symmetry
decreases. The A and B sites of perovskite structure can be doped by ions, that is, both
A and B sites can be occupied by one or several ions. In addition, there are also
cations or anions vacancy in the perovskite crystal sometimes.

1.2 B-site ordered double Perovskite-type structure

The name of double perovskite corresponds to that of single perovskite ABO3. Its
general chemical formula can be expressed as A2BB′O6.[9-16] The most important
point that is different from ABO3 is that the B-site of perovskite oxide is occupied by
two different transition metal elements. The standard B-site ordered double perovskite
has an orderly arrangement of oxygen octahedrons composed of different elements in
B-site, as shown in Figure 1-2. B- and B′-site ions are the ion with smaller ion radius,
generally the transition metal element ions, and A site is the ion with larger radius, thesame as ABO3. The B-site ions at the top angle form the face centered cubic structurewith the oxygen ions, the B-site ions are located in the body center of the octahedron
formed by the O anions, and the A-site ions are located in the body center of the cube
formed by the B- and B′-site ions. In the B-site ordered double perovskite-type
structure A2BB′O6, the tolerance factor formula is also applicable, that is4

RA +RO =√2

RB +RB'
2

+RO t

(1-3)

In most cases, the B-site ordered double perovskite structure is not ideal.
Different elements in B- and B′- site, electronic configuration and interaction between
ions will cause structural distortion. In the B-site ordered double perovskite, thesubstitution of B-site elements provides a great possibility to find new materials. In
Fig. 1-2, it can be seen that B-site ion and B'-site ions are separated by O ions to form
B-O-B' bond, which is 180 ° in ideal case.

Figure. 1-2 B-site ordered double perovskite oxide

B-site ordered double perovskite oxides have been widely investigated in the
recent years owing to its rich interesting physical properties. [17-22] Compared withthe simple disordered ABO3 perovskite, both the B- and Bc-site in this orderedstructure can adjust different kind of transition metals ions arranged with arocksalt-type manner. As a consequence, some intriguing magnetic or electricalphenomenon can be observed through the interactions among B-B, B-Bc and Bc-Bc
sites, leading to many different kind of intriguing physical properties, such as
ferroelectricity,

colossalmagnetoresistance,

half

metallic

behavior,or

high-temperature ferrimagnetic order and so on. One of the famous example is5

Sr2FeMoO6,[16-17] people observed a high ferromagnetic Curie temperature (TC ~420 K) and half-metallic behavior in this compound, which makes the material to behighly potential for application in spintronic devices above room temperature. This
intriguing property inspired the investigations of 3d-4d(5d) orbitals hybridized
through B-Bc pathway in B-site ordered double perovskites. Another similar sample is
Sr2CrOsO6,[12] people found a ferrimagnetic (FiM) phase transition occuring withmuch higher TC (725 K) through simply changing the B-site transition metal ions. In
addition, People have found interesting spin interactions in the ordered perovskite
family of A2FeOsO6 (A = Ca, Ca0.5Sr0.5, and Sr). Among these compounds, the Fe3+
and Os5+ ions are ordered at B-sites. With decreasing the size of A-site ions, the crystal
symmetry can be reduced owing to the increasing octahedral distortion, where theaverage Fe-O-Os bond angle is about 165.3° for Sr, 158.9° for Sr0.5Ca0.5 and 150.9°
for Ca. Sr2FeOsO6 shows the tetragonal I4/m symmetry, and (Ca0.5Sr0.5)2FeOsO6shows the monoclinic P21/n sysmetry.[18-22] Two antiferromagnetic phase transitions
were observed in Sr2FeOsO6 owing to the lattice instability and the competing
magnetic superexchange interactions. In contrast, one FiM ordering is found in the
monoclinic

phase of Ca2FeOsO6 and CaSrFeOsO6 showing the Curie temperature

about 320 K and 210 K, respectively, which is owed to the AFM ordering between the
Fe3+ and Os5+ ions. In contrast, when the lead ions occupy the A-site, Pb2FeOsO6shows an interesting cubic double ordered perovskite structure with Pm3m symmetry.
A long-range ferrimagnetic order is observed below 280 K between the neighbored
Fe3+ and Os5+ octahedron through straight Fe-O-Os pathway.

1.3 Charge order

Generally, transition-metal oxides with mixed valence states of ions show very
interesting physical properties in some compounds. The compounds with Fe ionssometimes will show many exotic physical phenomena owing to rich valence state of6

Fe ions. For instance, an intriguing charge order state was found in the compound
Fe4O5 that is recently obtained using the high-pressure synthesis method, where thecompeting Fe2+ and Fe3+ ordering is revealed.[23,24] Another interesting sample is an
A-site ordered quadruple perovskite LaCu3Fe4O12. Charge transfer is observed
between Cu and Fe ions via changing the temperature or pressure showing theunusually high Cu3+ and Fe3.75+ valence states, meanwhile, the sharp variations of unit
cell, changes of magnetic and electrical transport properties are observed
accompanied with negative thermal expansion.[2,25,26] Charge ordering is usuallyeasy to be observed in transition metal oxides with mixed-valent ions.[27-29] The
perovskite with Mn ions is especially potential to observe long-range charge orderingof the Mn3+ and Mn4+ ions in these materials owing to the antiferromagnetic spinordering, or the orbital ordering of Mn3+ ions accompanied with lattice distortions.
After the long-range ordering, people usually can observe a series of changes in theresistivity, magnetization, and lattice parameter. The charge ordering transition are
generally associated with the effects of chemical pressure introduced by cation size,
temperature, pressure, or magnetic field.

1.4 Phase transition
In many kinds of perovskite type oxides, structural phase transitions occur when
the temperature changes. From high temperature to low temperature, the symmetry
usually decreases. Structural phase transition is usually accompanied by visible
changes in atomic position and lattice distortion in the crystal. For example, phasetransition in ferroelectricity perovskite oxide appears when the crystal phase become
low symmetric phase with non-centro symmetry via temperature decreasing. The
spontaneous atomic displacement causes the corresponding structural phasetransitions. During the process, the fundamental vibration of a transverse wave among
the lattice vibrations is frozen with decreasing temperature, which is essentiallysensitive to the temperature. As the temperature decreases, the crystal symmetry7

decrease, at the same time, the frequency of the fundamental vibration decreases,
where the softening of phonon occurs, and this kind of softening fundamental
vibration is called the soft mode. The anharmonicity of the lattice vibrations isbelieved to play a critical role in the mechanism of softening process. When the lattice
vibration is anharmonic, with decreasing temperature the frequency of fundamental
vibration decrease owing to the restoring force of lattice vibration. Below the phasetransition temperature, the frequency of the soft mode becomes zero, the atomic
displacement pattern corresponding to this soft mode is frozen, and a ferroelectricphase with low symmetric can be realized. The soft mode of lattice vibrations largely
depends on the different mass of atoms consisting of compounds, crystal structure
parameters, thermal vibration of atoms, the bonding nature and the interaction
between atoms, including the dipole field, etc. Soft mode has been used not only to
explain the phase transition of ferroelectrics, but the phase transition ofsuperconductivity, or the metal-insulator phase transition.[30]When people try to investigate the phase transition of perovskite oxide, two kindof soft modes are most frequently used to explain the process of phase transition. One
mode is the Γ15 mode that is a mode of octahedron consisting of six oxygen occupying
the center of the Brillouin zone. The other modes are M3 and R25 modes at theboundary of Brillouin zone, which are the rotation modes of oxygen octahedron.
Figure 1-3 shows the Brillouin zone in the simple cubic structure of the perovskite
oxide. Rotation of Γ, M, R means the phonon mode revealing each reciprocal point in
the Brillouin zone.[31] People can observe different kind variation of soft mode in thephase transitions of perovskite-type materials.

8Figure 1-3 Brillouin zone of cubic perovskite-type structure.

For example, the Γ15 mode is a transverse wave optical mode of infrared active.
when the mode is softened and frozen, the phase transition takes place with the atomic
displacement occurring along the [001] direction, which usually allows theappearance of spontaneous polarization for ferroelectric materials. Figure 1-4 shows
three different kind of atomic displacement of the Γ15 mode,[31] (a) indicates therelative motion between the B-site ions and the surrounded oxygen octahedron called
the Slater mode, (b) between A-site ions and oxygen octahedron with B-site ionscalled the Last mode, and (c) shows the transformation of oxygen octahedron. For
example, the spontaneous polarization of ferroelectric BaTiO3 can be explained usingslater mode. In comparison, the mechanism of phase transition of ferroelectric PbTiO3
can be attributed to the combination of the Slater mode and Last mode.

Figure 1-4 Atomic displacement when Γ15 mode is frozen; (a) slater mode, (b) lastmode, (c) O octahedron displacement mode.
9Differently, Figure 1-5 shows the rotational displacement of oxygen octahedronin a crystal structure when the M3 and R25 modes are frozen,[31] which is a transverse
acoustic mode. The rotation of oxygen octahedron is around the <001> axis. In the M3
mode, the adjacent oxygen octahedrons of up and down take the rotation along thesame direction, however, in the R25 mode, the directions of rotation of oxygenoctahedrons are opposite, which will cause the occurring of super lattice structure
owing to the alternate twisting of octahedrons. These two modes are usually easy to
be observed in the low temperature antiferroelectric phases.

Figure 1-5 Atomic displacement when M3, R25 mode is frozen; (a) M3 mode and (b)R25 mode.

For the perovskite-type structure, these three different kinds of modes can all be
the candidates to explain the phase transition. The choice of the soft modes is found to
be strongly depending on different perovskite materials. In general, there seem toexist a competition between the Γ15 mode, and M3 and R25 modes. In the perovskite
10material, when phase transition take place with the Γ15 mode, the M3 and R25 modes
seem to not exist, which conversely makes sense in most cases. This phenomenon of
separation behavior of phase transition mode can be revealed using the tolerance
factor of perovskite-type structure.[32]The ideal tolerance factor should be equal to unit in the cubic perovskite structure.
The tolerance factor was proposed to classify the stability of different of theperovskite-type structure consisting of different element ions. In some point, thetolerance factor can also be used to explain the competition among different kind ofsoft mode. The Shannon ion radius is often used for ionic radius. In general, when thetolerance t > 1, there exists a large enough space around the B-site ions rather thanaround the A-site ions, which benefits the occurring of Γ15 mode. However, when thetolerance factor t < 1, it means that there exists space around the A-site ions instead. Insuch case, the phase transition is accompanied with the M3 or R25 modes. There also
exist some special cases, for example, LaAlO3 was found to have a phase transition
with the R25 mode, and phase transition of SrTiO3 showed the combination of Γ15
mode with the decreasing process and R25 mode at very low temperature. In addition,
Table 1-2 shows some existing examples of perovskite-type compounds with different
tolerance factor and soft modes.[31,33,34] The soft modes of different perovskite
compounds almost obey the tendency related to the tolerance factor mentioned above.
Generally, with tolerance factors t > 1, the Г15 mode, which is at the Brillouin zone
center, is easily to occur. On the other hand, with tolerance factor t < 1, the M3 and R25
modes, which are at the Brillouin zone boundary, are easily to get frozen.

11Table 1-2 Tolerance factor and the soft modes of various perovskite-type compounds
at low temperature.
Materials

Soft mode

Tolerance factor t

BaTiO3
PbTiO3
KNbO3
KTaO3

Г15
Г15
Г15
Г15

1.071
1.027
1.062
1.062

LaAlO3KMnF3
SrTiO3

R25
M3
Г15

1.017
0.980
1.009

CsPbBr3
PbZrO3

R25

0.870
0.970

The phase transition usually accompanies with the distortion from the ideal
perovskite structure. The distortion is generally attributed to three factors, that is theoctahedra distortions, ions displacements within the octahedra and octahedra tilting.
In early years, Glazer reported the Glazer notation system to classify the distortion.
The Glazer notation tilt system can be described using the rotation of the octahedra
correspond to three axes. The description of rotations is presented with twoparameters. For example, a rotation system a+a+a+ indicates the same rotation about
each axis. The first parameter a describe the magnitude of the rotation. Hence, therotation system a+a+c+ shows a different rotation angle about z axis. The second
parameter is the +, - or 0 superscript. The positive superscript indicates the samerotation direction between the adjacent layers of octahedra, the negative superscript
reveals the opposite rotation of the neighbored octahedra, and zero superscript is
applied with no rotations. There are total 23 types of Glazer notation divided into fourgroups with the number of tilt parameters as shown in Table 1-3.[35,36]
Correspondingly, M3 mode and R25 mode can also be defined using Glazer tilting
system as a0a0c+ and a0a0c− as shown in Fig. 1-5, respectively.

12

Table 1-3 Classification of space groups by Glazer notation.
Tilt system

Tilt systemSpace

Tilt system

Tilt systemSpacenumbersymbolgroup

numbersymbolgroup

Three-tilt systems

Two-tilt systems

1

a+ b+ c+Immm

15

a0 b+ c+Immm

2

a+ b+ b+Immm

16

a0 b+ b+

I4/mmm3

a+ a+ a+Im3

17

a0 b+ c−Cmcm

4a+ b+ c−Pmmn

18

a0 b+ b−Cmcm

5a+ a+ c−P42/nmc

19

a0 b+ c−I2/m

6

a+ b+ b−Pmmn

20

a0 b− b−Imma

7

a+ a+ a−P42/nmc

8

a+ b− c−P21/m

9a+ a− c−

P21/m

21

a0 a0 c+P4/mbm10

a+ b− b−Pnma

22

a0 a0 c−I4/mcm11

a+ a− a−Pnma

12

a− b− c−P1

13

a− b− b−I2/a14

a− a− a−R3c―One-tilt systems―

Zero-tilt systems―

23

a0 a0 a0―

Pm3m

1.5 Electron charge density study by synchrotron radiation
X-ray diffraction
The discovery of X-rays was one of the most important discoveries of physics inthe late 19th and early 20th centuries, which provides an effective research method formany scientific fields. The discovery and research of X-rays play a huge importantrole in the impact on the development of physics and the entire science andtechnology since the 20th century. In 1895, Rontgen reported a discovery of new kindof ray named X-rays. Barkla found the polarization of the X-rays, which is a big step
13

forward in understanding the nature of X-rays. In 1912, Laue found the X-ray
diffraction phenomenon when a beam of X-rays hitting on a crystal, which captured
the scattering on the plate. The wave properties of X-rays were proved for the firsttime. Hence, Laue was awarded the Nobel Prize in 1914 year for this discovery. In thefollowing year 1913, Bragg father and son designed the first X-ray spectrometer,
which was successfully used to analysis the structure of NaCl crystal. X-rays wasproved to have effectiveness for analyzing crystal structure. The formation of X-rays
crystal diffraction was clearly explained, and the most famous Bragg formula wasproposed. In 1916 year, Debye and Scherrer developed a method to study crystal
structure using X-rays with powder polycrystal samples, and they observed thediffraction pattern of concentric rings called Debye-Scherrer ring on the imaging plate.
Compared with single crystal samples, polycrystalline samples are much easier toobtain, which facilitates the study of crystal structural analysis. When a charged
particle with a speed close to the speed of light makes a circular motion in a magnetic
field, it emits a continuous spectrum of electromagnetic waves along the tangent
direction of the deflection orbit. In 1947, people first observed this kind ofelectromagnetic wave on an electron synchrotron and called it the synchrotron
radiation. Since then, people started to use the synchrotron-radiation X-ray diffraction(SXRD) to determine the crystal structure of materials. In the past 100 years, X-ray
diffraction analysis of crystal structure has been greatly developed. Since X-ray
diffraction has a correlation with the electrons, the obtained diffraction peak contains
information about the electron density around the atoms. X-ray diffraction has alreadybeen recognized as an excellent method in the direct observation of chemical bonds incrystals structure in the real space.
Electron charge density study using synchrotron-radiation X-ray diffraction plays
an important role to reveal the relationship between crystal structure and physicalproperties. In recent years, accuracy of experimentally obtained charge density
distributions has made spectacular advances through analyzing the synchrotronradiation powder X-ray diffraction data using the maximum entropy method(MEM)/Rietveld method. Generally, the Rietveld structural analysis is limited to the14atomic level, which gives some limited structural information, i.e. lattice constants,
atomic position, thermal vibration, bonding angle, and bonding distance, etc. However,these structural parameters of atomic level sometimes fail to give an accurate
connection between structure and physical properties. The MEM/Rietveld method isrecognized as a combination between the MEM method and the Rietveld analysis,
which has successfully been applied in the charge density studies to complementary
the structural analysis.
The charge density study can be used in investigating the bonding nature in the
crystal structure. For instance, a novel superconductor MgB2 shows a geometricthree-dimensional (3D) nested structure with boron honeycomb sheets sandwiched
between Mg triangular sheets. Furthermore, the corresponding MEM charge density
clearly reveals the bonding nature around B ions and Mg ions. The two-dimensional(2D) strong covalent bonding network of boron ions and the full ionization of Mg ionsare observed in the MEM charge density distribution map as shown in Fig. 1-6.[37,38]
This kind of 2D layered structure usually benefits the superconductivity, which isrelated to the superconductive mechanism.

Figure 1-6 The MEM charge density distribution of MgB2 at 15 K. The isosurface
shown here is 0.75 e Å-3

Another sample to show the difference in bonding nature using MEM/Rietveld
method is a spin-crossover complex, that is Fe(phen)2(NCS)2 shows a reversible
low-spin to high-spin transition induced by laser at low temperature about 92 K as
15

shown in Fig. 1-7.[39] The Fe2+ ions are surrounded by six N atoms in crystal
structure at both low-spin and high-spin phase, therefore, Fe2+ ions show moreisolated from the neighbored N atoms at high-spin phase with a much lower charge
density in a longer Fe2+-N bond.

Figure 1-7 (a) Crystal structure of Fe(phen)2(NCS)2 with Ball and stick model. The
MEM charge density distribution of (b) the LS phase at 92 K, (c) the on phase using
laser at 92 K, and (d) the HS phase at 260 K, Isosurface is 0.45 e Å−3.

Our group has been revealing the structural characteristics of perovskite-typeoxide ferroelectrics using MEM/Rietveld method, such as BaTiO3 and PbTiO3. The
precise charge density distributions of BaTiO3 and PbTiO3 have been evaluated tounderstand the chemical bonding nature from an electronic viewpoint. BaTiO3 forms
an ordinary three-dimensional (3D) covalent network of Ti-O6 octahedron in allphases as shown in Fig. 1-8. In comparison, Strong covalence on the Pb-O bond isobserved in the tetragonal PbTiO3, which forms a two-dimensional (2D) layered
structure of a covalent bonding network of the Ti-O5 pyramid and the covalent Pb-O
16

bonds, however, in cubic PbTiO3, the bonding nature of Pb-O bonds shows distinct
ionic as shown in Fig. 1-9.[40,41] The possible existing orbital hybridization between
Ti-O and Pb-O predicted by first-principles calculation is successfully visualized. It
seems like the phase transitions are accompanied by changes in the bonding nature ofsome specific bonds.

Figure 1-8 MEM charge density distribution in tetragonal of BaTiO3; (a) (1 0 0) planeand (b) (2 0 0) plane. Contour lines are drawn at intervals of 0.2 e Å-3 from 0.4 e Å-3
to 2.0 e Å-3.

Figure 1-9 MEM charge density distribution in PbTiO3 tetragonal; (a) (1 0 0) planeand (b) (2 0 0) plane. Contour lines are drawn at intervals of 0.2 e/Å3 from 0.4 e Å-3 to
2.0 e Å-3.

Besides, the MEM charge density study can also be used to study the charge
17order or the orbital order. For example, the charge density distribution study has also
been performed in the Nd1-xSrxMnO3 system to investigate the charge and orbital
ordering. In the half-doping, charge ordering of Mn3+ and Mn4+ in Nd1/2Sr1/2MnO3 issuccessfully visualized at 18 K. A zigzag pattern of the anisotropy charge density
around Mn3+ sites were observed, which is owing to the px(y)-dx2-y2 hybridization ofbonding electron as shown in Fig. 1-10.[42] Meanwhile, the charge density aroundMn4+ sites is isotropic. At x = 0.65, charge ordering of Mn ions disappear, instead, anlong-range ordering of d3z2-r2 orbital appears in Nd0.35Sr0.65MnO3 at low
temperature(˘TN). An anisotropic charge density distribution around Mn ions wasclearly observed with a higher charge density in the shorter Mn-O1 bonds as shown in
Fig. 1-11.[43]Figure 1-10 MEM charge density distribution of Nd12Sr12MnO3 on the (010) plane at18

(a) 300 K and (b) 18 K.

Figure 1-11 MEM charge density distribution of Nd0.35Sr0.65MnO3 on the Mn–O1–O2
plane at 30 K.

On the other hand, the excellent counting statistics and the high angularresolution of SXRD data also enable us to visualize the electron charge density oflight elements such as Li and H in the compounds with heavy elements at the sametime.

1.6 BaBiO3 and BaBiO2.5

BaBiO3, with a perovskite-type structure, has attracted extensive researchinterest after the superconductivity discovered at low temperatures in the doped solid
solutions (Ba, K) (Pb, Bi) O3. The superconductive oxide BaPb1-xBixO3 was firstly
found in 1975 as shown in Fig. 1-12,[44] BaPb0.8Bi0.2O3 shows a transition
temperature about 12 K accompanied with a low density of states around the Fermi
19level. The doping the electronically inactive Ba donor sites induce thesuperconductivity in Ba0.6K0.4BiO3 with a higher Tc about 30 K in 1988 as shown in
Fig. 1-13.[45-47]

Figure 1-12 Eectrical resistivity with dependence of temperature of BaPb0.8Bi0.2O3.

Figure 1-13 Temperature-dependent magnetization for Ba0.6K0.4BiO3-x samples
decreasing the temperature with a magnetic field of 19 Oe. Inset shows enlarged
region near Tc of the sample annealed at 475 °C in oxygen.
20Additionally, BaBiO3 is expected to be used in photocatalytic applications.
BaBiO3 serves as an efficient catalyst under visible light in water splitting reactions,carbondioxidereduction,

and

the

decompositionofvariousorganic

contaminants.[48-52] The high activity of BaBiO3 is owing to the electron
configuration of Bi ions and chemical bonding, which facilitate charge mobility andnarrow the band gap. BaBiO3 photocatalysts have inspired much attention owing to
the special electronic structure consisting of orbital hybridization between O 2p andBi 6s obtials. The 6s orbitals of Bi ions show the high dispersion, which encourages
the reduction of the band gap and charge mobility.
Assuming that the valence state of Ba ions is Ba2+ in BaBiO3, that of Bi ionsshould be Bi4+ to maintain charge neutrality. However, as a Bi ion is referred to as avalence skipper, it typically does not assume the Bi4+ valence state in compounds.
Hence, Bi ions may exhibit charge disproportionality from the Bi4+ valence state to
the Bi3+ and Bi5+ valence states. The typical chemical formula of oxide materials witha perovskite-type structure is denoted ABO3, where A and B are occupied by cations.
BaBiO3 does not exhibit a B-site disordered simple perovskite-type structure, however,
it is supposed to exhibit a B-site ordered double perovskite-type structure to occupy
two different B sites for Bi3+ and Bi5+ ions with equal occupancy with the formula ofBa2Bi3+Bi5+O6. The charge order attributes the fact that BaBiO3 is electrically asemiconductor.[53-56] This compound have been proved to undergo a successive
structural phase transitions under lowing temperature from cubic (Fm 3 m),
rhombohedral (R3), and monoclinic (I2/m) to monoclinic (P21/n) phases as shown in
Fig.1-14.[57-60]

21

Figure 1-14 Phase transition diagram of stoichiometric BaBiO3.
BaBiO3 undergoes several temperature dependent phase transitions accompaniedwith BiO6 octahedral tilting. The charge ordered BaBiO3 have four different phases.
From cubic phase to rhombohedral phase and monoclinic to monoclinic phase, these
two-phase transitions seem to adopt the second-order phase transition. In contrast, thephase transition between rhombohedral and monoclinic phase shows thediscontinuous transition, that means the first-order phase transition. The detail ofevery crystal structure is shown below in Fig. 1-15.

Figure 1-15 The crystal structure of BaBiO3 at different phases.
22Bismuth is one of the most studied ions existing in many different kinds offunctional materials processing different local coordination environments, leading tothe occurrence of large amount of intriguing properties such as the ferroelectricity,
superconductivity and so on.[61-65] These functional materials seem to be strongly
corelated to the lone pair of Bi3+ ions, which usually show the stereoactivity.[66-68]
The Bi ions with various oxidation states can exhibit in many compounds, which richthe functional diversity. Among these Bi-containing compounds, perovskite BaBiO3
has been widely investigated, which shows an octahedral of [Bi3+O6 and Bi5+O6]
breathes out and in mode.[69-71] Generally, the modification of local coordination
environments of cations in the network of crystal can emerge many interesting
properties.
The modification of local oxygen coordination environments of cations indifferent oxides have been widely investigated. Through the part of oxygen atoms
extraction, the phase transformation of structure can be achieved, which sometimes
enable the apperance of many kinds of novel oxides with unexpected physicalproperties

different

from

those

of

corresponding

oxygenrich

parentcompounds.[72-82] One of the most well-known samples is the transformation fromperovskite structure SrFeO3 to layered structure SrFeO2, which shows the dramaticchanges in crystal lattice and magnetic spins.

1.7 Purposes
BaBiO3 as the parent sample of the superconductive doped solid solution havealready been investigated in the crystal structure and phase transition using neutron
diffraction. We have already learned that is one of the best papers on the structure
analysis. However, we have been supposing that if we can analyze the crystal
structure precisely by using synchrotron radiation X-ray diffraction (SXRD), in23

addition, we can provide the experimental direct evidence on the chemical bonding
between atoms. The MEM/Rietveld analysis combined with SXRD enabled us tovisualize the electron charge density distribution around the Bi ions, hence, we could
directly observe the charge order of Bi3+ and Bi5+ in BaBiO3 from the difference in theionic radii. The nature of chemical bonds could be also observed to summarize them.
We believe that we are the first ever to experimentally observe the electron charge
density distributions accurately in BaBiO3. Our study enriches the understanding of
the structural characteristics, charge order, and chemical bonding nature of BaBiO3 ina viewpoint of electron charge density distribution, which may provide avenues fordeveloping low dimensional BaBiO3-derived functional materials such as
superconductors.
Since it is difficult to prepare the stoichiometry oxygen failing compounds in awell-controlled behavior, the synthesis of BaBiO3-derived phases (such as BaBiO2.5)
by reductive method is challenging. Previous work often resulted in obtaining mixed
phases with impurities, that usually made crystal structural analysis and property
characterization extremely difficult. Besides, the structure and physical properties of
the BaBiO2.5 are still poorly investigated until now. Encouraged by the discovery ofsuperconductivity in the doped solid solution of BaBiO3, we suggested that there may
be new physical characteristics in its reductive phase, which may have an interesting
Bi local coordination geometry and unusual chemical bonding nature.

Chapter 2
Experimental method
2.1 Synchrotron radiation X-ray diffraction

Synchrotron-radiation light source refers to a physical device that generates
synchrotron radiation. It is a type of high-performance strong light source thatgenerates synchrotron radiation when relativistic electrons (or positrons) deflect in a24

magnetic field as shown in Fig. 2-1. The emergence of electronic synchrotrons,
especially the development of electronic storage rings, has promoted the widespread
application of synchrotron radiation.

Figure 2-1 The generation of synchrotron radiation. When electrons make a motion ina magnetic field, it emits synchrotron radiation along the tangent direction of thedeflection orbit.

The synchrotron-radiation light source has experienced three generations ofdevelopment, and its main body is an electronic storage ring. The early research ofsynchrotron radiation was carried out on the electron synchrotron. The electronicstorage ring of the first generation of synchrotron radiation light source was designed
for high-energy physics experiments. It only used the parasitic synchrotron radiationfrom the deflection magnet, so it was also called "combined light source". The firststorage ring installed in a synchrotron-radiation light source was at the Synchrotron
Radiation Center of Tantalus in 1968. The electron storage ring of thesecond-generation synchrotron radiation light source is specially designed for the useof synchrotron radiation, which mainly draws synchrotron radiation light from thedeflection magnet. The device has been optimized to make the electron beamemittance much smaller than that of the second generation, so the brightness ofsynchrotron radiation is greatly improved, and high-brightness, partially coherent
quasi-monochromatic light can be extracted from inserts such as undulators. The
third-generation synchrotron-radiation light source can be further subdivided into
high-energy light sources, medium-energy light sources, and low-energy light sources
according to the differences in photon energy coverage and electron beam energy inthe electron storage ring. With its excellent light quality and irreplaceable role, the25third-generation synchrotron-radiation light source has become the best light sourcefor basic research and high-tech development and application research in many
disciplines today. The schematic of a third-generation synchrotron is shown in Fig.
2-2.[83] In the future, the fourth-generation light sources are under development thatwill produce ultra-brilliant and pulsed time-resolution X-rays for extremely
experimental demanding.

Figure 2-2 Schematic of a third-generation synchrotron.

The synchrotron radiation presents several characteristics. The synchrotron
radiation light has a wide wavelength band. The wavelength coverage of synchrotron
radiation light is large, and it has a continuous spectrum from far infrared, visible light,
ultraviolet to X-ray range, and it can obtain a specific wavelength of light according
to the needs of users. The synchrotron radiation light shows the high collimation. The
emission of synchrotron radiation light is concentrated in a very narrow cone centeredon the direction of electron motion, with a very small angle, almost parallel beam,comparable to laser. The synchrotron radiation light presents the high polarization.
The synchrotron radiation light from the deflection magnet is completely linearlypolarized on the electron orbit plane. In addition, people can obtain the light of any
polarization state using some specially designed insert. The synchrotron radiation
light is a kind of light with high purity and brightness. Since the synchrotron radiation
26

light is produced in the ultra-high vacuum without any pollution caused by any
impurities. it is very pure light. The synchrotron radiation light source provides ahigh-energy light with small wavelength. The X-ray brightness of the third-generationsynchrotron radiation light source is hundreds of billions of times that of the normal
X-ray machine. The synchrotron radiation light can be a narrow pulse light withexcellent pulse time structure. Its width is adjustable from 10-11 to 10-8 seconds. Theinterval between pulses is tens of nanoseconds to microseconds. This characteristic isvery useful for the study of "change process", such as chemical reaction process, life
process, material structure change process and environmental pollution micro process.
In addition, the synchrotron radiation has some other unique and excellent properties
such as high stability, high flux, micro beam diameter, quasi coherence, etc.

2.2 SPring-8 BL02B2
All our experiments of X-ray diffraction data were all collected in Spring-8
BL02B2 beamlines. SPring-8 is one of largest three-generation synchrotron radiation
facility in the world until now as shown in Fig. 2-3. It is located at Harima Science
Park City, Hyogo Prefecture, Japan. The wavelength of the emitted radiation has awide energy range from vacuum ultraviolet rays to hard X-rays with the highest
intensity in the world. Furthermore, the high energy γ-ray can be achieved with theenergy from 1.5 to 2.9 GeV, meanwhile, the infrared rays can also be obtained.

27

Figure 2-3 The bird eye view of the synchrotron radiation facility Spring-8.

Spring-8 can provide the high-energy synchrotron radiation that can be used inmany different scientific fields, including observing the mobility of chemical
reactions, structural analysis, life science, medicine, industry and so on. Figure 2-3
shows the bird eye view of SPring-8. There are several systems included in thesynchrotron-radiation Spring-8. During the whole process of generation ofsynchrotron radiation, firstly, the electron gun generates an electron beam. Then, theelectron beam can be accelerated to about 1 GeV by the linear accelerator. In order toobtain much higher energy electrons with 8 GeV, further accelerate process can beachieved in the synchrotron. The storage ring plays the important role in storing thehigh energy electrons. Then, the radiation light is generated using the polarizationelectromagnet combined with the insertion light source. The emitted light is finally
guided to every different hatch installed outside the storage ring all through the beamline, which can be used in various experiments for different purposes.

28

Figure 2-4 Schematic view of beamline of BL02B2 in Spring-8.

SPring-8 BL02B2 is a beam line for powder diffraction using high-energy
synchrotron radiation as shown in Fig. 2-4.[84] The beamline is designed to performseveral scientific studies. One hand, it can analysis the crystal structure using thehigh-resolution X-ray diffraction data. Based on the precis structure analysis, thecharge density of the polycrystal sample structures can be obtained that are related tothe properties of functional materials. Structure phase transitions can also be clearly
determined. On the other hand, Ab initio structure determination can also be
performed. Besides the polycrystal samples, the structure analysis of thin films canalso be obtained.
Figure 2-5 shows a schematic figure of the optical system installed in BL02B2
beamline.[84] The optical system is used in BL02B2 beamline, which is mainlycomposed of a curved mirror and a double Si crystal monochromator. The Mirrors aredesigned for the purpose of removing the unnecessary harmonics, there are three setsof options corresponding to different demand of the energy of synchrotron radiation.
Demand of lower than 15.5 keV energy, Si mirror is preferred. Ni mirror can be usedin the region from 15.5 keV to 28.2 keV energy, and Pt mirror is required for higher
energy than 28.3 keV. The double Si crystal monochromator is installed to improve
the parallelism of X-ray and energy resolution, and the Si crystal with parallel (111)plane is used as a standard. The BL02B2 beamline can provide high energy X-ray
with a wide energy region from 12 to 37 keV. The energy resolution ΔE/E is about
2u10−4.
29

Figure 2-5 The schematic of the optical system in SPring-8 BL02B2.

In the experiment hatch of BL02B2 beamline, a large Debye-Scherrer camera is
installed. Figure 2-6 shows a schematic view and photograph of the large
Debye-Scherrer camera.[85,86] A curved imaging plate (IP) camera and asemiconductor detector multi-connected diffractometer (MYTHEN) camera on the 2θ
axis are installed to collect diffraction pattern. In the measurement, the sample sealedin the glass or quartz capillary is fixed on the ω-axis exposed in the X-ray. Duringexposure process, the sample keep rotating around the ω-axis to obtain thehomogenous diffraction intensity on Debye rings, which balances the influences ofinhomogeneous particle size distribution in the sample. This rotating measureguarantees the high quality of powder diffraction pattern. To make the sample locatedat center, the 5-axises goniometer head is used, so that the sample does not move
during the rotation around the ω-axis. When the phase transition is investigated,different temperature diffraction data are required. Hence, a temperature controllingsystem is installed which is controlled using flowing gas. Below room temperaturefrom 15 - 300 K, He gas recycling type refrigerator is capable, N2 gas blowing device
can be used in measurement at a low temperature of 90 - 300 K. Higher than room
temperature, N2 gas is heated when measurement at a high temperature of 300 - 1100
K is performed. In our measurement, the IP and MYTHEN detector were both used,
and the temperature was controlled using a low temperature N2 gas blowing device.
30

Figure 2-6 The (a) schematic view and (b) photograph of the large Debye-Scherrer
camera installed in SPring-8 BL02B2.

MYTHEN detectors have six one-dimensional semiconductor detectors
manufactured by DECTRIS company, that are installed at multiple 2θ-axes. In themeasurement process, there are two kinds of collecting modes that can be used. One isthe double-step mode. The six detectors are aligned at intervals of one detector, and aseries of data can be obtained by measuring from the reference position and from thereference position by one detector and measuring at the same time. The other one is
the single-step mode, which is usually used in the measurement only a certain angularrange data needed. The measurement time of this mode generally is shortenedcompared with double-step mode, which is owing to no displacement of the detectors
through matching the position of the detector at the needed angular range.
The IP data can be obtained using the imagine plate, which is then read throughan IP reader. People can directly obtain the diffraction ring on the IP, then we need totransform this diffraction ring into one-dimensional pattern. However, compared with
the MYTHEN camera, the diffraction pattern can be directly obtained with
one-dimensional data, and the data of the diffraction pattern can be obtained mucheasier with shorter diffraction time. Besides, the MYTHEN diffraction angle
resolution is higher than that of IP, it is possible to combine the sweeping temperature31

measurement that is usually used in the determination of phase transition by observing
the absolute change in peak position while changing the temperature. it can collecteven a very small intensity of diffraction, therefore, the background on the low angle
side will become large. In contrast, the IP camera detects only the intensity higher
than a certain value, the background is much smaller than the MYTHEN data, and theintensity of signal to noise ratio is much better. Hence, IP data is more suitable for the
charge density study or full-pattern Rietveld refinement owing to the betterbackground.

2.3 Sample preparation

First, using the solid-state heating reaction method, the parent polycrystalline
BaBiO3 sample was successfully prepared. The starting powders were thoroughly
ground to form a homogeneous fine powder consisting of Ba(NO3)2 (Aladdin.,
99.99%) and Bi2O3 (Aladdin., 99.99%) powder with stoichiometric amounts. Then thestarting powder was then heated at 600 °C for 24 hours. After the first heating process,the obtained powder was ground again. Then second heating processs is performed
under 800 °C for 8 h in air. After that, the successfully obtained BaBiO3 sample wasslowly cooled to the room temperature.
Second, the parent BaBiO3 sample was then heated at different temperature for 5
h in flowing N2 gas atmosphere placed in the ceramic containers. The heating andcooling process are at the rates of 10 and 5°C/min, respectively. Several temperatures
were used in the heating process of 600, 700, 800 and 900 °C. The obtained BaBiO2.5
samples were found to be pure under 800 °C. The BaBiO2.5 samples are extremely
sensitive to air, where oxidation can occur. Hence, the BaBiO2.5 samples were stored
in a glovebox with N2 gas atmosphere.

32

The polycrystal samples are sealed into the capillary with a small inner diameter0.1 or 0.2 mm. The small size of capillary will almost eliminate the influence on thediffraction data. There are several kinds of capillary aiming at different temperature
region. Above 500 K, a quartz capillary is suitable for the measurement. Below that, aboro glass capillary is acceptable owing to the low melting temperature point.
However, the influence of boro glass capillary is much smaller than the quartz
capillary. People need to aware that the absorption of these two kinds of capillary issmall enough to ignore in the experiments.

2.4 Experiment condition
In the research work of BaBiO3, all the powder diffraction pattern was collectedusing the apparatus mentioned above. Electron charge density studies were performedat the BL02B2 powder X-ray diffraction beamline in SPring-8 using a large Debye–
Scherrer camera. High energy X-rays with a short wavelength of 0.414005(1) Å (30keV) were used. One may pay attention to the decision of synchrotron radiationenergy in the experiment, which should be slightly higher than the K edge absorptionenergy of the elements involved in the compounds. The powder sample was thensealed in a quartz capillary of 0.1 mm internal diameter for the high temperature33

measurements, meanwhile, the boro glass capillary of 0.1 mm inter diameter was used
for the low temperature measurements. The SXRD patterns were measured in thetransmission geometry. If the sample contains heavy elements such as Bi, theabsorption effect in X-ray diffraction must be considered. However, in thisexperiment, the absorption effect was negligibly small because high-energy X-rays
were used. The phase transitions were investigated during temperature sweeps from100 to 1100 K with a every 5 K interval and heating rate of 20 K/min using the
MYTHEN detectors.[86] Precise SXRD data with good counting statistics forelectron charge density studies were collected at 900, 600, 300, and 100 K in eachphase using an imaging plate IP. The temperatures can be controlled within 1 K by theN2 gas flow temperature controlling system. The crystal structure and electron charge
density distributions were determined by using the MEM/Rietveld method. The MEM
analyses were performed with the unit cell divided into small voxels. The volume of
one voxel corresponded to approximately 0.05 × 0.05 × 0.05 Å3. The MEM analysis
was performed using the program package ENIGMA.[87] The crystal structure andMEM electron charge density distribution maps were plotted using the program
VESTA.[88]The Vienna Ab initio Simulation Package (VASP) was used in First-principles
calculations.[89,90] The projector augmented wave method was applied in thetreatment of Electron–ion interactions.[91,92] Using the Perdew–Burke–Ernzerhof
function, the generalized gradient approximation calculation of exchange-correlationinteractions was modified. The valence electrons included Ba 5s2 5p6 6s2, Bi 5d10 6s2
6p3, and O 2s2 2p4 in the calculation. In all calculations, the plane-wave kinetic energy
cutoff is 500 eV, and k-point spacing for Brillouin zone sampling is 0.25 Å-1. The
crystal structures were determined by relaxing the structure, where the magnitudes of
all forces worked on the atoms should be less than 0.01 eV/Å.
In the experiment of BaBiO2.5 samples, the TG measurements were carried out
on a Netzsch STA 449C thermal analyzer, the BaBiO2.5 powder samples were heatedunder an oxygen atmosphere with a flow rate of 40 ml min-1, which will enable us to34

determine the amount of oxygen in the obtained BaBiO2.5. The heating treatment was
performed from 30 to 800 °C with a rate of 5 K min-1. The ordinary powder XRD
measurements with lower resolution were performed using a desktop diffractometer(D2 PHASER, Bruker, λ= 1.54056 Å) under 30 kV and 10 mA radiation. In theexperiment, the BaBiO2.5 samples were carefully sealed in two pieces of tapes toavoid the pollution of air. The high-resolution SXRD data were collected on an
imaging plate using a large Debye–Scherrer camera at SPring-8 BL02B2 beamline.
The wavelength used in the experiment is stabled at 0.414096(1) Å. The BaBiO2.5
samples used in the SXRD measurement were sealed in the Hilgenberg glass
capillaries under lower temperature below 400 K with an inner diameter of 0.2 mm. In
order to avoid the occurrence of preferred orientation in diffraction pattern, therotating of sample was necessary during the measurement to average the intensity ofdiffraction pattern. The SXRD pattern of BaBiO2.5 was collected in the 2θ range from
3.0° to 70.0° (resolution of d >0.36 Å in the d spacing). The low-temperature SXRD
data of BaBiO2.5 was collected using the low temperature N2 gas flow controllingsystem with 1 K resolution. Using the MEM/Rietveld method, the crystal structureand electron charge density distributions of BaBiO2.5 were obtained. The 148 × 152 ×
120 pixels of unit cell were used in the MEM electron charge density distribution
calculation of BaBiO2.5. The program VESTA was used in the drawing of the crystal
structure and MEM electron density distribution. The BaBiO2.5 samples were sealedin two pieces of quartz glasses in the collection experiments of PL, Raman scattering,
and absorption spectra. The emission and excitation spectra of PL experiments and atroom-temperature were collected using a FLS 980 spectrofluorometer (Edinburgh
Instruments Ltd.). The NIR PL spectra under the 658 nm laser diode at different
temperature were collected using an electrically cooled PMT (Hamamatsu,
H10330-75) equipment and analyzed by the monochromator (Horiba, iHR550). The
different excitation power densities from 0.05 to 1.2 W/cm2 of 658 nm laser wereused in the PL measurements. The 658 nm laser light was the third harmonic of a
Nd:YAG laser pumped by an optical parametric oscillator. The Raman scattering
measurements were performed under a 633 nm laser line with a power of 5 mW using35

amicro-Raman

spectrometer (Horiba

Jobin

Yvon, Labram HR 800).

The

backscattered radiation was collected by microscope optics and scattered by amonochromator installed with a holographic grating. The slit and pinhole sizes are
100 and 300 mm, respectively. The UV-vis-NIR spectroscope (Cary 5000, Agilent)with an integrating sphere was used in collecting of diffuse reflectance spectra of the
BaBiO2.5. The Kubelka-Munk transformation was used to convert the obtainedreflectance spectra into the pseudo-absorbance spectra. At the 1W2B beam line
located at the Beijing Synchrotron Radiation Facility, the X-ray absorption spectra
were collected at the Bi LIII edge of BaBiO2.5, Bi2O3 and Bi. All spectra were collected
at room temperature. The Bi and Bi2O3 powders were used as the referencescompared with BaBiO2.5 sample. The IFEFFIT software package was used in theanalyzing of the XAFS data.[89] The FEFF6 computational package was used insimulating the scattering amplitudes of Bi-O, Bi-Bi, and Bi-Ba paths used in fitting
the EXAFS spectrum of BaBiO2.5, originating from the experiment structure result.
Eight single scattering paths were included in the calculation model. The EXAFS
fitting results showed the 0.8 of amplitude reduction factor S02, and Bi2O3 sample wasused as a reference sample. The fitting was carried out in R space from 1 to 4 Å, on
the FT of the k3-weighted EXAFS.
The projector augmented wave (PAW) potentials in the VASP code was used inspin polarized density functional theory (DFT) calculations.[90] A single point at thegeneralized gradient approximation (GGA) lattice geometry was used in the Hybrid
DFT calculation. The structure used in the calculation is modified using the GGA
scheme with Perdew-Burke-Ernzerhof (PBE).[91] The 450 eV of the kinetic energy
cutoff for the plane-wave basis was determined. The Monkhorst-Pack k-point meshwith 5 × 5 × 5 was used to take sample in the Brillouin zone. When the maximum
forces were less than 0.01 eV·Å-1 and the total energy was converged to 1×10-4
eV/atom, the ions were relaxed. The band structure and density of state (DOS) of
BaBiO2.5 were analyzed by the hybrid exchange and correlation functional of
Heyd-Scuseria-Ernzerhof (HSE06).[92] Considering the fact of the weak chemical36

bonding among the crystal structures of BaBiO2.5, we used a damped van der Waals(optB86b-vdW functional) correction in the calculation.

Chapter 3
Analysis method
3.1 Rietveld analysis
In the early 1969 years, Rietveld reported his original research to analyze theneutron powder diffraction data.[93] Since then, the Rietveld refinement has beenrecognized as a powerful method to analyze the crystal structure. In the Rietveld
analysis, the parameter of profile and cell are carefully refined based on a given
structure model that is similar to the structure of your compound, meanwhile, thediffraction optics effects, and instrumental factors are also considered. After years ofdevelopment, it has been wildly used in the analysis of crystal structure nowadays
using the characteristic XRD or SXRD, and time-of-fight (TOF) neutron diffraction
from the spallation neutron source.
The Rietveld refinement results of the diffractions can simultaneously provide
many kinds of information including the lattice constants, atomic position, thermal
vibration, vacancy and so on. Based on these factors, lattice distortion and crystallite
size can be discussed. Meanwhile, the mass ratio of each phase in a mixture can be
determined by the ratio of scattering factors. Rietveld analysis is suitable for the
crystal structure analysis with the premise that people understand the structure of thetarget sample. If the crystal structure of the sample is unknown, it is necessary toanalyze the crystal structure using maybe the Transmission Electron Microscopes.
In the Rietveld analysis, the diffraction pattern calculated based on an
approximate structure model, then, measured diffraction pattern should match thecalculated diffraction pattern as closely as possible. After that, the structure
parameters can be optimized through eliminating the model deviation. The diffraction37

data measured from the X-ray diffraction experiment are fitted using a theoretical
function by nolinear-least-squares methods. The error sum of the squares can beobtained using the following function in the Rietveld refinement.

S ( x)¦ w [ y  f ( x)]

2

i

i

(3-1)

i

i

The S(x) can be minimized during the optimization with the variable parameter x.
Here yi (i = 1, 2, 3…) displays the observed diffraction intensity, and wi (=1/yi)corresponds to the statistical weight at the ith measurement point. fi(x) = f (2θi ; x1, x2,x3,…), which is the calculated diffraction intensity. The calculated diffraction intensity
has a function shown below.

( )=( ) ( ) ( )

|

|

(

) (Δ2

)+

(2 )

(3-2)

( ) is related to the roughness effect of the surface at

The scale factor refers to s.

low scattering angles in the Bragg-Brentano X-ray diffraction geometry.
shows an absorption in the Debye-Scherrer geometry,( )( ) is related to the effect ofa width of divergence slit in the Bragg-Brentano X-ray diffraction geometry. K showsrefers to the multiple factor,

the Miller index of the reflection.
structure factor,

shows a preferred orientation,

polarization factor, and (Δ2represents the) refers to the Lorentz() is a reflection profile factor.

(2 ) shows the

background intensity at ith point.
Generally, R factor acts as an index for evaluating the Rietveld analysis results.
There are several kinds of R factors involved in this paper, such as RWP, RI, and RF.
The definition is shown below.
­
2®¦ wi [ yi  f i ( x)]
¯ i

(3-3)

¦ I c (H )  I (H ) ¦ I c (H ) ,(3-4)¦ Fc (H )  F (H ) ¦ Fc (H ) ,

(3-5)

RWP

RI

obs

cal

H

RF

1

½2
¦i wi yi ¾¿ ,
2

obs

H

obs

cal

H

obs

H

RWP shows a weighted reliability factor of fitting result between the whole38

experimental diffraction with the calculated diffraction pattern, RI shows a reliability
factors comparing the calculated diffraction intensity and observed diffraction
intensity, and RF evaluates the difference of the crystal structure factor between
observed and calculated diffraction pattern. In these functions, Iʹobs(H) and Ical(H)refer to the observed and calculated integrated intensity of Bragg reflections,
respectively, and Fʹobs (H) refers to an estimated structure fator of observed
diffraction. The variable parameter H refers to (h, k, l). If these reliability factors are
smaller than 5%, which means the calculated crystal structure is consistent with theassumed structural model, it can be believed that reliable crystal structure are
obtained.
In the Rietveld refinement, apart from the fitting of lattice parameter, atom
position and thermal vibration, fitting of diffraction profile is also necessary. The
half-value width of profiles in the diffraction pattern is fitted using a half-width
function. The half-width function defines the half width W of the profile in thecalculated diffraction based on the structural model. The definition of the function isshown below.
1

W= u  v u tan θ+w u tan 2 θ  p u sec 2 T 2(3-6)

where u, v, w, and p factors describe the profile of calculated diffraction pattern.

3.2 Maximum entropy method (MEM)
The Maximum Entropy Method (MEM) is a model estimation method based on
the information theory.[94,95] In early 1967 year, it was proposed by J. P. Burg toanalysis the earthquake wave.[96] The MEM is recognized as the most potential
method to deal with the limited number of information. MEM has been successfully
used in various data analysis in many different fields. Nowadays, MEM has been
widely applied in a wide range of spectral analysis and image processing techniques.
In MEM analysis, we can obtain a solution that maximizes information entropy
under a constraint condition based on the information obtained in the experiment. In
39this paper, we used MEM method to determine the electron charge density
distribution. During the process of determination of electron charge density
distribution, the whole structure factor of the crystal structure can be estimatedbeyond the limitation of the experimental diffraction whose structure factor is usuallyzero without diffraction intensity. The MEM charge density study can only observe
the outer shell electrons distribution, such as localized bound electrons andconduction electrons rather than inner shell electrons. These outer shell electrons
contribute greatly to the diffraction intensity of low angle region, whose distribution
is spatially existing far away from the nucleus. MEM method is more suitable forpolycrystal samples rather than single crystal samples owing to its extinction effect in
the low angle region that cannot be ignored. The MEM analysis can provide the direct
visualization of chemical bonds between atoms. The attenuation effect in the low
angle region can be avoided in the angular dispersion type powder diffraction, and thepossible existed overlap of reflection in low angle region can also be ignored. The
first charge density study using MEM is performed for Si. Figure 3-1 shows thecomparison between structural factors of calculated MEM electron density analysis of
Si and the observed experimental structural factors.[97,98] The structural factor
obtained by MEM electron density analysis shows a good agreement with theobserved structural factor. A smooth curve of MEM structural factor is observed
extended to the high angle region. In contrast, the observed structural factor can only
be observed with sinθ/λ < 0.8 Å-1 owing to the high abortion.

40

Figure 3-1 Structural factors and observed structural factors calculated by MEM
electron density analysis of Si. Although the observed structural factor is only sinθ/λ =0.8 Å-1, structural factors can be predicted up to the high angle region by MEM
analysis.

The MEM analysis of Si gives the detailed structural information including thebonding characteristics between atoms. The two-dimensional electron charge density
distribution of the Si (110) plane using the ordinary direct Fourier method and MEM
is shown in Fig. 3-2.[99] Both electron charge density distributions can be obtained
based on the same experimental measured structural factor with a range of d > 0.58 Å(Q < 0.86 Å−1). It is obvious that electron density distribution around the atomic
position can be obtained using the conventional direct Fourier method, however, theelectron charge density distribution between atoms such as the bonding electronsappears to be irregular, which is because of the defect of truncation effect. Generally,
to observe the electron charge density distribution in the real space using conventional
direct Fourier method, the whole inverse space is required including the high Q data,
which usually mismatch the experimental result.

41

Figure 3-2 Difference in Si electron density distribution due to different analysis
method; (a) the conventional direct Fourier method (contour lines -3.0 to 3.0 eÅ-3,
with step 0.3 eÅ-3), (b) MEM electron density analysis (contour lines 0.1 to 2.0 eÅ-3,
with step 0.1 eÅ-3). (110) plane electron density distribution. It has been analyzed
using structural factors of the same range (d > 0.58 Å) measured by X-ray diffraction
experiments.

In contrast, the MEM can clearly obtain not only the electron charge density
distribution around atoms but also the existence of covalent bonds between Si atoms.
The overlap of the electron charge density is directly observed. This is because MEM
can successfully estimate the whole structural factor including high Q region whichcannot be actually experimental measured. Hence, MEM method has a goodadvantage in analysis of the electrons density distribution of bonds based on the finite
number of structural factors.

42

Figure 3-3 Structure structural factor of Si including the observed structural factor
Fobs and calculated structure factor Fsim by the Pendellosung Method. Si is assumed to
be a complete ion of 4+ valence state. Fobs and Fsim have slight differences in low
order diffraction.[99]Besides, the electron density distribution highly depend on the range of Q. Figure
3-3 shows a comparison between the experimental structure factor Fobs used in Fig.
3-2 and the calculated structural factor Fsim using the Pendellosung Method.[100] In
the range where Q is large with a larger diffraction index, the calculated Fsim structural
factors almost show no deviation from the measured Fobs structural factors. However,it should be understood that the difference of structure factor is much larger in thesmall Q range. This is owing to the information of the outer shell electrons and boundelectrons included in the small Q range. As a consequence, to investigate the charge
density distribution in the covalent bonding, the structural factors of low Q region
should be carefully be estimated.
On the other hand, as already mentioned above, the Fourier transform shows adisadvantage in structure factor analysis of high Q data is. Q is derived from theBragg's diffraction equation.
2d sinθ = λ,(3-7)

Q = 4π sinθ / λ = 2π/d.

(3-8)

The high Q data contain a smaller d value, which will give us more detail of theelectron charge density distribution in spatial crystal structure. In a sense, theparameter d can be used to measure the resolution of the crystal space. In MEM
43analysis, the structure factor of high Q region is necessary, otherwise, an error ofstructure factor estimation may occur in the calculation. So high resolution ofdiffraction data with a wide-angle range is needed. However, in the experiments of
X-ray diffraction, atomic scatterings and thermal vibration will sharply reduce theintensity in the high Q angle region, the statistical precision of high Q data is
generally poor. As a consequence, the analysis result might be incorrect. In order to
get a precise analysis results, it is obvious that the statistical accuracy of experimental
observed structure Fobs should be as high as possible in a wide Q range.
In the case of X-rays diffractions, considering the electron density distribution of
the unit lattice to be one whole scatterer, the structural factor can be defined using thefollowing function.

F (H )

V ³ U ( xyz )e 2Si hx  ky lz dxdydz(3-7)

The entropy can be defined as,

U c(r ).
U0c (r )(3-8)

U c(r ) U (r ) ³ U (r )dr ,(3-9)

U 0c (r ) U 0 (r ) ³ U 0 (r )dr ,(3-10)

S¦ U c(r ) lnrHere, r = (x, y, z).

where ρ(r) is the electron density at a certain pixel r, and ρ0(r) is the electron density
in a reference state. Here we introduce constrain C as given information.

Fcal ( H )  Fobs ( H )
1
,
¦
V H2
N
2

C

(3-11)

where Fobs (H) is the observed crystal structure factor by experiment, N is the numberof structure factors, and σH is the standard deviation for Fobs (H). Fcal (H) can beobtained using the following function.

Fcal ( H ) V ³ U (r ) exp  2Sir  H dr .

(3-12)

V is the unit cell volume. C is expected to be 1 from the error theory. Combined with44

the equations (3-7) and (3-10), the following entropy Q(λ) equation is obtained.

Q(O ) ¦ U c(r ) lnrU c(r ) O
 C 1 ,U 0c (r ) 2(3-13)

where λ is Lagrangian multiplier. Using the approximation ∂Q(λ)/∂ρ(r) = 0 which
maximizes equation (3-10), finally we have,
ª

1

º

¬

H

¼

U (r ) exp«ln U 0 (r )  OF0 ¦ 2 ^Fobs ( H )  Fcal ( H )`exp  2Sir  H »V(3-14)

where Fcal(H) is approximated as a Fourier transform of ρ0(r)Fcal ( H ) V ³ U 0 (r ) exp  2Sir  H dr .

(3-15)The right side of the expression (3-13) is a function that depends only on ρ0(r).
In an actually calculating process, firstly given a uniform electron charge density
distribution to the ρ0(r), then ρ(r) is obtained. The operation of setup values in ρ0(r) is
repeated until C no larger than one to finally obtain the electron density distribution.
The electron charge density distribution ρ(r) obtained by this method does not changewith different initial value ρ0(r). The ultimate goal is to get the accurate Fobs(H).

3.3 MEM/Rietveld method

MEM/Rietveld method is a combination of MEM method and Rietveld analysis
mentioned above to study the charge density distribution study. The electron charge
density distribution can be obtained using the synchrotron-radiation X-ray powder
diffraction data combined with MEM/Rietveld analysis. The method can be simply
described as the following process.[101]In the powder diffraction experiment, the Bragg reflections are projected on the2θ-axis, so the reflections overlap each other. Therefore, in order to obtain theelectron charge density distribution from the diffraction pattern, the following
processes should be taken.
(1) Separate the multiple overlaps of Bragg reflections and multiple every productof Bragg reflection.
45

(2) Various corrections are applied to obtain the absolute value of the crystal
structure factor based on the obtained integrated intensity.
(3) Assume a crystal structure model and determine the crystal structure factor
based on it.
(4) Perform the MEM analysis using the obtained crystal structure factor.
The first three steps are involved in the Rietveld analysis. The profile separation
can be obtained using the structural model that best matches the experimental powder
diffraction pattern in the Rietveld analysis. Various corrections involved in the crystal
structure model are needed to obtain the precise structural factor, such as the lattice
parameter, atomic position, thermal vibration, profile and so on. Finally, electron
charge density distribution analysis by MEM is carried out based on the crystal
structure factor obtained from the modified crystal structure model. One should noticethat comparison between the electron charge density distribution and crystal structureis needed. when the obtained electron density distribution shows no agreement withthe crystal structure model, the crystal structure model should be modified to matchthe result of the electron density distribution, and Rietveld analysis should be
performed again. Through these several repeating cycles, we can finally obtain the
crystal structure model and the corresponding electron density distribution. Figure 3-4
shows a flowchart of the MEM/Rietveld method.[101]46Figure 3-4 Flow chart of the MEM/Rietveld method.

Chapter 4
Electron charge density study of BaBiO3Perovskite-type oxide BaBiO3 has been recognized as an important mothercompound for superconductors. However, convincing experimental evidence
associated with the charge order of Bi ions in BaBiO3 is lacking. Through analyzing
synchrotron-radiation X-ray powder diffraction data combined the maximum entropy
method/Rietveld method, we present the charge order of Bi3+ and Bi5+ ions and thenature of chemical bonds in BaBiO3 that are visualized clearly in electron charge
density distribution maps. BaBiO3 undergoes successive phase transitions from the47

cubic phase with space group Fm3m to the rhombohedral R3, monoclinic I2/m, andanother monoclinic P21/n phases with decreasing temperature. The Bi3+-O bond isconsiderably more ionic than the Bi5+-O bond in all four phases. BaBiO3 can be
regarded as an ionic crystal comprising isolated Ba2+ and Bi3+ ions as well as[Bi5+O6]7- octahedra in the Fm3m symmetry; a three-dimensional covalent bonding
network is formed in the R3 and I2/m symmetries, and a two-dimensional layered
structure comprising Bi3+O4 planar squares and Bi5+O6 octahedra is established in the
P21/n symmetry at low temperatures. Our study provides a comprehensive
understanding of the structural characteristics, electron charge density distribution,and chemical bonds of BaBiO3, which may be useful for developing BaBiO3-derived
functional materials such as superconductors.
In this paper, we report the results from high energy synchrotron radiation
powder diffraction and charge density study of BaBiO3 to investigate the charge orderand phase transition behavior. We demonstrate the crystal structure of BaBiO3 withfour phases at temperature in fine steps from 100 to 1100 K. Using synchrotron
radiation XRD, we successfully find the direct evidence of charge order in BaBiO3 bydetecting the small superlattice reflection peaks. Specially, MEM analysis of SXRD
data have proven to be a potential way to determine the electron charge density
distribution and corresponding the bonding nature between neighbored atoms, whichenables us to directly visualize the electron charge densities around Ba, Bi, and O ions.
The difference in chemical bonding on the Bi3+–O and Bi5+–O bonds is clearlyvisualized. We found that the Bi3+–O bonding shows unusual characteristics with thedependence of temperature, revealing unusual hybridization between O 2p and Bi 6s
or 6p orbitals. Our works find the bond lengths are slightly diverse with thedependence of temperature, which is together with the transformation of the bonding
nature in charge density level. The phase transition mechanism has been disserted
from the charge density viewpoint.

48

We herein report the results of electron charge density distributions of BaBiO3
through analyzing SXRD data using the MEM/Rietveld method, the charge order of
Bi ions and the nature of chemical bonds in the four different phases are successfully
demonstrated. Small superlattice reflection peaks originating from the charge order of
Bi ions at B-sites with a double-perovskite-type structure are successfully detected in
the SXRD pattern. Bi3+ and Bi5+ ion sites in the structure could be successfully
distinguished, and the difference of bonding nature between Bi3+–O and Bi5+–O bonds
was clearly visualized in MEM electron charge density distribution maps. The
structural characteristics were summarized for the four different phases with the
viewpoint of chemical bonding nature. The observed nature of chemical bonds andphase transitions were evaluated by first-principles calculations.

4.1 Phase transition of BaBiO3

In order to confirm the phase transition behavior of BaBiO3, we then performed
the temperature-dependent synchrotron-radiation XRD measurements as shown in Fig.
4–1. Through observing the change of peak position and peak splitting, we candefectively determine the possibility of phase transition occurring. The different color
shows the intensity of peaks with the lowest blue as background and highest red as
peak position. During the measurement, we control the temperature from 100 to 1100
K with a rate of 20 K/min through the flowing N2 gas. It is apparent that there exist
four different phases in BaBiO3 from 100 to 1100 K.

49

Figure 4–1. Temperature-dependent SXRD patterns of BaBiO3 in the 2T ranges of
15.0°-17.8° upon heating from 100 to 1100 K with a rate of 20 K/min. The fourphases are separated by broken lines at the phase transition temperatures.

Unfortunately, BaBiO3 sample is destroyed by the high temperature around 1100
K owing to the evaporability of bismuth which avoid us to observe the possible
evolution of Bi ions ordered state. In the beginning, we assume that the orderedstructure will be destroyed at higher temperature than 1100 K, which will show adisordered structure with a average Bi4+ valence state. We note the diffraction peaksof Fm3m cubic phase at ~15.4° (440)c, 16.3° (442)c and 17.2°(620)c split at ~867 K,
which can be attributed to the R3 rhombohedral structure. Further decreasing thetemperature at ~443 K will lead to the appearance of more peaks split corresponding
to the I2/m monoclinic structure. At lower temperature around 116 K, anothermonoclinic phase with space group P21/n occurs, accompanied with these splitting
peaks shrink. All these observations unambiguously confirm three phase transitions
occurring over a broad temperature window (100-1100 K) with four different stuctur,
which give us a clear crystal structure evolution. Generally, the temperaturedependence of the lattice parameter is expected to reveal the appearance of phasetransitions. As a consequence, we perform this comparison among these four different
crystal structures. Figure 4–2 clearly presents the lattice parameters in all four phaseswith the dependence of temperature, which is scaled to compare the primitive cubic
50

parent subcell.

Figure 4–2 The reduced lattice parameter of the primitive cubic subcell in BaBiO3
with the dependence of temperature.

One may notice that the continuous transition from the primitive P21/nmonoclinic to the second I2/m monoclinic phase has an obvious impact on thetendency of the lattice parameter. In P21/n monoclinic phase, the lattice parameter bm
and cm shorten with increasing temperature, the lattice parameter am lengthened at thesame time. In contrast, the discontinuous phase transition around 440 K reveal afirst-order transition from I2/m monoclinic phase to R3 rhombohedral phase. With
increasing temperature, the two phases get the larger lattice parameter. Besides, there
is a two-phase coexistence temperature region from 439–443 K, which is able toobtain lattice parameter in the monoclinic and rhombohedral phase at the same time.
Above 867 K, another continuous phase transition to cubic Fm3m occurs.

4.2 Crystal structure and charge density at 900 K

To gain a deep insight into the crystal structure of every phase, we hence tookhigh-resolution synchrotron-radiation XRD experiments of BaBiO3 at different
temperatures, which have been used in Rietveld analysis and MEM analysis. Figure51

4–3 shows the SXRD pattern of BaBiO3 measured at 900 K and the related structurefrom the Rietveld refinement results. The inset shows the superlatice peaks, which
reveals the B-site charge order structure.

Figure. 4–3 SXRD pattern and structure refinement results of BaBiO3 at 900 K (2T <70°, d > 0.22 Å). The observed (circles), calculated (line), and difference (bottomline) are shown. The ticks indicate the Bragg reflections allowed with space group
Fm-3m. Rwp = 4.78%, Ri = 2.85%, Rf = 2.38%. The inset shows the supperlattice
peaks.

The refined crystal-structure parameters using the Rietveld refinement are listed inTable 4-1. The diffraction peaks can be assigned on a cubic Fm3m ordered double
perovskite with lattice parameter double times of the lattice constant of simple
disordered perovskite.

52

Table 4–1 Refined Structural Parameters of BaBiO3 at 900 K in cubic phase.aaAtom Site

x

y

z

U11

U22

U33

Ba

8c

0.25

0.25

0.25

3.18(1)

=U11

=U11

Bi3+

4a

0

0

0

1.21(2)

=U11

=U11

Bi5+

4b

0.5

0.5

0.5

1.67(2)

=U11

=U11

O

24e

0.2619(32)003.37(63) 9.67(47) =U22

The space group is Fm3m (No. 225). The lattice parameter is a = 8.7721(2) Å. The

isotropic thermal parameter U11 = U22 = U33 is refined for Ba, Bi3+ and Bi5+. The
anisotropic thermal parameters U11 and U22 = U33 are determined for O. Off diagonal
elements, U12 = U13 = U23 = 0. Uij are given in units of 10-2 Ǻ2.

The absence of some diffraction peak suggests the face-centering of the orderedunit cell, whose index shows the h + k, k + l, and h + l = odd [102] The appearance ofdiffraction peaks of (531) and (533) peaks shown in Fig. 4-3 indicates the chargeordering of Bi3+ and Bi5+ ions. The Rietveld refinement results clearly prove thatBaBiO3 forms a B-site ordered double perovskite structure. In this cubic space group,
Ba ions are located at 8c (0.25, 0.25, 0.25) position, Bi3+ ions at 4a (0, 0, 0) position,
Bi5+ ions at 4b (0.5, 0.5, 0.5) position, and the O ions at a special site 24e (x, 0, 0),
respectively.[103] The only changeable position parameter is the Ox, which attributes
the Bi3+–O and Bi5+–O bond length. The Bi–O bond lengths were found to be Bi3+
2.298 Å and Bi5+ 2.088 Å. Based on the refined bond length, bond valence sum (BVS)
calculations give a valence state to be 3.46 for Bi3+ site and 5.56 for Bi5+ site,[104]suggesting a charge order of Bi3+and Bi5+ ions. One may notice the thermal vibration
factor of the oxygen ions along the direction perpendicular to the Bi-O bonds,
UO22(=UO33), is larger than UO11 along the direction parallel to the Bi-O bonds, which
is owing to the existence of covalent bonding between Bi5+ and O shown in Fig. 4–4.
The reliable factor of structure factors RF determined from Rietveld analysis was 2.38%
53

used in the final MEM charge density analysis. There are 436 observed structure
factors from the Rietveld analyses for 900 K data that were used in the MEM analyses.
The MEM analyses were performed in an unit cell divided into 176 × 176 × 176
pixels. The volume of one pixel is equal to about 0.05 × 0.05 × 0.05 Å3.
The charge-density distributions of Three–dimension (3D) and Two–dimension(2D) on (100) plane at 900 K are shown in Fig. 4–4(a) and 4–5(a), respectively. Herethe isosurface represents the average distribution of electron clouds. Compared withthe core electron density size of Bi ions, Bi5+ ions are more localized than Bi3+ shownin Fig. 4–4 (b) and (c), which is consistent with the fact that the radii of Bi5+ ions(0.76 Å) are smaller than Bi3+ ions (1.03 Å). From this point, we can easily
distinguish the Bi ions with different valence state at different site.

Figure 4–4 (a) 3D MEM electron density distribution at 900 K with space group
Fm-3m. The isosurface is 0.5 e Å-3. 1D charge density around (b) Bi5+, and Bi3+ (c)cut at the minimum charge density point in O-Bi3+/Bi5+-O bond along [010] direction.

The electron charge density distributions around O in cubic BaBiO3 are quite
anisotropic as shown in Fig. 4–4(a), extending along the direction perpendicular to the
Bi5+–O bond direction, which is in good agreement with the larger UO22(=UO33)
thermal parameters in the Rietveld analysis results. The electron charge density
54

distributions around Ba2+ in cubic BaBiO3 are essentially ionic, there is no electron
density overlap observed between Ba2+ and O2-. The minimum charge density on thelonger Bi3+–O bonding is 0.40 e Å-3. This is the same value as the background (~0.36
e Å-3) level, which is owing to the strong thermal vibration at 900 K. On the other
hand, the electron charge densities of Bi5+ and O are found to be highly overlapped.
The minimum charge density on the shorter Bi5+–O bonding in Fig. 4–5(a) is charged
up to 0.62 e Å-3, which is much larger than the Bi3+-O bond. This demonstrates thatthe Bi3+-O bonding nature of the cubic phase should be ionic, however, that of Bi5+–O
is found to be strong covalent, which forms an special ionic crystal consisting of two
Ba2+ and a Bi3+ ions, and an isolated (Bi5+O6)7- octahedron. In order to confirm therationality of the experiment results, Figure 4–5(b) shows the electron charge density
distribution on the (100) plane determined by the first principles calculation.

Figure 4–5 (a) 2D MEM electron density distribution of BaBiO3 on the (100) plane at
900 K in cubic phase, contour lines are drawn from 0.0 to 4.0 e Å-3 at intervals of 0.5
eÅ-3 ,and (b) electron density distribution determined by the first–principles
calculation based on the atomic coordinates.

In the First-principle calculations, the actual cubic lattice parameters (see Table 1)
at 900 K obtained from Rietveld analysis were used. The First-principle calculation
results quantitatively reproduces MEM charge density compared with theexperimental results. As for the minimum charge density between Bi3+/Bi5+ and O,55there is not much difference between the experimentally obtained value and thetheoretically calculated value using First-principle calculation shown in Fig. 4–5(b).
One may notice that the electron charge density distributions around oxygen are quite
isotropic without considering the thermal vibration in the calculations. The minimum
electron density is 0.48 e Å-3 and 0.36 e Å-3 in the Bi5+–O and Bi3+–O bonds,
respectively. No overlap of the charge densities is observed between Bi3+ and O.
Meanwhile, covalent bond nature of Bi5+–O bonding is found, showing good
agreement with the experiment results. The ionic crystal structure with two Ba2+ and a
Bi3+ ions, and an isolated (Bi5+O6)7- octahedron is clearly determined.

4.3 Crystal structure and charge density at 600 K

With decreasing the temperature, first phase transition occurs from cubic torhombohedral structure with R3 space group through the a-a-a- Glazer tilting. Figure
4–6 and Table 4–2 shows the Rietveld refinement fitting results and refined structuralparameter at 600 K, respectively. The inset shows the high angle fitting results.

Figure 4–6 XRD pattern and structure refinement results of BaBiO3 at 600 K (2T <70°, d > 0.22 Å). The observed (circles), calculated (line), and difference (bottom line)
56

are shown. The ticks indicate the Bragg reflections allowed with space group R3. Rwp= 3.77%, RI = 3.65%, Rf = 6.35%. The inset shows the high angle fitting result.

Table 4–2 Structure parameters of BaBiO3 at 600 K in rhombohedral phase.a

a

Atom

Site

x

y

z

B

Ba

6c

0

0

0.2494(2)1.57(1)

Bi3+

3a

0

0

0

0.92(1)

Bi5+

3b

0

0

0.5

0.44(1)O

12f

0.5172(19)-0.0340(20)0.2580(7)2.75(17)

The space group is R3 (No. 148). The lattice parameter is a = 6.1850(1)Å, andc=15.0858(2)Å.

Based on the coordinates, the Bi–O bond lengths in two octahedrons are 2.282 Å
and 2.125 Å for Bi3+ and Bi5+ sites, respectively, which is slightly different from two
octahedrons obtained at 900 K cubic phase, giving the BVS valence state to be 3.61
for Bi3+ and 5.03 for Bi5+, respectively. There were 2484 structure factors that wereused in the MEM analysis, which were determined from the Rietveld refinement in a
d-space range larger than 0.36 Å. The MEM analysis was carried out with the unit cell
divided into 126 × 126 × 300 voxels.
Figure 4–7 shows the 2D MEM electron density distribution on the (112) planeat 600 K. The electron densities are both isotropic and highly overlapped in the
Bi3+/Bi5+–O bonds. The minimum of the charge densities is charged up to 0.78 e Å-3
and 0.86 e Å-3 in the Bi3+–O and Bi5+–O bonding, respectively, which confirm theformation of covalent bonds. The minimum charge density is much larger than thevalue of the background level. In contrast, Ba2+ ions are highly ionic without any
charge density overlap with the coordinated O ions. All of this information suggests
57

that BaBiO3 forms a normal 3D covalent network through the connection of Bi3+–
O-Bi5+ bonds at 600 K. The special ionic state of Bi3+ ions at 900 K is broken insteadwith a covalent state at 600 K, owing to the rotation of the oxygen octahedron.
Meanwhile, Bi5+ ions still hold stronger covalent bonds with the coordinated O atoms
than the Bi5+–O bonds in cubic phase.

Figure 4–7 2D MEM electron density distribution of BaBiO3 on the (112) plane at
600 K in rhombohedral phase. Contour lines are drawn from 0.0 to 4.0 e Å-3 atintervals of 0.2 e Å-3.

4.4 Crystal structure and charge density at 300 K

At 300 K, BaBiO3 shows a monoclinic structure with the space group of I2/m,
and the fitting result of the Rietveld analysis are shown in Fig. 4-8 and summarized inTable 4–3. Through the a0b-b- tilt system, the oxygen positions are modified into two
different sites. As a consequence, the octahedrons around the Bi ions include twodifferent sets of Bi-O bonds, that is, Bi-O1 (Bi3+-O1 2.276 Å and Bi5+–O1 2.135 Å)and Bi–O2 (Bi3+–O2 2.262 Å and Bi5+–O2 2.149 Å) bonds.

58

Figure 4–8 XRD pattern and structure refinement results of BaBiO3 at 300 K (2T <70°, d > 0.22 Å). The observed (circles), calculated (line), and difference (bottom line)
are shown. The ticks indicate the Bragg reflections allowed with space group I2/m.
Rwp = 1.90%, Ri = 3.21%, Rf = 3.62%. The inset shows the high angle fitting result.

Table 4–3 Structure parameters of BaBiO3 at 300 K in monoclinic phase.a

a

Atom

Site

x

y

z

B

Ba

4i

0.5013(3)00.2465(1)0.72(0)

Bi3+

2a

0

0

0

0.30(2)

Bi5+

2d

0

0

0.5

0.23(1)O1

4i

0.0657(10)00.2584(12)1.33(14)O2

4j

0.2673(10)0.2453(16)-0.0376(4)1.25(9)The space group is I2/m (No. 12). The lattice parameter is a = 6.1871(1)Å, b =6.1399(1)Å, c=8.6710(1)Å, and β=90.179(1)°.

The number of structure factors determined from the Rietveld refinement was59

5353 structure factors, which were transformed in the MEM analysis. The MEM
analysis was performed in a unit cell divided into 124 × 120 × 176 voxels. In Fig. 4–9,
the MEM charge density at 300 K on the (020) plane and between Bi3+/Bi5+ and O2 isvisualized. The MEM electron charge density clearly exhibits the Bi3+–O and the
Bi5+–O bonding features along any directions in the octahedrons. Similar to the MEM
result of rhombohedral structure, the monoclinic phase at 300 K also forms a normal
3D covalent network structure at the charge density level. The minimum charge
density of Bi5+–O1 (=1.04 e Å-3) and Bi5+–O2 (=0.96 e Å-3) bonds is larger than the
Bi3+–O1 (=0.93 e Å-3) and Bi3+–O2 (=0.86 e Å-3) bonds, respectively. This bonding
electron distribution is consistent with the MEM result of rhombohedral structure.

Fig. 4–9 2D MEM electron density distribution of BaBiO3 (a) on the (020) plane
between Bi3+/Bi5+ and O1, (b) between Bi3+ and O2, and (c) between Bi5+ and O2 at
300 K in monoclinic phase, contour lines are drawn from 0.0 to 4.0 e Å-3 at intervalsof 0.2 eÅ-3.

4.5 Crystal structure and charge density at 100 K
60

In the second monoclinic phase at 100 K, the space group of double perovskite isP21/n and the tilt system is a+b-b-. The refinement results and structure parameters areshown in Fig. 4–10 and Table 4-4. One may notice that oxygen positions are modified
into three different sites. Hence, the Bi-O1/O2/O3 bond lengths in the larger Bi3+O6
octahedron are 2 × 2.450, 2 × 2.256, 2 × 2.211 Å, while bond lengths in the smaller
Bi5+O6 octahedron are 2 ×1.989, 2 × 2.127, 2 × 2.188 Å. The Bi3+–O1 bond is much
longer than Bi5+–O1 bond, which therefore shows a very different bond nature. Basedon the bond length BVS calculation, the valence state of the Bi ions are 3.5 and 5.5
for Bi3+ and Bi5+, respectively.

Figure 4–10 XRD pattern and structure refinement results of BaBiO3 at 100 K (2T <70°, d > 0.22 Å). The observed (circles), calculated (line), and difference (bottom line)
are shown. The ticks indicate the Bragg reflections allowed with space group P21/n.
Rwp = 3.28%, Ri = 2.36%, Rf = 3.02%. The inset shows the high angle fitting result.

61

Table 4–4 Structure parameters of BaBiO3 at 100 K in monoclinic phase.a

a

Atom

Site

x

y

z

B

Ba

4e

0.5037(2)-0.0099(1)0.2473(2)0.24(0)

Bi3+

2a

0

0

0

0.13(3)

Bi5+

2b

0

0

0.5

0.13(3)O1

4e0.0796(12)0.0047(24)0.2777(10)1.23(19)O2

4e

0.2632(51)0.2513(43)-0.0296(46)1.20(19)O34e

0.2681(48)0.7660(39)-0.0326(47)1.20(19)

The space group is P21/n (No. 14). The lattice parameter is a = 6.1811(1)Å, b =6.1279(1)Å, and c=8.6544(1) Å.

The number of structure factors determined from the Rietveld refinement were7272. The MEM analysis was then performed in an unit cell divided into 124 × 124 ×
172 voxels. Figure 4–11 shows the 2D MEM electron charge density distribution onthe (020) plane at 100 K.

62

Figure 4–11 2D MEM electron density distribution of BaBiO3 (a) on the (020) plane
between Bi3+/Bi5+ and O1, (b) between Bi3+ and O2/O3, and (c) between Bi5+ andO2/O3 at 100 K in monoclinic phase, contour lines are drawn from 0.0 to 4.0 eÅ-3 atintervals of 0.2 e Å-3.

Obviously, the electron charge densities around Bi3+ and O are quite anisotropic.
The electron densities are overlapped along the directions of Bi3+–O2 and Bi3+–O3with short bonds as shown in Fig. 4–11(b). In a contrast, Figure 4–11(a) shows that
Bi3+ atom is isolated with the coordinated O1 atom without any charge density
overlap. The minimum of the electron densities in the short Bi3+–O2 and Bi3+–O3bonds are charged up to about 0.41 and 0.47 e Å-3, respectively, revealing theappearance of covalent bonds. One may notice the minimum charge density along thelonger Bi3+–O1 bond is about 0.30 e Å-3, that is the same value as the charge densityof background level, revealing the ionic bonding nature. As a consequence, Bi3+ ionsform a special planar square structure. In contrast, Bi5+ ions show a normal 3D
covalent octahedron. We can observe the electron charge density overlap along the
Bi5+–O2 and Bi5+–O3 bonds in Fig. 4–11(c), where the minimum charge density is
charged up to 0.47 and 0.40 e Å-3, respectively. Besides, the shortest Bi5+–O1 bondshows the stronger covalent bonds than the other two longer Bi5+–O bonds with alarger minimum charge density of 0.73 e Å-3. All of this information suggests that63

BaBiO3 forms an intriguing 2D low dimensional covalent layer through theconnection of Bi3+O4 planer square and Bi5+O6 octahedron at 100 K.

4.6 Phase transition mechanism

To gain a deep investigation of the phase transition mechanism of BaBiO3, we
next try to reveal the phase transition mechanism at the charge density level as shownin Fig. 4–12. The evolution of electron charge density and correspond crystal structureof BaBiO3 at different temperature are clearly observed, which is that expected for theevolution of the Bi–O bonds and its minimum charge density.

Figure 4–12 The evolution of electron charge density and correspond crystal structureof BaBiO3 with the dependence of temperature.

We found the Bi–O bond length to be distinctly different in two octahedral sites
as shown in Fig. 4–13. The average Bi3+–O bonds slightly increase upon heating64

process, whereas, the average Bi5+–O bonds decrease. The Bi3+–O bonds persist larger
than the Bi5+–O bonds in the whole temperature range.

Figure 4–13 The bond length of Bi–O with the dependence of temperature.

In other word, the Bi5+–O bonds show stronger covalency, which are not easily
affected by the temperature. We can always observe the charge density overlap of Bi5+
and O in all four phases of BaBiO3 in Fig. 4–12. However, the bond nature of Bi3+–Obonds shows intriguing evolution with the dependence of temperature. At 100 K
monoclinic phase, Bi3+ ions hold a special square planner charge density
configuration with four covalent bonds and two ionic bonds. Differently, Bi3+ ionsshow a normal octahedral structure with six covalent bonds at 300 K monoclinic and600 K rhombohedral phases. The covalent nature of the Bi3+–O bonds is destroyed in
cubic phase at 900 K, instead with an ionic structure isolated with the coordinatedoxygen atoms. All these facts suggest that the phase transition accompanied with thedeformation of the charge density around the Bi3+ ions. We show the correspondstructure at the charge density level using the ball–and–stick model in Fig. 4–12, here
the sticks reveal the covalent nature of the bonds. Accordingly, upon heating, theBaBiO3 structure represent a novel evolution from a 2D covalent-bonding network65

consisting of the Bi3+O4 planner square and Bi5+O6 octahedron, to a normal 3D
covalent bonding network consisting of two BiO6 octahedrons, and to an ionic
structure consisting of the isolated [Bi5+O6]7- octahedron and Bi3+ ions.
It becomes important to see whether the change in symmetry from monoclinic torhombohedral, and to cubic is possible or not. For this, the energy hierarchy of
BaBiO3 using the first–principle method is shown in Fig. 4–14. The calculated energy
difference between different symmetry is obvious. From cubic to monoclinic
symmetry, the energy is monotonously decreasing, which reveals the more stablestructure. The ground state is found to be monoclinic phase with P21/n symmetry.

Figure 4-14 Energy hierarchy of BaBiO3 using First-principle calculation method.

4.7 Conclusion

The crystal structure and phase transitions of BaBiO3 have been studied by highresolution synchrotron-radiation X-ray powder diffraction. The charge order and thechemical bonding nature are investigated based of the electron charge density study
by MEM/Rietveld method. We find that BaBiO3 undergoes phase transitions frommonoclinic, to rhombohedral, and to cubic symmetry upon heating progress.
Importantly, we clearly reveal the charge order of Bi3+ and Bi5+ ions located at
different octahedron in the charge density distribution map. The phase transitions are66

accompanied with the variation of the bonding nature in Bi3+–O bond. We find thedeformation of charge density distribution around Bi3+ ions, which leads to thevariation of correspond structure from two-dimensional covalent layer, tothree-dimensional covalent network, and to a mutually isolated structure with Bi3+
ions and [Bi5+O6]7- octahedra. We highlight that charge density study using
MEM/Rietveld method provide a pathway to investigate the charge order and phasetransition mechanism.

Chapter 5
Electron charge density study of BaBiO2.5

Engineering environments of cations in oxides surrounded by oxygen
coordination has received much intense interest owing to many intriguing discoveriesof novel oxides with unexpected physical properties. In this work, we successful
synthezed the layered BaBiO2.5, which is acheived by a phase transformation from theparent perovskite BaBiO3. Through analysing the synchrotron-radiation X-ray
diffraction data combined with the MEM/Rietveld method, we investigated theunusual chemical bondings nature of Bi-O bonds consisting of Bi ions and fouroxygen coordinations,where an inhomogenous coordination geometry was found.

As a consequence, this special asymmetry structure results in the near-infrared
luminescence, which is different from the other ordinary Bi-containing systems. The
mechnism of excitonic nature in luminescence were proposed based on theexperimental and theoretical results. This work opens a pathway to the discover new
materials systems with intriguing functionalities in these compounds with uncommon
Bi-O bonding and Bi coordination environments. This work could inspire intenseinterest to explore functional materials with unusual properties containing heavier
elements.

67

5.1 Crystal structure of BaBiO2.5

Layered BaBiO2.5 sample was prepared by using heat treatment of parent BaBiO3
sample at 800 ºC temperature in a reductive N2 (O2<10-6) atmosphere, based on an
oxygen reduction transformation process. The polycrystalline BaBiO3 sample wasprepared using the solid-state reaction method with stoichiometric amounts ofBa(NO3)2 powder (Aladdin., 99.99%) and Bi2O3 powder (Aladdin., 99.99%). Initial
compounds were thoroughly ground for the next heating treatment. The groundpowder was heated at 600 °C for 24 hours. After cooling to room temperature, thepowder was ground and heated again at 800 °C for 8 hours in air.
During the collection of XRD diffraction pattern process, the succeeded samples
were tightly sealed sandwiched with two pieces of tapes to prevent the polluion ofwater and oxygen, and the XRD pattern of the tape was also shown in the XRD
pattern. Obviously, heating treatment of BaBiO3 at 600 ºC fails to transform theperovskite-type structure of BaBiO3. The structure of BaBiO2.5 phase did not occur,
which signifies the failure of the bond dissociation divided from the oxygenoctahedron coordination. Heating treatment at 700 and 800 ºC can both results in theoccurrence of BaBiO2.5 structure phase. Here the BaBiO2.5 phase obtained from 800ºC heating treatment shows much higher quality in the crystallinities. Increasing thetreatment temperature to 900 ºC, an unknown structure phase will occur. Besides, theother Note that the reductive atmosphere is necessary to obtain the BaBiO2.5 from
BaBiO3 phase. The attempt of heating treatment at air atmosphere failed to obtain the
BaBiO2.5 structure phase. The synthesis condition plays a critical important role in theformation of BaBiO2.5 structure phase.

68Figure 5-1 XRD patterns of the yielded products by thermal treatment of BaBiO3 atdifferent temperatures (600, 700, 800, and 900 ºC) in flowing reductive high-purity N2
gas including the XRD of the tapes.

The resulting product we obtained is dark green powder. The process ofsynthesis of BaBiO2.5 is recoverable reverted back to BaBiO3 phase. The result oftreatment in an oxygen atmosphere are shown in Fig. 5-2. The oxidation process
occurs from BaBiO2.5 to BaBiO3 phase. The oxygen vacancy concentration of
BaBiO2.5 was determined using thermogravimetric data taken in flowing oxygen as
shown in Fig. 5-3. Based on the weight difference between obtained BaBiO2.5 andBaBiO3, the oxygen vacancy concentration of BaBiO2.5 was found to be exact 2.5.
The synthesis of BaBiO2.5 has been performed using other methods in some papers,
however, some impurity phases or oxygen-deviated phases are usually found. We
successfully synthesized the stoichiometric BaBiO2.5 owing to the controlling of
annealing temperature and the used atmosphere.

69

Figure 5-2 XRD patterns of synthesized BaBiO3 perovskites (black) and compounds
obtained from the BaBiO2.5 treated in flowing O2 atmosphere(red).

Figure 5-3 Thermogravimetric data collected during the transformation of BaBiO2.5
to BaBiO3 in oxygen atmosphere. Insets show the structures of BaBiO2.5 (left) andBaBiO3 (right). The red, purple and green balls represent O, Bi, and Ba atoms,
70respectively.

Figure 5-4 Synchrotron XRD patterns of BaBiO2.5 at room temperature (RT) and 100
K. No phase transition occurs when cooling BaBiO2.5 down to 100 K. Inset shows theenlarged XRD patterns.

In order to confirm the possible phase transition behavior of BaBiO2.5, we thencollected the SXRD pattern at room temperature and 100 K as shown in Fig. 5-4.
Through the comparison between XRD pattern of BaBiO2.5, no peak splitting or extra
peaks are observed, revealing no phase transition occurring in the temperature range
from room temperature to 100 K. The data taken at 100 K showed a better
signal-to-noise ratio, we then used it in the following charge density study of
MEM/Rietveld analysis. BaBiO2.5 shows a monoclinic structure with a P21/c spacegroup, and the fitting results of the Rietveld analysis are shown in Fig.5-5 andsummarized in Table 5-1. The reliability factors of the intensities RI and the average
weighted profile Rwp are about 3.73% and 3.69%, respectively.

71

Figure 5-5 Fitting result of the high-resolution synchrotron-radiation XRD pattern of
BaBiO2.5. The inset shows the enlarged high angles region. The solid lines indicatesthe calculated diffraction, and round spots indicates the observed diffraction. The
short lines indicate the Bragg reflections position. The difference between theobserved and calculated diffraction is shown at the bottom in blue lines.

Table 5-1 Structure parameter from the Rietveld refinement for BaBiO2.5 at 100 K.a
Atom Sitexyz

B (Å2)

Ba4e

0.8644(1)0.0770(1)0.2293(2)0.29(1)Bi4e0.3701(1)0.1210(0)0.2758(1)0.36(0)O1

2b0.50000.00000.00001.00(0)O24e

0.1788(9)-0.0756(8) 0.3011(15)1.06(21)O34e0.1728(11) 0.2526(11) 0.0497(13)1.26(22)

The space group is P21/c. Lattice constant is a= 7.31256(8) Å, b=7.56805(10) Å, andc=6.03889(7) Å. Rwp, Rp, Ri, and Rf are 3.69%, 2.33%, 3.73%, and 2.00%,
respectively. The bond lengths of Bi-O1, Bi-O2, short Bi-O3 and long Bi-O3 bondingare 2.24066(0), 2.06474(0), 2.07554(0), and 2.54854(0) Å, respectively.

72

5.2 Charge density distribution
The MEM/Rietveld analysis of SXRD data is usually recognized as a direct
forward way to investigate the electron charge density distribution and the bonding
nature between ions. In the MEM/Rietveld analysis, any kind of electron charge
density distributions is enabled if the symmetry requirements are met. To perform the
MEM/Rietveld analysis, high-quality of SXRD data with good counting statistics andhigh angular resolution are required, the SXRD pattern were herin collected at
BL02B2 in SPring-8 using a large Debye-Scherrer camera with an imaging plate. The
sample temperature was controlled using the N2 gas flow system within 1 K
resolution. No phase transition was observed with BaBiO2.5 sample cooling down to
100 K. Owing to good signal-to-noise ratio for the data taken at 100 K with IP, weused the data in the following MEM/Rietveld analysis process. The number ofstructure factors determined from the Rietveld refinement is 7345. The
MEM/Rietveld analysis was then performed in an unit cell with 148 x 152 x 120
pixels. The reliability of structure factor used in the final MEM charge density is
2.00%.
As shown in the Fig. 5-6, the features of Bi-O and Ba-O bondings are
successfully visualized in the MEM electron charge density map. Obviously, theelectron charge densities between Bi and O are observed to be anisotropic, which arefound to be strongly overlapped between the Bi and O1, O2, and O3 ions. The Bi-O1,
Bi-O2, and short Bi-O3 bondings shows the minimum electron charge densities of
0.35, 0.48, and 0.48 e/Å3, respectively, which reveals the appearance of the Bi-O
covalent bonds. In contrast, the longer Bi-O3 bond is found to have a 0.22 e/Å3
minimum charge density, that is almost the same value of the charge density ofbackground level, revealing its ionic bonding nature. All information mentioned
above suggest that the Bi-O bonds in BaBiO2.5 form not only the covalent but also theionic bond, showing a clearly experimental evidence for unusual orbital hybridizationof Bi and O ions. In marked contrast, it is obvious that Ba possess ionic bonds with73

neighbored coordinated oxygen ions. In view of the special electron charge density
distribution and asymmetric local coordination of Bi ions in BaBiO2.5, it can be
expected that the lattice is easily deformed under external stimulation.

Figure 5-6. 3D electron charge density distribution maps of BaBiO2.5 along different
axis. The isosurface is 0.3 e Å-3.

EXAFS is a well-known powerful method for examining the local coordination
environment around the absorbed ions, we further performed the extended X-ray
absorption fine structure spectroscopy (EXAFS) at the L3 edge of Bi in BaBiO2.5 tosupplement the MEM/Rieveld analysis result as shown in Fig. 5-7. In the EXAFS
experiments, the structure parameter obtained by the MEM/Rietveld method was used
as a primitive model. The model and experimental spectra in the k or R space show agood agreement. Best-fit parameters and the quality of the fit are collected in Table
5-2. The goodness-of-fit parameter, the Rfactor, of this fitting is 0.0083. R is half of the
global scattering path length, and σ2 is the mean square displacement factor; bothwere obtained from the fitting. We also show the Reff values for each path, which wereobtained by using as input the structure as determined by the MEM/Rietveld analysisof the SXRD pattern of BaBiO2.5 taken at 100 K. One may note that the EXAFS
spectrum of BaBiO2.5 was performed at room temperature, which attribute to larger R
compared to Reff.

74

Figure 5-7. a) Bi L3-edge k3-weighted EXAFS spectra and b) the Fourier transform(FT) spectra of the k3-weighted EXAFS of BaBiO2.5. The fitting range in R space is
1-4 Å.

Table 5-2. Main results of the fit performed on the k3-weighted EXAFS spectrum of
BaBiO2.5.
Path

Reff (Å)

R (Å)σ2 (Å-2)

Bi-O2.1

2.070

2.104±0.016

0.0040±0.0008

Bi-O1.1

2.241

2.266±0.007

0.0059±0.0048

Bi-O3.2

2.549

2.577±0.008

0.0025±0.0024

Bi-O1.2

3.248

3.285±0.010

0.0129±0.0027

Bi-Bi.1

3.556

3.597±0.011

0.0071±0.0008

Bi-Ba.1

3.581

3.621±0.011

0.0097±0.0015

Bi-Bi.2

3.596

3.636±0.011

0.0072±0.0008

Bi-Ba.2

3.682

3.724±0.011

0.0097±0.0015

5.3 Photoluminescence mechanism
BaBiO2.5 shows the unique electron density distribution and bonding nature,75

which have prompted us to examine its photophysical properties using a variety ofspectroscopic techniques. Compared with non-luminous BaBiO3, the room
temperature NIR PL peak of BaBiO2.5 is 1186 nm, the full width at half maximum is
27 nm, and it is accompanied with two weak emission bands at 1238 and 1296 nm asshown in Fig. 5-8. When various excitation power densities were used, the intensity
ratio or emission curve of the three emission bands remain almost unchanged asshown in Fig.5-9. We noticed that the compound showed no other emission bands inthe range of 300-1000 nm. All these results indicate that there is only one class ofluminescent substances in the system, which is in sharp contrast to other
Bi-containing materials that usually contain multiple types of emitters.

Figure 5-8. a) Excitation-emission graph of BaBiO2.5. b) PL spectra of BaBiO2.5
performed at 293 and 5 K under the excitation of 658 nm.

76

Figure 5-9. Normalized room-temperature PL spectra of BaBiO2.5 under 658 nm
excitation with different excitation power densities.

We find that the excitation or emission nature of BaBiO2.5 are obviously different
from those of other observed systems containing Bi ions, such as Bi3+, Bi2+ or Bi+ ions,which allows us to exclude the possibility of emission from those species. There are
two kind of possible emitters, permanent material defects (such as surface defects or
dopants) and transient light emitting defects (such as excitons) that may cause theobserved PL.[105,106] It is recognized that the emission of permanent material
defects usually exhibits a sublinear dependence on the excitation power intensity,
which eventually saturates when all defects are filled. Therefore, we performed the PL
spectrum of BaBiO2.5 with dependence of excitation power density. The PL intensity
was observed to increase linearly with the excitation power density from 0.05 to 1.2
W cm-2, indicating no PL saturation as shown in Fig. 5-10.
Although material degradation usually prevents the application of higher
excitation power densities, this linear correlation indicates that the luminescence does
not come from surface defects. In addition, for the synthesized and reground powders,
we observed almost same emission curves and PL intensities as shown in Fig.5-11,
further indicating that the emission is from an overall bulk effect, not a surface defect.
77Figure 5-10. Room-temperature emission intensity of BaBiO2.5 under 658 nm
excitation with dependence of excitation powder densities.

Figure 5-11. Room-temperature PL spectra of the synthesized and ground BaBiO2.5
powders under 658 nm excitation.

As we all know, self-trapped excitons (STEs) are one kind of transient defects
78

formed during light excitation, in which photo-generated charge carriers can bestabilized by large elastic lattice distortion driven by electron-phonon coupling. In
other word, when external stimuli is subjected , the deformable lattice is conducive tothe formation of STE. Since there is no PL saturation as shown in Fig. 5-12, almost
the same emission curve at various excitation energy and the asymmetric coordinationgeometry of Bi, we speculate that the observed NIR emission may originate from
STEs.

Figure 5-12. Room-temperature PL spectra of BaBiO2.5 under different excitation
wavelengths.

As shown in Fig. 5-13, BaBiO2.5 shows an indirect band gap about 2.48 eV and abelow-gap absorption band, which is consistent with the PL excitation lineshape asshown in Fig.5-8 a. Obviously, the room temperature PL spectrum is divided into
three peaks, whose energies are 323 and 403 cm-1, respectively, which is equivalent to
the phonon energy of 321 cm-1 and the sum of phonon energy of 321 and 81 cm-1 asshown in Fig. 5-14. Therefore, we assume that the splitting is owing toelectron-phonon coupling, that is the nature of vibration development. We note thatsimilar multi-band exciton emission is also observed in the layered perovskite
79containing lead, such as (C4H9NH3)2PbI4.

Figure 5-13. Tauc plot of (hvF(R∞))1/2 as a function of the photon energy, which isindicative of an indirect bandgap. The below-bandgap absorption can be assigned tothe exciton absorption as discussed in the main text.

Figure 5-14. Room-temperature PL spectrum of BaBiO2.5 under 658 nm excitation.
Black and colored curves are experimentally measured and decomposed PL curves,
80respectively.
Interestingly, lowering the temperature will result in sharply resolved peaks fromthe room temperature PL bands as shown in Fig 5-8 b and Fig. 5-15. The spectracollected at 5 K show about nine peaks, and the energy interval between neighboredpeaks is less than 230 cm-1 as shown in Fig. 5-15 b and Table 5-3.

Table 5-3. The decomposed peak position and energy spacing between adjacent peaks
for PL at 5 K. The spacing is calculated by subtracting the peak energy of
low-frequency emission from that of the adjacent high-frequency emission. We also
listed the phonon energy of BaBiO2.5 that is comparable to the corresponding energy
spacing. We note that only the spacing of 39.9 cm-1 shows a relatively large differencewith respect to the phonon energy of 53 cm-1, which may be due to the strong
overlapping of the emissions in this spectral region or the existence of the vibrationalmode at ca. 40 cm-1 for BaBiO2.5.

Peak position (cm-1)

Energy spacing (cm-1)

Phonon energy (cm-1)

8425.751.1

538359.766.0

678278.581.2

818110.1168.4

1718037.472.7

677997.539.9

537924.473.1

677702.1222.3

2278476.881

Figure 5-15. a) PL spectra of BaBiO2.5 with dependence of temperature. b)
Decomposed PL spectra. Black and colored curves indicate the experiment anddecomposed PL spectra, respectively.

We recommend that the Raman mode at 321 cm-1 at low temperatures is frozen,
which results in the appearance of radiation recombination involving low-frequency
modes. In addition, BaBiO2.5 shows almost the same attenuation at different
82wavelengths of 5 K as shown in Fig. 5-16, further indicating the vibration
characteristics of PL. We noticed that narrow-band PL was quite rare in thecompounds containing Bi.[107] The special structure of PL in this layered
Bi-containing structure is well investigated for the first time, which may be owing to
the weak coupling between emissive state and lattice vibration, compared with thestrong coupling in those compounds with three-dimensional crystals or glass-likelattices.

Figure 5-16. PL decay curves of BaBiO2.5 at 5 K monitored at different emission
wavelengths.

In order to gain a deeper understanding of the microscopic mechanism of STEs
formation in BaBiO2.5, we next performed the band structure and density of states
calculations using the density functional theory (DFT). When using the general
gradient approximation (GGA)/Perdew-Burke-Ernzerhof (PBE) functional, BaBiO2.5
is calculated to have an indirect bandgap of 2.85 eV, that is slightly larger than theexperimental value of 2.48 eV. The valence band maximum is found to located at the83G point, and the conduction band is located at the B point of the Brillouin zone asshown in Fig. 5-17.
Partial state density (PDOS) shows that O 2p orbital mainly contribute to the
maximum of valence band at the G point, meanwhile, Bi 6p and O 2p orbits mainlycompose the conduction band, which indicates a strong hybridization of Bi 6p and O
2p orbitals. As the experiment shows, this is found to be very consistent with thespecial bonding nature between Bi and O.
Based on experiment results, we then suggest that the photo-generated
self-trapping holes should be originated from the state of O and/or Bi. We emphasizethat more works are necessary to deepen the understanding of relationship of thecrystal structure and physical property in this system. Particularly, calculation
research on the allocation of various vibration modes of BaBiO2.5 and the details ofelectron-phonon coupling is useful to obtain a deep understanding of these properties,
which is still an open question for more studies.

Figure 5-17. Calculated band structure and spin-up and spin-down states PDOS of
BaBiO2.5.

84

5.4 Conclusion
In summary, we have successfully synthesized the pure layered BaBiO2.5transformed from the perovskite-type oxide BaBiO3. Using the MEM/Rietveld
method combined with the high-resolution synchrotron-radiation X-ray diffraction
data, the electron charge density distributions of BaBiO2.5 are evaluated for the firsttime. The unusual chemical bonds between Bi and O ions are observed, which
consists of three covalent bonds and one ionic bond. Hence, an unusual coordination
environment is formed. We found that this special asymmetric structure shows the
NIR PL that is different from the conventional Bi-containing light-emitting system.
The combination of extensive spectral analysis and theoretical calculations has
inspired us to suggest that the observed PL may be derived from the phonon-assisted
radiation recombination of STEs associated with O and/or Bi states. Our workemphasizes that synthesis of materials with unique Bi-O bonds and unusual Bi
coordination geometry would provide a way to discover systems with emerging
properties. We believe that this work may stimulate a strong interest in exploring
various materials containing Bi or other heavier p-block elements, which provides
attractive prospects for finding new systems with intriguing characteristics.

85

Acknowledgment
First, I would particularly like to thank my supervisor Professor Yoshihiro
Kuroiwa, who gave me the opportunity to do research works in such a favorable
environment and participate in the electron charge density distribution study
combined with the high resolution synchrotron-radiation X-ray diffraction on the
BaBiO3 related materials and the related materials. Professor Kuroiwa’s excellent
advice and useful support are not only valuable to my academic level, but also to mydaily life and personality. Second, I especially want to thank Professor Chikako
Moriyoshi. During my doctoral study in Hiroshima University, her patience andguidance help me a lot, and inspired me to promote scientific research. They show therich knowledge, rigorous academic attitude, keen scientific insight and scientificexploration spirit who will become my models in life. Third, I would like to thankProfessor Hong-Tao Sun from Soochow University for his great help of sample
supporting and kind advice for my research works.
I would like to thank Dr. Kawaguchi, a beamline scientist at SPring-8, for his
great support for the synchrotron X-ray powder diffraction experiment on beamline
BL02B2.
I would like to thank all the staff of the Division of Physical Sciences andStudent Support Office of the Graduate School of Science for their help in writing,information sharing and scholarship recommendation.
I would like to thank all the members of our materials structure physics lab: Y.
Nakahira, D. Tsuru, T. Abe, L. Wu, Y. Sunada, Y. Morida, D. Ichihashi, G. Kawamura,Y. Yokoi, T. Suematsu, S. Tsugita, S. Noda, K. Hagura, M. Hattori, M. Muta, H.
Shirakuni. As an international student, with their kind help and friendship, I canquickly adapt to the study and live in Japan and do my research works. I am very
happy to get along with you, and I will never forget our happy time working andplaying together. I would also like to thank the Japanese Student Services
Organization for providing scholarships to support me.
86

Last, I am honored and happy to be a member of our laboratory and a student of
my two lovely teachers.

87References[1] B. Martínez, X. Obradors, A. Gou, V. Gomis, S. Piñol, J. Fontcuberta, and H. VanTol, Phy. Rev. B 53 (1996) 2797.
[2] Y. Long and Y. Shimakawa, New J. Phys. 12 (2010) 1906.
[3] J. Blasco, J. García, J. M. de Teresa, M. R. Ibarra, J. Perez, P. A. Algarabel, C.
Marquina, and C. Ritter, Phys. Rev. B 55 (1997) 8905.
[4] W. T. Chen, Y. Long, T. Saito, J. Paul Attfield, and Y. Shimakawa, J. mater. Chem.
20 (2010) 7282.
[5] R. Shannon, Acta Crystallogr., Sec. A 32 (1976) 751.
[6] A. V. Hippel, R. G. Breckenridge, F. G. Chesley, and L. Tisza, Ind. Eng. Chem. 38(1946) 1097.
[7] B. Wul and I. M. Goldman, C. R. Acad. Sci. USSR 51 (1946) 21.
[8] S. A. Hayward, S. A. T. Redfern, and E. K. H. Salje, J. Phys.: Condens. Mat. 14(2002) 10131.
[9] G. King and P. M. Woodward, J. Mater. Chem. 20 (2010) 5785.
[10] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima and Y. Tokura, Nature 426(2003) 55[11] M. B. Salamon and M. Jaime, Rev. Mod. Phys. 73 (2001), 583.
[12] Y. Krockenberger, K. Mogare, M. Reehuis, M. Tovar, M. Jansen, G.
Vaitheeswaran, V. Kanchana, F. Bultmark, A. Delin, F. Wilhelm, A. Rogalev, A.
Winkler and L. Alff, Phys. Rev. B 75 (2007) 020404.
[13] T. Alamelu, U. V. Varadaraju, M. Venkatesan, A. P. Douvalis and J. M. D. Coey, J.
Appl. Phys. 91 (2002) 8909.
[14] H. Kato, T. Okuda, Y. Okimoto, Y. Tomioka, Y. Takenoya, A. Ohkubo, M.
Kawasaki and Y. Tokura, Appl. Phys. Lett. 81 (2002) 328.
[15] Y. Tokura, K. I. Kobayashi, T. Kimura, H. Sawada and K. Terakura, Nature 395(1998) 677.
[16] Y. Tomioka, T. Okuda, Y. Okimoto, R. Kumai, K. I. Kobayashi and Y. Tokura,Phys. Rev. B 61 (2000) 422.
88

[17] D. Serrate, J. M. De Teresa and M. R. Ibarra, J. Phys.: Condens. Mat. 19 (2007)
023201.
[18] A. K. Paul, M. Jansen, B. Yan, C. Felser, M. Reehuis and P. M. Abdala, Inorg.
Chem. 52 (2013) 6713.
[19] H. L. Feng, M. Arai, Y. Matsushita, Y. Tsujimoto, Y. Guo, C. I. Sathish, X. Wang,
Y. Yuan, M. Tanaka and K. Yamaura, J. Am. Chem. Soc. 136 (2014), 3326.
[20] R. Morrow, J. W. Freeland and P. M. Woodward, Inorg. Chem. 53 (2014) 7983.
[21] A. K. Paul, M. Reehuis, V. Ksenofontov, B. Yan, A. Hoser, D. M. Tobbens, P. M.
Abdala, P. Adler, M. Jansen and C. Felser, Phys. Rev. Lett. 111 (2013) 167205.
[22] Y. S. Hou, H. J. Xiang and X. G. Gong, Sci. Rep. 5 (2015) 13159.
[23] S. V. Ovsyannikov, M. Bykov, E. Bykova, D. P. Kozlenko, A. A. Tsirlin, A. E.
Karkin, V. V. Shchennikov, S. E. Kichanov, H. Y. Gou, A. M. Abakumov, R. Egoavil,
J. Verbeeck, C. McCammon, V. Dyadkin, D. Chernyshov, S. van Smaalen and L. S.
Dubrovinsky, Nature Chem. 8 (2016) 501.
[24] B. Lavina, P. Dera, E. Kim, Y. Meng, R.T. Downs, P. Weck, S.R. Sutton and Y.
Zhao, Proc. Natl. Acad. Sci. USA. 108 (2011) 17281.
[25] Y. W. Long, N. Hayashi, T. Saito, M. Azuma, S. Muranaka and Y. Shimakawa,Nature 458 (2009) 60.
[26] Y. W. Long, T. Kawakami, W. T. Chen, T. Saito, T. Watanuki, Y. Nakakura, Q. Q.
Liu, C. Q. Jin and Y. Shimakawa, Chem. Mater. 24 (2012) 2235.
[27] T. Mizokawa and A. Fujimori, Phys. Rev. B 56 (1997) R493.
[28] D. H. Kim, H. M. Christen, M. Varela, H. N. Lee and D. H. Lowndes, Appl. Phys.
Lett. 88 (2006) 202503.
[29] J. Fontcuberta, V. Laukhin and X. Obradors, Phys. Rev. B 60 (1999) 6266.
[30] S. Aoyagi, PhD thesis, Okayama University, 2002 [in Japanese].
[31] T. Nakamura, Ferroelectricity involved Structural Phase Transitions (Shokabo,
Tokyo, 1988) [in Japanese].
[32] J. Harada, J. Cryst. Soc. Jpn 20, 1 (1978) [in Japanese].
[33] W. Zhong and D. Vanderbilt, Phys. Rev. Lett. 74 (1995) 2587.
[34] G. Pavel and J. B. Andrew, J. Phys.: Condens. Mat. 19 (2007) 176201.
89[35] A. Glazer, Acta Crystallogr., Sec. B 28 (1972) 3384.
[36] P. Woodward, Acta Crystallogr., Sec. B 53 (1997) 32.
[37] M. E. Jones and R. E. Marsh, J. Am. Chem. Soc. 76 (1954) 1434.
[38] E. Nishibori, M. Takata, M. Sakata, H. Tanaka, T. Muranaka and J. Akimitsu, J.
Phys. Soc. Jpn 70 (2001) 2252.
[39] K. Kato, M. Takata, Y. Moritomo, A. Nakamoto and N. Kojima, Appl. Phys. Lett.
90 (2007) 201902.
[40] Y. Kuroiwa, S. Aoyagi, A. Sawada, J. Harada, E. Nishibori, M. Takata and M.
Sakata, Phys. Rev. Lett. 87 (2001) 217601.
[41] H. Tanaka, Y. Kuroiwa and M. Takata, Phys. Rev. B 74 (2006) 172105.
[42] P. Maselli, D. Nicoletti, A. Nucara, F. M. Vitucci, A. Irizawa, K. Shoji, T. Nanba
and P. Calvani, EPL (Europhysics Letters) 92 (2010) 47001.
[43] K. Kato, M. Takata, E. Nishibori, M. Sakata, N. Hamada and Y. Moritomo, J.
Phys. Soc. Jpn. 74 (2005) 2137.
[44] A. W. Sleight, J. L. Gillson and P. E. Bierstedt, Solid State Commun. 88 (1993)
841.
[45] R. J. Cava, B. Batlogg, J. J. Krajewski, R. Farrow, L. W. Rupp Jr, A. E. White, K.
Short, W. F. Peck and T. Kometani, Nature 332 (1988) 814.
[46] D. G. Hinks, B. Dabrowski, J. D. Jorgensen, A. W. Mitchell, D. R. Richards, S.
Pei and D. Shi, Nature 333 (1988) 836.
[47] S. M. Kazakov, C. Chaillout, P. Bordet, J. J. Capponi, M. Nunez-Regueiro, A.
Rysak, J. L. Tholence, P. G. Radaelli, S. N. Putilin and E. V. Antipov, Nature 390(1997) 148.
[48] A. M. Huerta-Flores, D. Sanchez-Martinez, M. del Rocío Hernández-Romero, M.
E. Zarazúa-Morín and L. M. Torres-Martínez, J. Photoch. Photobio. A 368 (2019) 70.
[49] J. Tang, Z. Zou and J. Ye, J. Phys. Chem. C 111 (2007) 12779.
[50] M. Khraisheh, A. Khazndar and M.A. Al-Ghouti, Int. J. Energy Res. 39 (2015)
1142.
[51] J. Tang, Z. Zou and J. Ye, Angew. Chem., Int. Ed. 43 (2004) 4463.
[52] J. Sato, H. Kobayashi and Y. Inoue, J. Phys. Chem. B 107 (2003) 7970.
90

[53] A. W. Sleight, Physica C 514 (2015) 152.
[54] M. Yasukawa and N. Murayama, Physica C 297 (1998) 326.
[55] A. Ghosh, Solid State Commun. 112 (1999) 45.
[56] S. H. Lee, W. H. Jung, J. H. Sohn, J. H. Lee and S. H. Cho, J. Appl. Phys. 86(1999) 6351.
[57] D. E. Cox and A. W. Sleight, Solid State Commun. 19 (1976) 969.
[58] Q. Zhou and B. J. Kennedy, Solid State Commun. 132 (2004) 389.
[59] B. J. Kennedy, C. J. Howard, K. S. Knight, Z. Zhang and Q. Zhou, Acta Cryst. B
62 (2006) 537.
[60] S. Pei, J. D. Jorgensen, B. Dabrowski, D. G. Hinks, D. R. Richards, A. W.
Mitchell, J. M. Newam, S. K. Shiha, D. Vaknin and A. J. Jacobson, Phys. Rev. B 41
(1990) 4126.
[61] H. Krakauer and W. E. Pickett, Phys. Rev. Lett. 60 (1988) 1665.
[62] S. Sanna, V. Esposito, J. W. Andreasen, J. Hjelm, W. Zhang, T. Kasama, S. B.
Simonsen, M. Christensen, S. Linderoth and N. Pryds, Nat. Mater. 14 (2015) 500.
[63] H. Taniguchi, A. Kuwabara, J. Kim, Y. Kim, H. Moriwake, S. Kim, T. Hoshiyama,
T. Koyama, S. Mori, M. Takata, H. Hosono, Y. Inaguma and M. Itoh, Angew. Chem.
Int. Ed. 52 (2013) 8088.
[64] G. Catalan and J. F. Scott, Adv. Mater. 21 (2009) 2463.
[65] Y. Hosaka, F. Denis Romero, N. Ichikawa, T. Saito and Y. Shimakawa, Angew.
Chem. Int. Ed. 56 (2017) 4243.
[66] I. D. Brown, Chem. Rev. 109 (2009) 6858.
[67] H.-T. Sun, J. Zhou and J. Qiu, Prog. Mater. Sci. 64 (2014) 1.
[68] Z. Huang, J. E. Auckett, P. E. Blanchard, B. J. Kennedy, W. Miiller, Q. Zhou, M.
Avdeev, M. R. Johnson, M. Zbiri, G. Garbarino, W. G. Marshall, Q. Gu, C. D. Ling,
Angew. Chem. Int. Ed. 53 (2014) 3414.
[69] D. E. Cox and A. W. Sleight, Solid State Commun. 19 (1976) 969.
[70] G. Kim, M. Neumann, M. Kim, M. D. Le, T. D. Kang and T. W. Noh, Phys. Rev.
Lett. 115 (2015) 226402.
[71] N. C. Plumb, D. J. Gawryluk, Y. Wang, Z. Ristic, J. Park, B. Q. Lv, Z. Wang, C. E.
91

Matt, N. Xu, T. Shang, K. Conder, J. Mesot, S. Johnston, M. Shi and M. Radovic,
Phys. Rev. Lett. 117 (2016) 037002.
[72] M. A. Hayward, M. A. Green, M. J. Rosseinsky and J. Sloan, J. Am. Chem. Soc.
121 (1999) 8843.
[73] M. A. Hayward, E. J. Cussen, J. B. Claridge, M. Bieringer, M. J. Rosseinsky, C. J.
Kiely, S. J. Blundell, I. M. Marshall and F. L. Pratt, Science 295 (2002) 1882-1884.
[74] Y. Tsujimoto, C. Tassel, N. Hayashi, T. Watanabe, H. Kageyama, K. Yoshimura,
M. Takano, M. Ceretti, C. Ritter and W. Paulus, Nature 450 (2007) 1062.
[75] H. Kageyama, T. Watanabe, Y. Tsujimoto, A. Kitada, Y. Sumida, K. Kanamori, K.
Yoshimura, N. Hayashi, S. Muranaka, M. Takano, M. Ceretti, W. Paulus, C. Ritter and
G. Andre, Angew. Chem. Int. Ed. 47 (2008) 5740.
[76] S. Tominaka, Y. Tsujimoto, Y. Matsushita and K. Yamaura, Angew. Chem. Int. Ed.
50 (2011) 7418.
[77] a) G. K. Behrh, H. Serier-Brault, S. Jobic and R. Gautier, Angew. Chem. Int. Ed.
54 (2015) 11501; b) H. Barroux, T. Jiang, C. Paul, F. Massuyeau, R. Génois, E. E.
Gordon, M.-H. Whangbo, S. Jobic and R. Gautier, Chem. Eur. J. 23 (2017) 2998.
[78] a) B. M. Liu, Z. G. Zhang, K. Zhang, Y. Kuroiwa, C. Moriyoshi, H. M. Yu, C. Li,
L. R. Zheng, L. N. Li, G. Yang, Y. Zhou, Y. Z. Fang, J. S. Hou, Y. Matsushita and H.-T.
Sun, Angew. Chem. Int. Ed. 55 (2016) 4967; b) K. Zhang, C.-G. Ma, J.-Y. Zhang,
B.-M. Liu, Y. Zhou, S.-Q. Guo, J.-Y. Xu, J.-S. Hou, Y.-Z. Fang, L.-R. Zheng and H.-T.
Sun, Adv. Optical Mater. 5 (2017) 1700448.
[79] S. Inoue, M. Kawai, N. Ichikawa, H. Kageyama, W. Paulus and Y. Shimakawa,Nat. Chem. 2 (2010) 213.
[80] H. S. Kim, J. B. Cook, H. Lin, J. S. Ko, S. H. Tolbert, V. Ozolins and B. Dunn,
Nat. Mater. 16 (2017) 454.
[81] F. Lei, Y. Sun, K. Liu, S. Gao, L. Liang, B. Pan and Y. Xie, J. Am. Chem. Soc.
136 (2014) 6826.
[82] L. Tian, H. Zou, J. Fu, X. Yang, Y. Wang, H. Guo, X. Fu, C. Liang, M. Wu, P. K.
Shen and Q. Gao, Adv. Func. Mater. 20 (2010) 617.
[83] P. Willmott, An Introduction to Synchrotron Radiation (Wiley, West Sussex,
92

2011).
[84] Outline Schematic view of beamline at source and optics of BL02B2
http://www.spring8.or.jp/wkg/BL02B2/instrument/lang-en/INS-0000000318/instrument_summary_view[85] E. Nishibori, M. Takata, K. Kato, M. Sakata, Y. Kubota, S. Aoyagi, Y. Kuroiwa,
M. Yamakata and N. Ikeda, Nucl. Instr. Meth. Phys. Res. A 62 (2001) 2095.
[86] S. Kawaguchi, M. Takemoto, K. Osaka, E. Nishibori, C. Moriyoshi, Y. Kubota, Y.
Kuroiwa and K. Sugimoto, Rev. Sci. Instrum. 88 (2017) 085111.
[87] H. Tanaka, M. Takata, E. Nishibori, K. Kato, T. Iishi and M. Sakata, J. Appl.
Crystallogr. 35 (2002) 282.
[88] K. Momma and F. Izumi, J. Appl. Crystallogr. 41 (2008) 653.
[89] B. Ravel and M. Newville, J. Synchrotron Radiat. 12 (2005) 537.
[90] G. Kresse and J. Furthmuller, Phys. Rev. B 54 (1996) 11169.
[91] J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.
[92] J. Heyd, G. E. Scuseria and M. Ernzerhof, J. Chem. Phys. 124 (2006) 219906.
[93] H. Rietveld, J. Appl. Crystallogr. 2 (1969) 65.
[94] C. E. Shannon, Bell Syst. Tech. J. 27 (1948) 408.
[95] E. E. Jaynes, IEEE Trans. Syst. Sci. Cyb. SSC 4 (1968) 227.
[96] J. P. Burg, 37th Ann. Int. Meeting, “Soc. Explor. Geophys.” Oklahoma City, OK,
1967[97] D. M. Collins, Nature (London) 298 (1982) 49.
[98] M. Sakata, R. Mori, S. Kumazawa, M. Takata and H. Toraya, J. Appl. Cryst. 23(1990) 526.
[99] M. Sakata and M. Sato, Acta Crystallogr., Sec. A 46 (1990) 263.
[100] T. Saka and N. Kato, Acta Crystallogr., Sec. A 42 (1986) 469.
[101] M. Takata, Acta Crystallogr., Sec. A 64 (2008) 232.
[102] F. Mezzadri, D. Delmonte, F. Orlandi, C. Pernechele, G. Calestani, M. Solzi, M.
Lantieri, G. Spina, R. Cabassi, F. Bolzoni, M. Fittipaldi, M. Merlini, A. Migliori, P.
Manuel and E. Gilioli, J. Mater. Chem. C 4 (2016) 1533.
[103] S. J. Patwe, S. N. Achary, M. D. Mathews and A. K. Tyagi, J. Alloys Compd.
93

390 (2005) 100.
[104] I. D. Brown, Chem. Rev. 109 (2009) 6858.
[105] E. R. Dohner, A. Jaffe, L. R. Bradshaw and H. I. Karunadasa, J. Am. Chem. Soc.
136 (2014) 13154.
[106] a) T. Hu, M. D. Smith, E. R. Dohner, M. J. Sher, X. Wu, M. T. Trinh, A. Fisher,
J. Corbett, X. Y. Zhu, H. I. Karunadasa and A. M. Lindenberg, J. Phys. Chem. Lett. 7(2016) 2258; b) D. B. Mitzi, Chem. Mater. 8 (1996) 791; c) K. M. McCall, C. C.
Stoumpos, S. S. Kostina, M. G. Kanatzidis and B. W. Wessels, Chem. Mater. 29 (2017)4129.
[107] a) Y. Fujimoto and M. Nakatsuka, Jpn. J. Appl. Phys. 40 (2001) L279; b) E. M.
Dianov, Light: Sci. Appl. 1 (2012) e12; c) H.-T. Sun, Y. Sakka, H. Gao, Y. Miwa, M.
Fujii, N. Shirahata, Z. Bai and J.-G. Li, J. Mater. Chem. 21 (2011) 4060; d) H.-T. Sun,
Y. Matsushita, Y. Sakka, N. Shirahata, M. Tanaka, Y. Katsuya, H. Gao and K.
Kobayashi, J. Am. Chem. Soc. 134 (2012) 2918; e) B. Xu, S. Zhou, D. Tan, Z. Hong,
J. Hao and J. Qiu, J. Appl. Phys. 113 (2013) 083503; f) L. S. Chi, R. S. Liu and B. J.
Lee, J. Electrochem. Soc. 152 (2005) J93.

94