広島大学 学位論文
Photoluminescence Properties of Distorted Titanates
Investigated by X-ray Absorption Spectroscopy
Ｘ線吸収分光法による歪んだチタン酸化物のフォトルミネセンスの研究

2020 年
広島大学大学院理学研究科
物理科学専攻Dongxiao Fan

目次
• 主論文
Photoluminescence Properties of Distorted Titanates Investigatedby X-ray Absorption Spectroscopy
Ｘ線吸収分光法による歪んだチタン酸化物のフォトルミネセン
スの研究
Dongxiao Fan• 公表論文Photoluminescence mechanism of self-activated titanate phosphorsinvestigated by X-ray absorption spectroscopy under UV
irradiation
Dongxiao Fan, Nobuo Nakajima, and Seiya Kato
Journal of Physics: Condensed Matter, 2020.

• 参考論文
1. Enhanced afterglow property of BaAl2 O4 : Eu2+ , Dy3+ phosphorsby adding Li2 CO3 .
Ma, Junfeng, Dongxiao Fan, Bingbing Niu, Xuena Lan.
Functional Materials Letters, 10(03), 1750020 (2017).

2. The in situ growth of 3D net-like CNTs on C fiber.
Junfeng Ma, Xuena Lan, Bingbing Niu, Dongxiao Fan.
Materials Chemistry and Physics, 192, 210-214 (2017).

主論文

• Abstract
Titanium–oxygen polyhedra, such as TiO4 with a tetrahedron structure, TiO5 with asquare pyramidal structure, and TiO6 with an octahedron structure, commonly exist intitanate materials from the perspective of microscopic scale. The physical properties oftitanates greatly depend on the local structure of titanium–oxygen polyhedra. For example,
Ba2 TiO4 , which contains a series of isolated TiO4 tetrahedra, has excellent CO2 sorption
properties. Fresnoite structure materials containing TiO5 polyhedron exhibit strong second
harmonic generation response and have been widely used in second-order nonlinear opticalmaterials. The TiO6 octahedron is more well-known than TiO4 and TiO5 ; further, it iscontained in titanates with perovskite structure, which is currently a research hotspot. A
typical example of physical property related with TiO6 octahedron is the ferroelectricity
in BaTiO3 and PbTiO3 , which is produced by the displacement of a titanium ion in an
octahedron cage. In terms of structural similarity, TiO5 can be considered to be resulting
from the TiO6 octahedron with Ti ion displacement along the z-axis. The local structuralcharacteristics of these polyhedra in materials contribute to their physical properties.
Self-activated titanate phosphors with TiO5 polyhedra exhibit excellent photon-emission
property under UV irradiation, i.e., photoluminescence (PL). This phenomenon is also observedin BaTiO3 with distorted TiO6 octahedron. Interestingly, both PL and ferroelectric properties
result from the distortion of TiO6 octahedron. The term “self-activated phosphors” is assigned
to non-ion-doped phosphors, whereas conventional phosphors are created by doping ions ina matrix as the color center. The function of the doped ions is to form in-gap states. To
determine the role of the TiO5 polyhedron in self-activated titanate phosphors, the local
electronic states in the TiO5 polyhedron must be investigated. The incipient ferroelectricmaterial SrTiO3 , which is known to be centrosymmetric over the entire temperature range,
also exhibits PL with faint blue light emission at temperatures less than 37 K. This fact
motivated us to investigate the local structure of TiO6 based on the influence of temperatureand UV irradiation.

Further, for clarifying the reports about the appearance of dipole

moment and the increase in dielectric constant of SrTiO3 under UV irradiation, the local
structure information is required. Due to its sensitivity to specific atomic species and the highresolution of spatial distribution, X-ray absorption fine structure (XAFS) is an ideal approach
for investigating the electronic states of self-activated phosphors and the local structure of
SrTiO3 under UV irradiation.
The Ti K-edge X-ray absorption near edge structure of self-activated titanate phosphors(Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , and Na2 TiSiO5 ) were analyzed with UV irradiation, from which
two features in the pre-edge region were observed: a large pre-edge feature and a hump. The
electronic states of the features are determined by theoretical calculations. The electronic
states of the pre-edge, which originate from the hybridization of Ti 3d, Ti 4p, and O 2p, act
as in-gap states to emit visible light in the PL process. The electronic state of the hump,
which originates from the sub-band of Ti 4p, acts as an electron trap for the electrons excited
by UV irradiation. These results confirm the color center nature of TiO5 polyhedron. An
energy diagram of self-activated titanate phosphors was proposed to clarify the PL property.
To investigate the local structure that is advantageous to realize the PL property, a Ti
K-edge extended XAFS of SrTiO3 was performed at 22–300 K under UV irradiation. By using
the tetragonal structure model of SrTiO3 to fit the data, the dynamic Ti ion displacement
along the z-axis is confirmed, which illustrates the instantaneous existence of TiO5 polyhedron.
With UV irradiation, the Ti ion vibration along the z-axis is significantly enhanced, which
explains the appearance of dipole moment and the increase in the dielectric constant of SrTiO3 .
The in-gap states are necessary to produce the PL property in non-ion-doped titanates.
Moreover, the TiO5 polyhedron can create in-gap states. The distortion of TiO6 octahedronin SrTiO3 results in TiO5 polyhedron. Therefore, the PL property can be adopted as aneffective characterization method to investigate the in-gap electronic states and determine
the local structure distortion in non-ion-doped titanates. The simultaneous existence of PL
and the ferroelectric property caused by the local structure distortion reveal a new direction
for investigating new multifunctional materials.

Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1.1

Basic characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1.2

Titanium–Oxygen polyhedra . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

1.3

TiO5 polyhedron containing self-activated titanate phosphors . . . . . . . . . .

6

1.4

TiO6 Octahedron distortion in SrTiO3 . . . . . . . . . . . . . . . . . . . . . . . 10

1.5

Objective of this study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 Experimental Techniques . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1

2.2

2.3

17

Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.1

Self-activated titanate phosphor powder . . . . . . . . . . . . . . . . . . 17

2.1.2

Fine strontium titanate powder . . . . . . . . . . . . . . . . . . . . . . . 20

Synchrotron radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.1

Basic information about synchrotron radiation . . . . . . . . . . . . . . 21

2.2.2

Beamline 9A of Photon Factory . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.3

Beamline 12C of Photon Factory . . . . . . . . . . . . . . . . . . . . . . 23

X-ray absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3.1

X-ray absorption near edge structure . . . . . . . . . . . . . . . . . . . . 28

2.3.2

Extended X-ray absorption fine structure . . . . . . . . . . . . . . . . . 29

7

3 Investigation of the electronic states of TiO5 polyhedron containing
self-activated titanate phosphors by XANES . . . . . . . . . . . . . . .

36

3.1

Experimental setup and configuration . . . . . . . . . . . . . . . . . . . . . . . 36

3.2

Ti K pre-edge XANES of titanate compounds . . . . . . . . . . . . . . . . . . . 38

3.3

Ti K pre-edge XANES of self-activated titanate phosphors . . . . . . . . . . . 40

3.4

UV effect for Ti K pre-edge XANES of self-activated titanate phosphors . . . . 41

3.5

Theoretical calculation of Ti K pre-edge XANES of self-activated titanate
phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.6

Energy diagram of self-activated phosphors . . . . . . . . . . . . . . . . . . . . 47

3.7

Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4 Investigation of local distortion of TiO6 octahedron in SrTiO3 with UV
irradiation by EXAFS . . . . . . . . . . . . . . . . . . . . . . . . . . .

51

4.1

Experimental setup and Configuration . . . . . . . . . . . . . . . . . . . . . . . 51

4.2

XAS of SrTiO3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.3

EXAFS Data fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.4

Temperature dependence of Ti K-edge EXAFS spectrum . . . . . . . . . . . . 57

4.5

UV dependence of Ti K-edge EXAFS spectrum . . . . . . . . . . . . . . . . . . 61

4.6

Conclusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

66

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

68

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70

1Introduction

1.1

Basic characteristics

Titanate materials play an important role in the field of modern condensed matter physics.
Various types of titanate compounds exhibit excellent and diverse physical properties, suchas photocatalysis, ferroelectrics, photoluminescence, and surperconductivity [1–4]. Titanate
oxides have been most popular since the discovery of ferroelectricity in perovskite-type oxides(e.g., BaTiO3 ) in the first half of the 20th century. In 1972, the report on photochemical
catalysis of TiO2 by Fujishima significantly expanded the research on titanate materials andtheir potential for practical applications. Currently, utilizing advanced techniques such as
sample preparation and characterization to explore the novel physical functional properties
and investigate the profound physical mechanisms behind the physical phenomena has become
very common.
The defining factor of the physical properties of any material is its constitutive structure.
The image of materials science tetrahedron (figure 1-1), which concisely depicts the relationshipof interdependence between the structure, properties, performance, and processing, showsthat the structure of materials occupies the irreplaceable top position and contacts all theother terms [5]. In terms of scale, the structure of the material is of two types, namely,
macroscopic structure and microscopic structure; the macroscopic structure is average resultsof microscopic structure. For example, the conventional interpretation of ferroelectricity inperovskite-type titanates is Ti ion displacement from the symmetric center, whereas theferroelectricity in BaTiO3 along the [001] direction in the macroscopic scale is an average
result of the Ti ion displacement along the [111] direction in the microscopic scale [6]. In
addition, the microscopic structure of a material can strongly influence the physical properties
such as strength, toughness, ductility, hardness, corrosion resistance, high- or low-temperature
behavior, and wear resistance [7]. Therefore, revealing the local structure of a material cansignificantly enhance the understanding of its physical properties. Generally, the macroscopic
structure of a specific material can be measured by conventional X-ray diffraction (XRD)
and Raman spectroscopy, which are sensitive to the long-range order of the materials. For

1

Figure 1-1: Schematic of materials science tetrahedron.
the microscopic structure, such as chemical bond distance, coordination number, and thesymmetric property, X-ray absorption fine structure (XAFS) is an ideal approach for probingthese local structures because of its characteristics of high resolution of the spatial distributionof atoms and sensitivity to specific atomic species and local structures [8, 9].

1.2

Titanium–Oxygen polyhedra

In terms of the local structure of titanate compounds, three types of titanium oxygen
polyhedra are available: TiO4 with tetrahedral structure, TiO5 with a square pyramidal
structure, and TiO6 with octahedral structure (figure 1-2). The different local structures
result in distinct physical properties on the macroscopic scale, and determine the practicalapplications in daily life. For example, Ba2 TiO4 , which contains a series of isolated TiO4
tetrahedra, has excellent CO2 sorption property and can be used for improving the air qualities[10]. TiO5 polyhedron, which is present in materials with fresnoite structure because of theexistence of the short Ti–O between the apical O and central Ti, exhibits strong second
harmonic generation response, and has been widely used for second-order nonlinear opticalmaterials [11]. TiO6 is not only a more common titanium–oxygen polyhedron among thetitanate compounds such as TiO4 and TiO5 , but also commonly exists in perovskite-type
titanates. Tremendous physical properties are associated with the TiO6 octahedron local
structure. Specifically, in the field of titanate ferroelectric materials, ferroelectricity is produced2

(a) TiO4(b) TiO5(c) TiO6

Figure 1-2: Structures of various titanium–oxygen polyhedra: (a) tetrahedral TiO4 , (b) squarepyramidal TiO5 , (c) and octahedral TiO6 .
by the displacement of titanium ions in an octahedron cage, such as BaTiO3 and PaTiO3 [2].
Further, another distortion of TiO6 octahedron is also available, which is the rotation of
the oxygen octahedron without Ti displacement. Because the centrosymmetry is not broken
during this rotation, no ferroelectricity is realized.
In the case of isolated Ti atoms, the 3d orbitals are five-fold degenerate with an equivalentenergy level (figure 1-3 (a)). In perovskite titanates, because a Ti ion is located at the centerof the octahedron, Ti4+ experiences an octahedral field with Oh symmetry. Based on theligand field theory, the five-fold degenerated 3d orbitals of a Ti ion with Oh symmetry will be
split into a triply degenerated t2g (dxy , dyz , and dxz ) energy level and a doubly-degeneratedeg (dx2 −y2 and dz 2 ) energy level, in which the energy level of eg is higher than that of t2g .
However, in a tetrahedral field, the order of energy level is reversed and the magnitude of
splitting is less [12], as shown in Figure 1-3 (b). In the case of 3d orbitals in the TiO5
polyhedron, because the symmetry breaks along the horizontal plane, the splitting of energy
levels becomes more complicated.
Considering the local characteristic nature of titanium–oxygen polyhedra, XAFS is adopted
as the common experimental approach to detect the electronic states and the local atomic
arrangement structure. It is well-known that the pre-edge of X-ray absorption spectroscopy(XAS) is related to the electronic transition from the core level to the unoccupied electronic
states of the conduction band; therefore, the pre-edge feature can provide useful electronic
information. Figure 1-4 shows the pre-edge of Ti K-edge XAS of strontium titanate (SrTiO3

3

(a) 3d electron orbitals of isolated atoms(b) 3d electronic orbitals under free, tetrahedral, and octahedral fields

Figure 1-3: Schematic of (a) 3d electron orbitals of isolated atoms, and (b) crystal-field
splitting of 3d electronic orbitals under different fields.
or STO). There are three peaks marked as A1 , A2 , and A3 in the pre-edge. The A1 peak iscorresponding to the Ti 1s to the t2g electronic transition and the A2 is corresponding to the Ti
1s to the eg electronic transition. Because of the 3d-type nature of t2g and eg , both transition
belong to the quadrupole transition. The electronic state of A3 peak originates from theinter-atomic hybridization from 3d orbital of the absorbing Ti and the 4p of the neighboring Ti,
so A3 peak is corresponding to the dipole transition. Therefore, the information of electronic
states and transition type can be obtained from the XAS experiment. The electronic states
are sensitive to the symmetry property of the absorbing atom, so XAS experiment can also
probe the microscopic structure of the materials. Thus, the sensitivity of XAS to the local
structure makes it a suitable technique for studying the local structure of titanate compounds.
Two types of titanium–oxygen polyhedra containing materials are studied in this thesis.

4Figure 1-4: The pre-edge of Ti K-edge XAS of SrTiO3 .
One is TiO5 polyhedron containing titanates, which exhibit intense photoluminescence (PL)under UV irradiation. Because these titanate phosphors do not contain dopants to act as color
centers, they can be termed as self-activated titanate phosphors. The materials belonging to
the fresnoite family with intrinsic square pyramidal TiO5 polyhedron, Ba2 TiSi2 O8 (BTSO),
Sr2 TiSi2 O8 (STSO), and Na2 TiSiO5 (NTSO) are typical self-activated titanate phosphors,and can emit intense visible light under UV irradiation. The TiO5 polyhedron structure plays
a crucial role in the PL process. To understand the PL mechanism of these self-activatedtitanate phosphors, the electronic states around the valence band are required. The electronic
states formed by the TiO5 polyhedron are studied by X-ray absorption near edge structure(XANES). The other material is SrTiO3 containing TiO6 octahedron. SrTiO3 exhibits various
physical responses to UV stimulation. At room temperature, cubic SrTiO3 does not possess
the PL property; however, once the TiO6 octahedron in SrTiO3 becomes distorted, SrTiO3
exhibits faint emission under UV irradiation [13]. Furthermore, SrTiO3 exhibits PL at low
temperatures of less than 37 K [14]. Because SrTiO3 maintains centrosymmetry over theentire temperature range, as observed from the XRD results, no substantial distortion exists
in the TiO6 octahedron; consequently, no PL should occur. Some studies have reported that
the Ti ion movement along the [001] direction in SrTiO3 can be excited under UV irradiation,
which will form the TiO5 polyhedron [15]. This phenomenon has a significant importance5

in investigating ferroelectricity in SrTiO3 . However, Ti ion motion under UV irradiation has
not been proved experimentally. Therefore, the local structure of SrTiO3 is investigated by
XAFS.

1.3

TiO5 polyhedron containing self-activated titanate phosphors

Photoluminescent materials have wide practical application prospects, such as lighting,photosensitive detectors, cathode ray tubes, bioimaging, and decoration [16–18]. Generally,
photoluminescent materials are obtained by doping ions in a host matrix. The doped ionsserve as color centers, and the host matrix provides a suitable crystal field environment. The
doped ions can create in-gap states between the valence band and the conduction band. The
electrons in the valence band can be excited to the conduction band by UV irradiation. The
excited electrons undergo energy and momentum relaxation toward the in-gap states. Visible
light with longer wavelength compared with UV light will be emitted after the electron jumpback the band gap minimum (Figure 1-5). During the PL process, the in-gap states play
an important role in inducing the emission of visible light. Because rare earth elements
contain abundant electronic energy levels and exhibit bright emission intensity and highquantum conversion efficiency, rare-earth ion-doped photoluminescent materials have been
most widely utilized in light-emitting materials in the past few decades. However, due tolimited availability and the continuously increasing cost of rare-earth elements, it is necessary
to find appropriate alternatives for rare-earth ion-doped phosphors. Non-doped titanatematerials with good photoluminescent property are garnering increasing attention owing totheir advantages of high emission intensity, wide emission band, and rare-earth-free nature[19].
Generally, titanate compounds with high crystal symmetry have wide energy gaps, e.g.,
3.3 eV for tetragonal BaTiO3 (space group P4mm) and 3.4 eV for cubic SrTiO3 (Pm3m),
and do not exhibit PL. If the structures are distorted, these perovskite titanates exhibit PL
under UV irradiation [13, 20]. The distortion of the TiO6 octahedron is shown in figure 1-6.
After the distortion, the linkage of adjacent TiO6 octahedron along the z-axis is broken.
Further, because a square pyramidal titanium–oxygen polyhedron TiO5 is produced, the6

Figure 1-5: Energy diagram of the photoluminescence process
TiO5 polyhedron can be regarded as a distorted TiO6 octahedron. During distortion, thesymmetry of the octahedron is reduced, and the energy levels of eg and t2g will be further
split [21], resulting in the creation of in-gap states. The creation of in-gap states causes
the PL phenomenon in the distorted perovskite titanates. Therefore, the existence of TiO5
polyhedron is indispensable for producing the PL property.
Many TiO5 polyhedron containing titanate compounds exhibit PL. Table 1-1 lists some
self-activated phosphors containing TiO5 polyhedron with different excitations and emission
wavelengths; among these, the white-light-emitting phosphors are more interesting than theothers. In recent years, the application of white-light-emitting rare-earth-free phosphors inwhite-light-emitting diodes has received considerable attention because of their high brightness,
low energy consumption, and environment-friendly property. Moreover, they can be used infield-emission displays, thereby realizing displays with thin panels, wide viewing angle, fast
response time, high contrast ratio, and low power consumption. Many oxide compounds
containing transition metal ions coordination polyhedron with d0 electron configurations such

7

Figure 1-6: Lattice structure of BaTiO3 with (a) Oh symmetry TiO6 octahedron and (b)distorted octahedron TiO5 [20].
as Ti4+ , V5+ , and W6+ have shown good white light emission intensity under UV irradiation.
Among such closed-shell transition metal containing phosphors, titanate phosphors have been
more extensively studied because of their advantageous optical properties; furthermore, thesematerials are used as tunable solid-state laser hosts, nonlinear optical materials, phosphors,and long-lasting phosphorescence materials. All these benefits make titanate phosphors moreadvantageous than the other rare-earth-free phosphors.
It is well-known that Ti4+ ions-doped or –containing materials exhibit blue and yellow
emission under UV irradiation. The blue emission is caused by the charge-transfer transition
from 2p orbits of O2− to 3d orbits of Ti4+ , i.e., O2− -Ti4+ → O− -Ti3+ , whereas the yellow
emission results from the titanium octahedron associated with oxygen defects [3]. In the caseof self-activated titanate phosphors, because there are no doping ions to act as color centers inwhich electron excitation and de-excitation occur, the color responsible for the PL property isunclear. The methods for producing in-gap states differ between rare-earth-doped phosphorsand self-activated titanate phosphors; therefore, the electronic states and the microscopic
structure of self-activated titanate phosphors essential for realizing the photoluminescentphenomenon should be different. It has been reported that a single short Ti–O bond in a TiO5
polyhedron is essential for PL [22], but this theory has not been studied in detail. Although8Table 1-1: Various TiO5 polyhedron containing self-activated titanate phosphors and their
PL properties.
Wavelength (Ex.)

Wavelength (Em.)

(nm)

(nm)

Ba2 TiSi2 O8

260

450

[23]

Sr2 TiSi2 O8

260

492

[24]

Na2 TiSiO5

250

460

[3]

Ba2 TiP2 O9

254

490

[25]

Li2 TiSiO5

250

505

[26]

LiTiPO5

254

490

[26]

Na2 TiGeO5

255

425

[27]

Titanate phosphorsrefthe electronic states have been evaluated using the Raman spectroscopy data in previous
works, there are no reports that correlate the in-gap states and the orbital characteristicsof the relevant atoms. Thus, the electronic states of self-activated titanate phosphors areinvestigated via UV irradiation.
To investigate the electronic states essential for realizing the PL property in self-activatedtitanate phosphors, an appropriate technique must be used. As discussed above, XANES has
the advantageous characteristics of high resolution in measuring the electronic states aroundthe valence band, and is sensitive to particular atomic species and local structures [8, 9]. The
origin of XANES is the transition of the Ti 1s electrons to the unoccupied electrons statesnear the bottom of the conduction band. Because the conduction band mainly originates fromthe vacant 3d atomic orbitals of the Ti atoms in the compound, the lowest conduction band
can be a battle field for UV and X-ray excited electrons. Thus, XANES is an ideal technique
for studying the photoluminescent phenomenon of self-activated phosphors. The doping ratioof ion-doped phosphors is only several percentage or less, detecting which is difficult using
XANES.

9

1.4

TiO6 Octahedron distortion in SrTiO3Perovskite-type SrTiO3 has perfect cubic structure at room temperature and undergoes
an antiferrodistortive phase transition at 105 K, which is the rotation of the TiO6 octahedron
along the z-axis [28]. An Sr subunit cell of SrTiO3 with two phases is shown in Figure
1-7. Despite undergoing phase transition, the stability of the centrosymmetry within theentire temperature range makes SrTiO3 a very stable structure. SrTiO3 has the space group
P m3m with 3.905 Å lattice constant at room temperature and I4/mcm space group at low
temperatures.
SrTiO3 possesses several excellent physical characteristics such as high thermostability andchemical stability, large dielectric constant, photocatalysis, superconductivity, and sensitivity
to external stimulation such as pressure or photon. [29, 30]. SrTiO3 is a typical incipient
ferroelectric material and has quantum paraelectricity below 37 K [31]. With decreasing
temperature, the dielectric constant increases gradually, but the ferroelectric phase neverappears, which is a result of quantum fluctuations and the antiferrodistortive phase transition
at low temperature [32]. Nevertheless, various physical approaches have been used to obtain
ferroelectricity in SrTiO3 in the macroscopic scale even at room temperature, such as ion
doping, isotropic exchange, strain performance, low dimension treatment, and coherent motive
modulation [33–37].

(a)

(b)

Figure 1-7: Sr subunit cell of SrTiO3 : (a) tetragonal structure and (b) cubic structure; therotation of the TiO6 octahedron in (a) corresponds to antiferrodistortive phase transition.

10

As discussed above, cubic SrTiO3 do not exhibit PL, and only the distorted SrTiO3 shows
faint emission under UV irradiation. However, SrTiO3 exhibits PL below 37 K (Figure 1-8),
which has an important significance in investigating new physical properties of SrTiO3 [14].
Because SrTiO3 maintains centrosymmetry over the entire temperature range, as revealed by
XRD results, no large distortion exists in the TiO6 octahedron; thus, no PL should occur.
With the statement in the last section, the existence of TiO5 polyhedron is very important for
the PL property of self-activated titanate phosphors. The TiO5 square pyramid is frequently
considered as the Ti ion displacement from the octahedral center along the c-axis. In addition,
the point group of the distorted TiO6 by the way of Ti ion displacement is C4v , which is sameas the TiO5 . It is reasonable to suspect that small distortion of the TiO6 in SrTiO3 exist
to produce the PL property. To resolve this problem, the local structure of TiO6 in SrTiO3
must be investigated. Because the UV irradiation is the necessary for the PL property, thetechnique used for the detection must be available to combine the UV irradiation.

Figure 1-8: Photon emission spectra of SrTiO3 at less than 37 K; no PL property appears at
high temperatures [14].
Various methods have been reported to detect the distortion of the TiO6 octahedron
related with the off-center Ti ion in the octahedral cage. The conventional techniques of11

XRD and Raman can provide local structure information about the average positions of theatoms in the crystal lattice. Thus, they are not suitable to detect the microstructure of
SrTiO3 . One of the reports about the Ti displacement in cubic SrTiO3 was measured by
nuclear magnetic resonance spectroscopy (NMR). A model of eight off-center displacements
along with the eight [111] direction in SrTiO3 was proposed (figure 1-9). In this model, theTi displacement caused by the soft mode combined with the 8-site order-disorder model gavea good agreement with the experiment data [38]. However, the NMR technique is difficult tocombine the UV irradiation.

Figure 1-9: Intrinsic Ti disordered along the eight [111] directions off-center displacements
[38].

12

On the microscopic scale, XAFS is one of the best techniques to detect Ti displacement.
It is well-known that the intensity of the pre-edge of Ti K-edge XANES is a good indicator of
the appearance of Ti ion displacement. In addition, extended X-ray absorption fine-structure
spectroscopy (EXAFS) can directly measure the magnitude of Ti ion displacement in perovskite
materials. The quasiamorphous phases of SrTiO3 do not show macroscopic scale polarization,
but both XANES and EXAFS results reveal clear Ti displacement in quasiamorphous SrTiO3(Figure 1-10) [39]. It is interesting that the Ti K pre-edge XANES has some intensity even forthe crystalline cubic SrTiO3 , which indicates that the Ti displacement in cubic SrTiO3 may
exist. This assumption has been verified by the EXAFS results [40], which have shown thatthe displacement value is approximately 0.1 Å. However, the quality of data in this report is
not good. Generally, the longer of the data range in k-space, more precise of the measurement
can be obtained for the distance between the atoms. In this report, only k range (2.5 to 9Å−1 ) was used. The direct consequence of this shorted data is that only one peak (Ti-Ti)
appears, the peak of Ti-Sr is overlapped in the Ti-Ti peak, which will reduce the accuracyof the measurement. In my present study, more long k range data were obtained with morehigher resolution in energy.

(a)

(b)

Figure 1-10: (a) Ti K-edge XANES spectra of crystalline, amorphous and quasiamorphous
SrTiO3 , and (b) magnitudes of Fourier transforms of the k2 -weighted EXAFS spectra for
crystalline, amorphous, and quasiamorphous SrTiO3 [39].

13The advantage of EXAFS is not only the sensitivity of the local structure and atomic
species, but also the time scale. The lifetime of an excited state in XAFS is in the order of
femtoseconds, which far exceeds the periodic time of atomic vibration and the fluorescencedecay time of PL. Therefore, EXAFS is suitable for detecting the influence of UV on SrTiO3 .
The meaning of the measurement for the displacement of Ti ion off center is not limited
at revealing the origin of the PL property of SrTiO3 . It also can explained other physicalphenomenon of SrTiO3 related with the UV stimulation. A prominent physical property of
SrTiO3 is photosensitivity. Takesada reported a drastic increase in the dielectric constant of
quantum paraelectric SrTiO3 under UV irradiation and weak DC Electric Field (Figure 1-11)[41]. The influence of UV on the dielectric constant indicates that UV can affect the local
structure of SrTiO3 . It is considered that the enhancement of the dielectric constant is causedby the photoinduced polar domain. However, this conclusion is controversial, because after
the UV irradiation, the electrons in valence band can be excited to the conduction bands, theelectron migration in the conduction band can cause the leakage of electric current between
the electrode during the measurement, which can also increase the value of measured dielectric
constant. Nozawa investigated this phenomenon using XANES and found an enhancementof the pre-edge, which is considered as the vibration of Ti ions along the z-axis (Figure 1-12)[15, 42]. However, no experiments have been conducted to measure the vibration of Ti ionsdirectly.

Figure 1-11: UV-induced drastic increase in the dielectric constant of quantum paraelectric
SrTiO3 under weak DC Electric Field [41]14Figure 1-12: (a) Random vibration of Ti ions under thermal perturbation and (b) UV-induced
vertical vibration of Ti ion in SrTiO3 [15].

1.5

Objective of this study

Based on the response to UV irradiation, the changes in the physical property differ
depending on the titanium–oxygen polyhedra. Because the titanates belonging to the fresnoite
family with TiO5 polyhedron exhibit excellent PL property under UV irradiation, they canbe a good substitute for rare-earth-doped phosphors. However, the electronic in-gap states
essential for realizing the PL property are unclear. XANES, which is corresponding to theelectronic transition from the core level to the unoccupied electronic states, is a good approach
to in-gap states. SrTiO3 exhibits PL at low temperature as well with UV irradiation, which
speculates a small distortion of TiO6 octahedron in SrTiO3 . EXAFS, which is capable tomeasure the distance between atoms, is a suitable way to measure the distortion. Previous
studies on the pre-edge of Ti K-edge XANES have proposed the enhancement of Ti ion
vibration along the c-axis with UV irradiation. This physical phenomenon should be investigatedby EXAFS.
In this thesis, the influence of the disorder of titanium–oxygen polyhedra is examined
quantitatively, considering the UV effect. XAFS is adopted as the basic experimental approach.
The objectives of this research are listed below:
15(1) The electronic states of the titanate phosphors belonging to the fresnoite family are
investigated via XANES to evaluate the role of TiO5 polyhedron in the PL process. To
reveal the energy diagram of the PL process, the origin of in-gap states must be clarified
based on the experimental results and theoretical calculations.
(2) The local structure around the Ti atom in SrTiO3 is investigated via EXAFS to detectthe distortion of TiO6 octahedron. Then, the origin of PL property in SrTiO3 at low
temperature is investigated.

The titanium–oxygen polyhedra and related physical properties were discussed in thissection. The other four sections presented in this thesis are presented below. Sample preparation
and XAFS experiment are discussed in the “Experiment” section. To overcome the different
penetration of UV and X-ray, superfine powder was obtained by using a micron-sized sieve.
The influence of UV was analyzed by irradiating UV light on the sample during the XAFS
experiment. The electronic states of the self-activated titanate phosphors obtained by XANES
are described in the third section. Then, the role of TiO5 polyhedron in realizing the PL
property is presented. In the fourth section, the origin of PL property in SrTiO3 is studied bymeasuring the small distortion of TiO6 octahedron. EXAFS is adopted as the main technique
to detect this small distortion. In the “Conclusion” section, the relationship between the PL
property of titanates and the TiO5 polyhedron is summarized.

16

2

Experimental Techniques

2.1

Sample preparation

2.1.1

Self-activated titanate phosphor powder

The self-activatied titanate phosphor samples (Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , and Na2 TiSiO5 )were synthesized via the conventional solid-state reaction method.

Commercial BaCO3 ,SrCO3 , Na2 CO3 , TiO2 , and SiO2 powders with a purity of higher than 99% were used as
raw materials. They were weighed and mixed to obtain uniform stoichiometric mixtures ofthree samples. To obtain homogeneous compositions, all the mixtures were carefully ground,and then pressed into pellets. The pellets of Ba2 TiSi2 O8 and Sr2 TiSi2 O8 were annealed at
1000◦ C for 12 h under air atmosphere, and were ground again. After repelletization, thepellets were calcined at 1300◦ C under air atmosphere for 2 h. The final powder samples wereobtained by grinding the pellets. Na2 TiSiO5 was prepared following the same procedure, withthe exception of the annealing and calcination temperatures of 600◦ C and 850◦ C, respectively.
X-ray diffraction:
Because XAFS is very sensitive to the composition of measuring sample, the XRD patternsof these powder samples were recorded on an X-ray powder diffractometer with Cu K radiation(Rigaku, Miniflex) to check the purity and degree of crystallinity. A Rietveld refinement
analysis was performed to confirm and analyze the XRD data.
Figure 2-1 compares the observed XRD patterns of Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , and Na2 TiSiO5
with the calculated patterns obtained from Rietveld refinement analysis, which demonstrates
the high crystallinity of the samples. The intensities of the observed and calculated patterns
were normalized to unity. The crystal parameters obtained from our Rietveld analysis arelisted in Table 2-1, which are comparable to the previously reported values [25, 43, 44]. The
corresponding crystal structures are illustrated in the insets in each panel of Figure 2-1,
which highlight the TiO5 polyhedra. The top two panels exhibit similar diffraction patterns
because the crystal structures of Ba2 TiSi2 O8 and Sr2 TiSi2 O8 belong to the same fresnoite

17

structure with the tetragonal space group P4bm. A small amount of impurities exists in these
two samples, as indicated by the arrows. However, neither SiO2 nor SrTiO3 affect the Ti
K-edge XAS spectra because the signal is relatively weak. No impurity peak was observed for
Na2 TiSiO5 . As shown in the XRD patterns in Figure 2-1, Na2 TiSiO5 has another different
crystal structure with the orthorhombic space group Pmc21. This structure contains TiO5
polyhedral as well, as depicted in the inset.

Figure 2-1: Rietveld refinement results for XRD patterns of Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , andNa2 TiSiO5 . The experimental data are indicated by cross marks, and the calculated data
are represented by red line. The difference between the observed and calculated patterns areshown in the lower part of each panel. Impurities are indicated by arrows. Ba2 TiSi2 O8 and
Sr2 TiSi2 O8 show a similar crystal structure and contain the TiO5 polyhedron, Na2 TiSiO5
possesses a different crystal structure, but contains the TiO5 polyhedron.

18

Table 2-1: Refined crystal parameters of Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , and Na2 TiSiO5
SampleBa2 TiSi2 O8

Sr2 TiSi2 O8

Na2 TiSiO5a(Å)

8.519(3)8.323(5)9.128(1)b(Å)

8.519(3)9.323(5)4.801(4)c(Å)

5.211(7)5.024(7)9.823(5)

α=β=γ

90◦

90◦

90◦

Space Group

Pb4m

Pb4m

Pmc21

χ2

1.826

1.563

1.853

Fluorescence spectrophotometer:
The photoluminescence spectra were obtained using a fluorescence spectrophotometer with
Xe lamp as the light source (Hitachi, F-2700). The bandpass of the spectrophotometer was set
to 2.5 nm, thereby ensuring a high energy resolution (< 0.08 eV) for the entire spectrometer
range. A 300 nm longpass filter was used to block the excitation beam.
Figure 2-2 shows the photoluminescence excitation (PLE) and PL spectra of the threesamples. The monitored wavelengths of the PLE spectra were set to the peak wavelength ofthe corresponding PL spectra, and the excitation wavelengths of the PL spectra were set tothe peak wavelengths of the corresponding PLE spectra. All the spectra were normalized tounity at their respective maxima. All the spectral profiles are similar; specifically, the PLE
spectra of Ba2 TiSi2 O8 and Sr2 TiSi2 O8 overlap, because they are isomorphic to each other.
However, the PL spectra of these two samples show a slight difference in their peak positions:
450 nm for Ba2 TiSi2 O8 and 492 nm for Sr2 TiSi2 O8 . The energy difference is 0.23 eV, which
can be discerned by the naked eye, as shown in Figure 2-2(e) and 2(f). Ba2 TiSi2 O8 emits
bright bluish-white light, whereas Sr2 TiSi2 O8 emits yellowish light under UV irradiation.
Furthermore, Na2 TiSiO5 emits bluish-white light as well, similar to Ba2 TiSi2 O8 (Figure
2-2(g)). Although the crystal structure of Na2 TiSiO5 differs from that of the two otherphosphors, as explained in the previous passage, the PL and PLE spectra of Na2 TiSiO5
19

exhibit some similarities with those of Ba2 TiSi2 O8 and Sr2 TiSi2 O8 . These similarities confirm
that the TiO5 polyhedra, which are contained in all three samples, play an important role indetermining the PL properties of self-activated titanate phosphors.

Figure 2-2: PL property of Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , and Na2 TiSiO5 . (a) PLE and PL spectraof Ba2 TiSi2 O8 (black line), Sr2 TiSi2 O8 (red dot line), and Na2 TiSiO5 (blue dash line). Imagesof Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , and Na2 TiSiO5 under (b–d) natural light and (e–g) UV lightirradiation.

2.1.2

Fine strontium titanate powder

The raw material used for obtaining fine SrTiO3 powder is an SrTiO3 single crystal. First,
the single crystal was ground by using a agate mortar. The obtained powder was transferred20

Figure 2-3: 5 µm sieve was used for obtaining superfine SrTiO3 powder. The net of this sieve
is made of organic fiber and does not contain any metal.
to a beaker with ultrapure water to form a suspension of water and SrTiO3 powder underultrasonic treatment. Then, the suspended liquid was left still for 24 h, until the suspended
liquid was separated into two layers. The bottom layer contained relatively larger SrTiO3
particles and the top layer contained small particles.
To as-prepared superfine SrTiO3 powder and the top layer liquid was flowed across sievesof 5 µm (Figure 2-3). The superfine SrTiO3 powder was finally obtained after evaporating
the water.

2.2
2.2.1

Synchrotron radiation
Basic information about synchrotron radiation

Synchrotron radiation is a type of electromagnetic radiation, and is emitted when relativistic
charged particles move along curved trajectories when they are subject to a magnetic field[45]. A storage ring is used to produce electrons and maintain the electrons stored at constantenergy and radio-frequency cavities. The advantage of synchrotron radiation is its wide
spectral range, which covers infrared light to hard X-rays. Thus, synchrotron radiation is usedin many applications, such as determining the material structures and properties in physics,

21

Figure 2-4: Layout of the Photon Factory storage ring and beamline distribution
chemistry, and biology, and even in disciplines such as archaeology and environmental science[46].
The main properties of synchrotron radiation are high intensity, polarization (both linear
and circular), broad and continuous spectral range (from infrared light to hard X-ray region),
natural narrow angular collimation, pulsed time structure, high brightness of the source dueto the small cross-section of the electron beam and the high degree of collimation of theradiation, ultra-high vacuum environment, and high beam stability. All these properties canbe quantitatively evaluated.
To date, three generations of synchrotron radiation sources have been developed, eachwith an improvement in quality. All the XAFS experiments in this research were conducted22

at the Photon Factory (PF)(Figure 2-4), which is a second-generation synchrotron radiation.

2.2.2

Beamline 9A of Photon Factory

The experiments on Ti K-edge XANES of self-activated phosphors were conducted at thehigh-intensity, low-energy XAFS beamline BL-9A of the PF. The beamline layout of BL-9A isshown in Figure 2-5. An Si (111) double-crystal monochromator with water cooling was used
for obtaining a wide photon energy range of 2.1–15 keV. Both transmission and fluorescentmodes can be used in this beamline. The transmission mode can be used for powder samples,
along with two ionization chambers for X-ray detection, whereas the fluorescent mode can beused for crystal samples or bulk samples; a 19-element silicon drift detector is used for avoiding
self-absorption effects. To reduce high-order photons, a double flat mirror (Ni coated) is set
in front of the sample set-up stage.

Figure 2-5: The beamline layout of BL-9A at the PF

2.2.3

Beamline 12C of Photon Factory

The experiments on Ti K-edge EXAFS of SrTiO3 were conducted at the BL-12C: Highthroughput XAFS beamline BL-12C of the PF. The beamline layout of BL-12C is shownin Figure 2-6. An Si (111) double-crystal monochromator with water cooling was used for
obtaining a wide photon energy range of 2.1–23 keV. Both transmission and fluorescent modes

23

Table 2-2: Beamline parameters of BL-9A at the PF
Energy range

2.1 - 15 keV

Measurement mode

Transmission and fluorescent

Magnetic field

Bending magnet

Monochromator

Si(111) Water-cooled double crystal

High order light reduction

Double flat mirror (Ni coated)

Energy resolution∆E/E ∼2 × 10−4 (9 keV)

Photon flux

6 × 1011 phs/s (7 keV)

can be used in this beamline. Compared with that of BL-9A, the energy range of BL-12C issignificantly wider; therefore, XAFS for heavy element K-edge is available.

Figure 2-6: Beamline layout of BL-12C at the PF [47]2.3

X-ray absorption

X-ray absorption is involved in the electron transition between two quantum states,
namely, initial state and final state. Generally, the initial state is the core energy level,
and is specific for each element, whereas the final state is located above the Fermi level.
Because the final states can be modulated by the chemical and physical environments around24

Table 2-3: Beamline parameters of BL-12C at the PF
Energy range

4 - 23 keV

Measurement mode

Transmission and fluorescent

Magnetic field

Bending magnet

Monochromator

Si(111) Water-cooled double crystal

High order light reduction

Double flat mirror (Ni coated)

Energy resolution∆E/E ∼2 × 10−4 (9 keV)

Photon flux

9 × 1011 phs/s (7 keV)

the absorbing atom, measuring the absorption coefficient of X-rays can provide reliable anduseful information about the chemical and physical environments of the probe atom at the
microscopic level. The electron transition is based on the Fermi’s golden rule:µ≈X| ψf |H0 ψj | |δ(Ef − Ei − ~ω)(2-1)

Generally, the absorption coefficient µ is a smooth function of energy:

µ≈

ρZ4
ME3

(2-2)

This equation shows that the absorption coefficient µ depends on the sample density ρ, atomic
number Z, and atomic mass A. Because of this strong dependence of µ on both density ρ andatomic number Z, X-ray absorption is widely used in medical imaging, such as X-ray plateand X-ray computed tomography.
In an X-ray absorption experiment, we primarily measure the absorption coefficient µ,which produces the probability that X-rays will be absorbed according to the following
equation (Beer-Lambert law), where I0 represents the X-ray intensity incident on a sample,
t denotes the sample thickness, and I is the intensity transmitted through the sample. The
relationship between these parameters is given by the Beer-Lambert law:25

Figure 2-7: X-ray absorption spectra for several elements over the X-ray energy range of
1–100 keV. The spectra show strong energy dependence, and the sharp jumps in cross-section
correspond to core-level binding energies of the atoms [48].

I = I0 e−µt(2-3)

Generally, the absorption coefficient µ is a smooth function of energy. When the incident
X-ray equals to the binding energy of a core-level electron for a specific atom, a sharp increasein the absorption coefficient occurs. Customarily, the rising curve is named an absorption edge,
which corresponds to the core level to the continuum. Figure 2-7 shows several absorption
edges for several specific atoms.
Two different modes can be used in an XAFS experiment, namely, transmission mode andfluorescence mode. In the transmission mode, the intensity of the incident and the transmitted
X-ray beams are measured in an ionization chamber placed on both sides of the sample (Figure
2-8 (a)). The absorption coefficient of the transmission mode µ is expressed as:µ(E) = In

26

I0
I

(2-4)

In the fluorescence mode, the incident X-ray beam is generally measured in the ionizationchamber. Because of the existence of X-ray fluorescence or Auger electrons, a secondary
emission resulting from the absorption of X-rays can be measured by a multiple SSD detector,
as shown in figure 2-8 (b). The absorption coefficient of the fluorescence mode µ is expressedas:µ(E) ∝

If
I0

(2-5)

(a)

(b)

Figure 2-8: XAS experiment with (a) transmission mode and (b) fluorescence mode [9].
The equation of Fermi’s golden rule indicates that XAFS is a modulation of X-ray absorption
probability with the changing incident energy. Generally, the X-ray absorption spectrum isdivided into two regimes: XANES and EXAFS. XANES is the X-ray absorption from the corelevel to the electronic states above the Fermi level. Because these electronic states are highly
sensitive to formal oxidation states and the coordination chemistry of the absorbing atom,
XANES is used to interpret the electronic information. EXAFS is the interference between
the photoelectron wave and the scattering wave, which can be used to determine the distance,
coordination number, and species of the neighbors of the absorbing atom. Figure 2-9 shows
an Fe K-edge of XAFS [48].
27Figure 2-9: XAFS µ(E) for the Fe K-edge of FeO, showing the near-edge region (XANES)
and the extended fine structure (EXAFS) [48].
2.3.1

X-ray absorption near edge structure

XANES is involved in the transition from the core level to the unoccupied states above
the Fermi level (Figure 2-10). These unoccupied states are empty molecular orbitals, whichcorrespond to the chemical bond states. The empty molecular orbitals are decided basedon the local site symmetry and orbital occupancy. From the perspective of molecular orbital
theory, the intensity and energy level of the pre-edge peaks correspond to the linear combinationsof unoccupied empty molecular orbitals that have specific symmetry properties. Thus, the
XANES spectrum can provide the symmetry information of the absorbing atom. In addition,
because the possibilities of dipole transition and quadrupole transition differ greatly, XANES
can identify the orbital types above the Fermi level as well.
Theoretical Calculation of XANES
Spectral simulations were performed for Ti K-edge XANES of the three samples using
the finite difference method near edge spectroscopy (FDMNES) code [49]. In this code, a fullpotential calculation and relativistic corrections are employed for potential calculations witha self-consistent treatment, followed by the multiple scattering approach for XAS calculation.

28

Figure 2-10: Schematic energy diagram of XANES. The electron in the core level was excited
to the empty electronic states above the Fermi level, leaving behind a core hole
We assumed a cluster of radius 7.9 Å around a Ti ion using the crystal parameters obtained
by XRD measurement. The local density of states (LDOS) of all constituent atoms were also
calculated in the same runs.

2.3.2

Extended X-ray absorption fine structure

Because the incident X-ray energy is higher than the absorbing edge, the electrons will
escape the atom; this is known as the photoelectric effect, and the emitted electrons are
termed as photoelectrons because the absorbed X-ray is a photon. This process is illustrated
in Figure 2-11.
The photoelectrons can be described as spherical waves propagating outward from theabsorber atoms. These outgoing photoelectron waves can be scattered from the neighboring
atoms (Figure 2-12). The interference of the outgoing waves and the scattered waves causes
different features in the post edge, because the density of the unoccupied electron orbitals are29

Figure 2-11: Photoelectric effect, in which the core-level electron was excited by X-rays havingenergy higher than the binding energy, leaving behind a hole in the core electron level. The
photoelectron is promoted out of the atom [48].
modulated by the interference of the photoelectron waves [50].
Because the photoelectrons have the characteristics of waves, it is convenient to use thewavenumber to describe them. The converting equation from energy to wavenumber is givenby 2-6.

rkelectron =2m(Ephoton − E0 )
~2

(2-6)

Because of the scattering process in the scattering atom, the phase of the scattered waves
of a photoelectron is generally not the same as that of the outgoing wave. When the phase of
the outgoing waves and scattered waves is the same at the absorber atom position, a crest in
the post edge is produced, which corresponds to the maximum absorption coefficient (2-13).
When the outgoing waves and scattered photoelectron waves are completely out of phase at
the center of the absorber atom, a valley is produced, which corresponds to a decrease in theabsorption coefficient.

30

Figure 2-12: Interference of outgoing photoelectron waves from the absorbing atom andscattered photoelectron waves from the scattering atom.
The occurrence of “in phase” or “out of phase” is determined by the photoelectron
wavelength and the interatomic distances between the absorber and the scattering atoms. The
average distance between the absorber and the neighboring atoms does not change. According
to the energy and wavenumber conversion equation, with the increasing incident X-ray energy,
the wavelength of the photoelectron wave decreases progressively, and the interference of thetwo waves will be in phase and out of phase periodically. This causes an oscillation of theabsorption coefficient in the post edge. Using these oscillations, the average distances between
the absorber and the scattering atoms can be measured. In addition, as the energy of theincident X-ray is increased above the edge, the photoelectron wavelength decreases; therefore,
increasing the energy range will improve the measurement accuracy.
EXAFS data analysis
To extract the EXAFS oscillation from the post edge, the data µ(E) must be processedby normalization and subtraction of the background. In this process, the pre-edge is fittedusing the Victoreen formula, and then extrapolated to all energies in the measurement rangeof the data [8]. By subtracting the fitted Victoreen formula from µ(E), the pre-edge portionof the data was reduced to 0. The pre-edge subtracted data were then divided by the edge
jump, following which the value of XAS in the post edge was normalized to 1. The conversionof µ(E) to the EXAFS oscillation χ(k) is based on the equation 2-7.

31

Figure 2-13: Schematic diagram of the modulation of the absorption coefficient of X-ray by
the scattering photoelectron. The in-phase modulation between the outgoing photoelectron
wave and the scattering photoelectron wave causes a crest in the EXAFS region, and theout-of-phase modulation causes a valley [48].

χ(k) =µ(E) − µ0 (E)
∆µ0

(2-7)

In actuality, because of the complexity of the surrounding environment of the absorber,many factors must be considered to model the interference between the outgoing waves andthe scattered waves . First, because the model of the spherical wave is used for describing
the photoelectrons, a sinusoidal term is used; thus, the phase shift caused by scattering canbe included in the sinusoidal term. Second, because each atom around the absorber can bea scatter, the sum of all the scattering paths is required to fit the experimental data. In
addition, the amplitude of each path is infected by the scattering process and atom vibration.
The theoretical EXAFS oscillation formula is expressed as follows:32χ(k) =X Nj S2 (k)Fj (k)
0

j

kR2j

2
4
exp[−2k2 σj2 + C4j k4 ]e−2Rj /λj sin(2kRj + ψj (k) − C3j k3 )
3
3

(2-8)

The meaning of each parameter is defined below [8]:
 Scattering amplitude Fj (k): proportionality constant, which is itself proportional to thepossibility of scattering elastically off of the atom.
 j is the path number of each scattering path, including single scattering paths andmultiple scattering paths. To modulate the absorption probability, the sum of all
the scattering paths will be included, and each of these scattering events contributes
separately, which results in a linear combination of each path.
 ψj (k): phase shift caused by the local potential around the absorber and the scatterer.
 λj : mean free path of the photoelectrons, which causes the inelastic scattering and decayof the core hole.
 N is the coordination number or degeneracy. Around the absorber, several scattering

atoms belong to the same species with an identical distance. To group them, degeneracy
is used to be multiplied on a single path. For single scattering paths, N is referred to as
the coordination number.
 S20 is the amplitude reduction factor. Once a photoelectron has been ejected fromthe atom, a core hole is created at the core level. The screening effect of the corelevel electrons reduces, causing all the other electrons to feel more positive charge fromthe nucleus, and the orbitals adjust to this change. Irrespective of the activity of thephotoelectron, the final state of the absorbing atom is not the same as the initial state.
The amplitude reduction factor is an element-dependent constant.
 σ 2 is defined by the variance in the half path length, i.e., the square of the standard

deviation of the half path length (r − r)2 , always termed as the mean square relative
displacement (MSRD) or Debye-Waller factor. It is used to correct for the difference
between the experimentally determined and the theoretical χ(k) functions.
33 R is the half path length. For single-scattering paths, the half path length is simply

the average distance between the absorbing and scattering atoms. To compensate forthe difference between the real value of half path length and the assumed value in thestarting structure, δR is used.
 ∆E0 : Equation 2-6 describes the relationship between E0 and wavenumber. E0 is theenergy used to calculate the momentum of the photoelectron. Because it is’ difficult toobtain an exact value of E0 , ∆E0 is used to adjust E0 .
 The higher-order cumulants C3 and C4 are the further correction of the EXAFS formula,which are defined by (r − r)3 and (r − r)4 − C22 , respectively. C3 and C4 are notgenerally used in the EXAFS data analysis.

During the EXAFS data analysis, the scattering amplitude Fj (k) ψj (k): the phase shift
λj : the mean free path of the photoelectrons is obtained from the assumed initial atom
structure and the calculation procedure of the path data [51]. Based on the effect on χ,
all the other parameters defined above can be categorized into two: (a) intensity-relatedparameters, namely, amplitude reduction factor S20 , Debye-Waller factor σ 2 , C4 , path numberj, and degeneracy N, which can affect the oscillation intensity of χ; (b) half path length R,∆E0 , and C3 , which can affect the phase of χ.
Because the parameters in each group are highly correlated, it is difficult to obtain areasonable value for several parameters simultaneously. Thus, the common approach is to
fix one of them and guess the other. For example, the amplitude reduction factor S20 and
Debye-Waller factor σ 2 are highly correlated and affect the intensity of χ. Therefore, to finda reasonable physical value of σ 2 , S20 must be fixed with a specific value. The same approachis equally applicable for the phase-related parameters as well.
Equation 2-8 describes the EXAFS oscillation caused by the interference of the outgoing
and the scattering photoelectron waves. To obtain the atomic distance, Fourier transformmust be used:1
χ(R) = F T (χ(k)) = √2πZ ∞k n χ(k)W (k)ei2kR dk

(2-9)

0

As mentioned above, the wider range of X-ray energy measurement can improve the accuracy34

of the experimental measurement. However, with the increasing energy, the wavenumber
becomes longer, the amplitude of EXAFS oscillation delay fast. To emphasize the oscillationsat large wavenumbers, χ(k) is multiplied by a power of k, as shown in the Fourier transform
equation above.
To perform an EXAFS data analysis from the raw data to the reasonable value of eachparameter, several software packages are required. The scattering paths are calculated by
FEFF [51], which is an ab initio multiple-scattering code for calculating the excitation spectra
and electronic structure. The background reduction process of experimental data can beaccomplished using the software Athena. The fitting procedure of the experimental raw data
and theoretical calculations can be performed using Artemis or FEFFIT. Artemis is containedin the Demeter package and IFEFFIT package along with Athena [52, 53]. FEFFIT has thesame algorithm as Artemis. In this study, FEFFIT is used in the fitting procedure [54].

35

3Investigation of the electronic states of TiO5 polyhedroncontaining self-activated titanate phosphors by XANES

3.1

Experimental setup and configuration

The XANES experiments were performed in the beamline 9A of the PF. The spectra were
measured in the transmission mode at 100 K. The powder samples were spread over adhesive
tapes at a sufficiently low thickness to accommodate the effective penetration depths of UV
and X-ray radiation. The tape was then fixed on a rectangular copper sample holder (Figure
3-1 a). Two factors were considered in the designing of experiments: temperature control
and UV irradiation. The temperature was controlled by a closed-cycle helium cryostat. The
sample holder was placed at the center of the chamber of the cryostat. To allow UV lightirradiation on the chamber, one side of the Kapton film window was replaced with SiO2 glass(Figure 3-1 b).

(a)

(b)

Figure 3-1: Schematic of the sample setup. (a) Spreading of powder over the Scorch tape,and (b) vertical view of the cryostat chamber.
To ensure that the sample is irradiated by X-ray and UV light simultaneously, an angle of45◦ was set between the direction of the X-ray and the plane of the sample holder (Figure 3-2(a)). The sample powder on the Scorch tape was faced toward the SiO2 glass window (Figure
3-2 (b)).
36

(a)

(b)

Figure 3-2: (a) Sample holder makes an angle of 45◦ to the incoming direction of X-rays, (b)the sample within the direction where light is allowed to irradiate the sample through an SiO2
glass window.
An Hg-Xe lamp (Lightningcure LC8, Hamamatsu, Japan) was used as the UV light source.
The UV light was guided by an optical fiber onto the sample, and the UV power on the sample
was controlled by a neutral density filter from 0 to 500 mW cm−2 . Because the excitationenergy of self-activated phosphors is approximately 250 nm, a 250 nm band-enhanced typelamp was selected. To ensure the irradiation of sufficient intensity of UV light on the sample,
a condenser lens was placed in front of the optical fiber.

Figure 3-3: Hg-Xe lamp used as the UV light source in our experiments. The condenser lens
were used in the tail of the optical fiber to focus the UV light.
37

Figure 3-4: Schematic of the XAFS experiment in BL 9A with UV irradiation.
The experiments on Ti K-edge XANES of self-activated phosphors were performed at
the beamline BL-9A of the PF at the Institute of Materials Structure Science. A schematic
illustrating the XAFS experiment with UV irradiation is shown in Figure 3-4. The angular
position of the monochromator crystals was calibrated using the Ti K-edge threshold at4985.3 eV of standard BaTiO3 powder. The X-ray energy was varied using an Si (111) double
monochromator, and higher-order harmonics were removed using a double flat Ni-coatedmirror. The overall energy resolution of the beamline was 0.1 eV. For each sample, the
XANES spectra with and without UV irradiation were measured. The UV irradiation on the
powder was controlled at 162 mW, 324 mW, and 487 mW.

3.2

Ti K pre-edge XANES of titanate compounds

Figure 3-5 shows the Ti K-edge XANES of Ba2 TiO4 , Ba2 TiSi2 O8 , and SrTiO3 . SrTiO3
contains TiO6 octahedron with Oh symmetry. As shown in section 1.2, the 3d orbitals willbe split into t2g and eg energy levels. The first two peaks of Ti K pre-edge of SrTiO3 arecorresponding to the t2g and eg electronic states. Ba2 TiSi2 O8 contains TiO5 square pyramid
polyhedron with C4v symmetry. The intensity of the pre-edge of the TiO5 -containing materials
is significantly higher than that of the TiO6 -containing materials. This can be explained by
the symmetry reduction. The symmetry reduction from Oh to C4v is generally regarded tobe caused by Ti ion displacement in a TiO6 polyhedron along the vertical direction. In the38

Figure 3-5: Ti K-edge XANES of Ba2 TiO4 , Ba2 TiSi2 O8 , and SrTiO3 . The structure oftitanium–oxygen polyhedra (TiO4 , TiO5 , and TiO6 ) contained in each material are inserted
in each figure, respectively. The pre-edge features are noted by the dotted rectangle.
C4v symmetry, the 3d3z 2 −r2 orbital can hybridize with 4pz orbital, which is parity forbiddenin the Oh symmetry. This hybridization makes the electronic transition dipole allowed andresults in high pre-edge peaks. It’s worth noting that the second pre-edge peak in the TiO6octahedron and the giant pre-edge peak in the TiO5 polyhedron have the same eg origin, so
the intensity of the pre-edge of the TiO5 polyhedron is interpreted based on the magnitude of
Ti displacement [55]. In the case of Ti K pre-edge of XAS of Ba2 TiO4 , not only the intensityof the pre-edge is much more higher than that of the other two polyhedra, the energy levelalso varies slightly toward lower energy. The Ba2 TiO4 contains TiO4 tetrahedron with Td
symmetry. In the Td symmetry, the 5-degenerate 3d orbitals and 3-degenerate 4p orbitals
are highly hybridized to form equivalent orbitals. These orbitals are used to bond with the 4oxygen coordination. The hybridization results in the high electronic transition and energy
level shifting.

39

3.3

Ti K pre-edge XANES of self-activated titanate phosphors

The Ti K-edge XAS spectra of the three samples are shown in Figure 3-6. All the spectra
were normalized to unity at the peak maxima after the linear pre-edge backgrounds weresubtracted. The overall spectral profiles are generally similar. The main edges above 4982
eV correspond to 1s–4p dipole transitions. Above the edges, the intensity ratio of the firsttwo peaks, labeled as A and B, are identical in Ba2 TiSi2 O8 and Sr2 TiSi2 O8 because thesetwo compounds are isomorphic; the sharp cusp (labeled A) is a characteristic of Na2 TiSiO5 .
These minor differences above the edges reflect a slight difference in the Ti 4p states; however,
the energy levels of the corresponding states substantially exceed (ca 15 eV) the Fermi level
and are not related to the PL properties.

Figure 3-6: Ti K-edge XANES of Ba2 TiSi2 O8 (black solid line), Sr2 TiSi2 O8 (red dotted line),and Na2 TiSiO5 (blue dashed line). The inset shows an enlarged view of the humps withoffsets; humps are indicated by the vertical line.
Another characteristic of these spectra is a distinct peak at 4968.5 eV in the pre-edge
region. Farges et al. reported that this peak can be attributed to the 1s–3d ‘dipole’ transition[56]. In general, the 1s–3d transition is dipole-forbidden, but quadrupole-allowed; therefore,
the spectral intensity of the pre-edge features is weak compared with that of the main edge.
For example, the pre-edge peak intensities of titanates with TiO6 octahedra, e.g., SrTiO3[57, 58], are less than one-tenth of that of the main edges. Through symmetry reduction, Ti

40

3d orbitals start to hybridize with O 2p orbitals, following which the 1s–3d transition becomes
dipole-allowed. The present three samples contain pyramidal TiO5 polyhedra with a short
Ti–O bond, which supports hybridization between the O 2p and Ti 3d orbitals. In addition,
the isolation of TiO5 polyhedra in the unit cell plays an important role in the creation of a
distinct pre-edge peak. Farges et al. noted that the Ti 3d orbitals form molecular orbitals
with O 2p orbitals in each isolated polyhedron, resulting in “bound states.” This processleads to the creation of quasilocalized states just above the Fermi level.
Before discussing the UV irradiation effects, we consider the small humps observed between
the pre-edge peaks and the main edges in all three samples. This region is enlarged in theinset of Figure 3-6. Because the hump is located above the pre-edge peak, it may serve as a
trap for UV-excited electrons.

3.4

UV effect for Ti K pre-edge XANES of self-activated titanate phosphors

Because the characteristic high intensity for the giant pre-edge peak, the UV effect werefirstly investigated. Figure 3-7 (a) shows the variation of the pre-edges of Ba2 TiSi2 O8 withincreasing the UV irradiation power. It is clear that the intensity of pre-edge peaks are
decreased as UV power increasing. This phenomenon can be explained by the occupancy ofthe electronic states of the pre-edge by the UV excited electrons (figure 3-7 (b)). During
the X-ray absorption, the occupancy of electronic states of UV excited electrons reduces thevolume of electronic states for the X-ray excited electrons and therefore suppress the X-ray
absorption. However, the UV energy is around 5 eV, so the electronic states for receiving theUV excited electrons should at the higher energy level than the giant pre-edge. The directelectron transition from the O 2p valence band to the in-gap pre-edge state is unlikely tooccur. It may be assumed that the decrease of the pre-edge peak is caused by the de-excitation
process of the UV-excited electrons mediated by the trap state. To find the electronic statesof directly receiving UV excited electrons, the investigation for the electronic states at thehigher energy level is required.
The UV effect on the hump of Ba2 TiSi2 O8 is shown in figure 3-8 (a). The hump is enlarged

41

(a)

(b)

Figure 3-7: UV effect for the intensity of the pre-edge of Ba2 TiSi2 O8 (a). Schematic energy
diagram of the occupancy of electronic states of pre-edge by the X-ray and UV excitedelectrons (b).
and underlined for indicating the variation of the intensity. It shows the intensity of hump
are also decreased with increasing the UV irradiation power. This phenomenon can also be
explained by the occupancy of the electronic states of the pre-edge by the UV excited electrons(figure 3-8 (b)). However, the amplitude of the decreasing intensity for hump is much morethan that of the giant pre-edge, the hump is almost diminishing under the high UV irradiation
power. This gives the proof that the electronic states of hump is directly occupied by the UV
excited electrons. During the photoluminescence process, the electrons in the valence band
was excited to the electronic states of hump by UV irradiation. The electron moves to theelectronic states of pre-edge by the relaxation process. The electronic states of pre-edge act
as in-gap states for producing the visible light emission. So the electronic states of hump act
as electron traps for receiving the UV excited electrons. Therefore, the TiO5 polyhedron is
the the color center of the self-activated titanate phosphors, all the electronic states createdby the TiO5 polyhedron dominate the photoluminescence process.
For comparison, the UV effects observed in the XAS spectra of all three samples Ba2 TiSi2 O8 ,Sr2 TiSi2 O8 and Na2 TiSiO5 are illustrated in Figure 3-9: the top three panels present the

42

(a)

(b)

Figure 3-8: UV effect for the intensity of the hump of Ba2 TiSi2 O8 (a). Schematic energy
diagram of the occupancy of electronic states of hump by the X-ray and UV excited electrons(b).
results for the pre-edge peaks, and the bottom three panels display the results for the humps.
For all three samples, the intensity of the pre-edge peaks decreases slightly with the increasing
UV irradiation; additionally, the hump intensity decreases with the same trend. This indicatesthe same mechanism of photoluminescence for all the three sample.

In addiation, The

electronic states of pre-edge act as in-gap states during the PL process. In addition, relativesmaller intensity of hump in Na2 TiSiO5 indicates the relative smaller electronic density ofelectron traps, which maybe explain the relatively lower emission than that of Ba2 TiSi2 O8 .

43

Figure 3-9: (a–c) show the UV effect for the intensity of Ti K pre-edge peaks for Ba2 TiSi2 O8 ,Sr2 TiSi2 O8 , and Na2 TiSiO5 , respectively. The data of different UV output powders areindicated by lines of different colors. (d–f) show the UV effect for the intensity of hump for
Ba2 TiSi2 O8 , Sr2 TiSi2 O8 , and Na2 TiSiO5 , respectively.

44

3.5

Theoretical calculation of Ti K pre-edge XANES of self-activated titanate
phosphors

Figure 3-10: Theoretical calculation of Ti K-edge XANES of Ba2 TiSi2 O8 . The observed andcalculated data are compared in the first two panels. The LDOS of Ti 3d, Ti 4p, and O 2p
states are shown in the bottom three panels, respectively. The hump and its corresponding
LDOS state are indicated by arrows.
To confirm the energy diagram proposed in the previous section, we calculated the Ti
K-edge XAS results for the three samples using the FDMNES code. As an example, acalculated Ba2 TiSi2 O8 spectrum is compared with the observed spectrum in Figure 3-10,
together with the LDOS of the related orbitals. The shaded LDOS represents the occupied
valence bands. The characteristic features are well-reproduced by the calculations. The
45

general spectral profile is similar to that of the Ti 4p LDOS, which can be simply understoodas the Ti 1s–4p dipole transition. Notably, the pre-edge peak consists of both Ti 4p and Ti
3d states. The 3d LDOS is 10 times larger than the 4p LDOS, and is highly localized in the
pre-edge region. In general, the 3d and 4p states are less-hybridized in the centrosymmetriccoordination environment. By reducing the symmetry, i.e., TiO6 to TiO5 , the centrosymmetryis broke with point group reducing from Oh to C4v , the 3d and 4p belong to the same
irreducible representation A1 in the character table of C4v . Therefore, the 3d and 4p statesof an identical Ti ion start to hybridize with each other [59, 60]. This simulation results
are consistent with the discussion in section 3.2. Thus, it can be concluded that the large
pre-edge primarily originates from the intra-atomic hybridization between the Ti 3d and 4p
states. The contribution of O 2p is less pronounced in this region, which is consistent witha previous study by Farges et al., in which destructive interference from nearest-neighbor
oxygen atoms was reported [56]. The lower degree of hybridization is beneficial for PL because
the UV-excited electrons from the O 2p valence band can survive longer in the unoccupied
conduction band. The small hump is also reproduced in the calculated spectrum, as indicatedby arrows, together with a corresponding component in the Ti 4p LDOS. Thus, the hump canbe assigned to the Ti 4p sub-band.
After the in-gap states are determined by the XAS experiment and the origin of the in-gap
states are illustrated by the theoretical calculation. The reason that Sr2 TiSi2 O8 possesses
different emission color with UV irradiation comparing to Ba2 TiSi2 O8 and Na2 TiSiO5 canbe explained. The normalized pre-edge peaks of the three samples are compared in Figure3-11. The peak position of Sr2 TiSi2 O8 shifts toward lower energy by 0.2 eV compared withthose of Ba2 TiSi2 O8 and Na2 TiSiO5 , which corresponds to the red shift observed in the PL
spectrum of Sr2 TiSi2 O8 (Figure 2-2(a)). On this basis, it may be speculated that the electronic
states of these pre-edge peaks are evidence of the color centers in these self-activated titanate
phosphors.
The different energy position of the pre-edge peaks for Sr2 TiSi2 O8 and Ba2 TiSi2 O8 can be
explained by the 3d orbitals splitting based on the crystal field theory. The energy level of 3dorbitals with octahedral and square pyramidal structure are shown in figure 3-12. As we have46

showed above, 3d orbitals will be split into eg and t2g energy levels with Oh symmetry. Whenthe symmetry is reduced to the D4h point group with the lengthening the distance between
the Ti ion and two apical O ions in the same extent or the shrinking the the distance between
the Ti ion and four equatorial O ions in the same extent, the eg and t2g energy levels will
be further split. Based on the crystal field theory, the potential strength between the ions is
decided by the distance of the ions. Therefore, with the symmetry reduction from Oh to D4h ,the energy level of horizontal orbitals (dx2 −y2 and dxy ) will increase, the The energy levelof vertical orbitals (dxz , dyz , and dz 2 ) will decrease. The point group will further reduced
to C4v with the Ti ion displacement along the vertical direction. If the amplitude of thisdisplacement is big, the bond between the Ti ion and the bottom O will be broken, which
results the formation of the TiO5 square polyhedron. The Ti ion displacement cause theshrinking distance between the Ti ion and the top O (noted by d in figure 3-12). The effect isthat the energy level of horizontal orbitals (dx2 −y2 and dxy ) will decrease, the The energy levelof vertical orbitals (dxz , dyz , and dz 2 ) will increase. Especially, the dz 2 orbital has the whole
vertical direction character, its energy level is easily influenced with the changing of the value
d in the TiO5 polyhedron. The value of d for Ba2 TiSi2 O8 is 1.67 Å, smaller than the value
1.75 Å in Sr2 TiSi2 O8 . The electronic states of Ti K pre-edge in TiO5 -containing materials
mainly originate from dz 2 orbitals. So the difference of energy position for pre-edge peaks of
Ba2 TiSi2 O8 and Sr2 TiSi2 O8 stems from the different bond length d. This result points out a
new way to characterize the Ti ion off-center displacement. In addition, the decreasing d also
increases the orbital hybridization Ti 3d with Ti 4p and O 2p. This is also the origin of theexistence of the giant pre-edge peak in TiO5 -containing materials.

3.6

Energy diagram of self-activated phosphors

Based on the observed and calculated XAS spectra, a schematic energy diagram of PL and
the Ti K-edge X-ray absorption process are illustrated in Figure 3-13. First, an incident X-ray
excites a Ti 1s core electron to the unoccupied 4p state through dipole transition. A small
amount of core electrons is transferred to the Ti 3d states through quadrupole transition, whichis enhanced by the intense 3d LDOS to form a large pre-edge peak. Under UV irradiation, the47

Figure 3-11: Normalized Ti K pre-edge peaks of Ba2 TiSi2 O8 (black solid line), Sr2 TiSi2 O8(red dotted line), and Na2 TiSiO5 (blue dashed line). The energy difference between the
pre-edge peaks is represented by ∆E.

Figure 3-12: The 3d orbitals splitting with the symmetry reduction from Oh , D4h to C4v .

48

valence electrons are excited to the Ti 4p sub-band, followed by non-radiative relaxation to thelower Ti 3d state. The difference in the UV power dependence of the hump and the pre-edge
peak is attributed to the different orbital characteristics of the relevant states. The humporiginates from the Ti 4p orbital, whereas the pre-edge peak arises from the Ti 3d orbital.
The intra-atomic 4p–3d hybridization should be beneficial for PL because it suppresses the
direct recombination of UV-excited electrons with valence holes.

Figure 3-13: Energy diagram for self-activated titanate phosphors. (a) Schematic of X-ray
absorption, (b) X-ray absorption and PL procedure. The competition for occupying theelectronic states above the Fermi level between X-rays and UV light is indicated by the region
in red.

3.7

Conclusion

The mechanism of photoluminescent procedure of self-activated titanate phosphors wasinvestigated by Ti K pre-edge XANES. Two features were observed in the pre-edge region:a large pre-edge peak and a small hump. The influence of UV on these features was studied
by irradiating UV light during the XANES experiment. The intensities of the large pre-edge
peak and hump decreased with UV irradiation. From the perspective of electronic states,
the decrease in the intensity of these features can be explained by the electron occupationof the corresponding electronic states with UV excitation, which explains the participationof these electronic states in the PL process. The relevant electronic states of these features
were assigned using theoretical calculations based on the FDMNES code. The large pre-edge
49

peaks and humps arise from the intra-atomic Ti 3d–4p hybridization and Ti 4p sub-band,
respectively. An energy diagram was proposed to explain the PL procedure. The electronic
states of the hump serve as a trap for UV-excited electrons, and those of the lower energy
pre-edge peak act as a color center for the self-activated titanate phosphors. We believe thatthe energy diagram proposed in this study will provide new insights into the development ofhigh-efficiency rare-earth-free phosphors.

50

4Investigation of local distortion of TiO6 octahedron in SrTiO3
with UV irradiation by EXAFS

4.1

Experimental setup and Configuration

The EXAFS experiments were performed in the beamline 12C of the PF. The spectra
were measured in the transmission mode at 22 K (the lowest temperature available on thebeamline) to 300 K. To accommodate the effective penetration depths of UV and X-ray
radiation, the powder samples must be prepared with a sufficiently low thickness. Considering
this, equivalent SrTiO3 superfine powder was deposited on three Millipore membrane filters.
During the EXAFS experiments, three Millipore membrane filters were paralleled in line
receiving X-ray, and each of them was irradiated by UV light. A schematic of the SrTiO3
sample setup is shown in Figure 4-1. The obtained sample thickness corresponded to anabsorption jump ∆µ=0.7 at the Ti K-edge. An optical fiber with three furcated ends wasused to meet the number of membranes.

Figure 4-1: Schematic diagram of SrTiO3 sample setup

51

Generally, a temperature gradient exists between the sample and the temperature sensor,
which causes experimental errors in terms of temperature. To ensure the consistency oftemperature between the sample and the thermal sensor, the membranes were sandwiched ina good heat conductor (copper) with an opening hole for receiving X-rays (inset of Figure4-2). Low temperature XAS measurements were performed at 22–300 K using a closed-loop
liquid helium cryostat with a heating resistor.
The beamline stage setup is the same as that in the XANES experiment described in
section 3. To facilitate UV light irradiation on the chamber, one side of the Kapton film
window was replaced with SiO2 glass (Figure 4-2).

Figure 4-2: Schematic of the EXAFS experimental setup of BL-12C at the Photon Factory.

52

4.2

XAS of SrTiO3

The normalized SrTiO3 Ti K-edge XAS data at 22 K are shown in Figure 4-3; the detailed
procedure of normalization was explained in the experimental section. The main edge iscaused by the Ti 1s-4p dipole transition. The inset in Figure 4-3 shows the pre-edge featureof SrTiO3 Ti K-edge XAS. The first two small peaks in the pre-edge region are caused by
the 1s-3d quadrupole transition. The interpretation of the pre-edge peaks of Ti K XANES isalready shown in section 1.2. Previous studies have reported that the intensity of eg is related
to the Ti ion displacements [55, 61]. Specifically, Nakajima investigated the UV-induced
one-dimensional vibration of Ti ions in SrTiO3 [62]. Because the XANES is very sensitive
too the local structure environment. Once there is Ti ion displacement along the z direction,
the hybridization of 3d3z 2 −r2 and 4p will be enhanced, resulting the variation of the eg peakin the pre-edge region. The post edge that contains the EXAFS oscillation has not been
comprehensively studied under UV irradiation, because controlling the penetration of UV
light and X-rays is difficult. In this study, superfine SrTiO3 powder was used to improve theUV effect.

Figure 4-3: Normalized Ti K-edge XAS of SrTiO3 at 22 K

53

4.3

EXAFS Data fitting

As discussed in section 1, the purpose of studying SrTiO3 in this thesis is to analyze thelocal structure of SrTiO3 and the variation of the local structure under UV irradiation. To
extract structural information from the EXAFS oscillation, the starting structure model of
SrTiO3 must be built, and the scattering paths used to fit the EXAFS oscillation must be
calculated by FEFF. Although the well-known structures of SrTiO3 are cubic (P3m3) andtetragonal (I4/mcm), these results give only the macroscopic structure of SrTiO3 based on
the results of XRD. The structure in the microscopic scale is quite different from that in themacroscopic scale, which has already been widely observed in many perovskite materials suchas BaTiO3 and PbTiO3 [6, 63].
The background subtractions of the raw XAFS data were performed using the AUTOBK
algorithm, which is embedded in the Athena software [64]. In the post edge, not all thefrequencies belong to the oscillation of EXAFS; low frequency corresponds to non-structural
Fourier components of the background signal. Thus, the hRbkgi parameter was set to 0.6 toreduce the low-frequency Fourier components of the background signal. The normalization
and background removal parameters are shown in Table 4-1.

Table 4-1: Normalization and background removal parameters used in the EXAFS data
extraction.
Edge

E0

Rbkg

Normalization order

Ti K

4978

0.6

3

During the fitting procedure, three structure model are tested: cubic, tetragonal, andrhombohedral.

After comparing the fitting results, the tetragonal model gave the best

statistical fitting results. So the structure model used in this study was a tetragonal model
based on the results of the experiments of total neutron scattering [65]. The lattice parameterused in the FEFF calculation is shown in Table 4-2. It is well-known that the property ofthese calculated scattering paths highly depends on the symmetry of the crystal structure.
54

Table 4-2: The lattice parameter of SrTiO3 used in the FEFF calculation.
Crystal System

Tetragonal

Space Group

P4mm

Cell parameters

a=3.905Å

c=3.93819Å

Atom

x

y

z

Sr

0.00000

0.00000

0.00000Ti

0.50000

0.50000

0.50025O1

0.50000

0.50000

0.00000

O2

0.50000

0.00000

0.50000

In this study, the investigation of the location property of Ti along the z-axis is important.
The atom position z of Ti set at 0.50025 corresponded to the Ti displacement along thez-axis with a value of 0.001Å (figure 4-4). The purpose of setting Ti displacement to thissmall value is to produce all the scattering paths required for the fitting procedure with thetetragonal model. During the fitting procedure, a parameter zTi was assumed to obtain thereal value of Ti displacement along the z-axis. The scattering amplitude Fj (k) and phaseshift ψj (k) of each scattering path were calculated using the FEFF code (version 9.6) using
the exchange-correlation Hedin-Lundqvist potential. The calculations were performed for acluster of radius 9 Å and centered at the Ti atom. The calculations of the cluster potentials
were performed via the muffin-tin self-consistent-field approximation using the radius valueof 7 Å.
Generally, in the microscopic level, the Ti ion displacement in perovskite-type titanates
has two preferable directions: (a) the [001] direction corresponding to the tetragonal PbTiO3[66, 67], and (b) the [111] direction corresponding to the rhombohedral BaTiO3 [6]. In thisthesis, the Ti displacement along the [001] direction is presented, mainly because we aim
to confirm the Ti vibration along the [001] direction under UV irradiation, which has beenreported by Nozawa [15, 42]. The main parameters used in the fitting of the tetragonal model
are listed in Table 4-3. Because of the high correlation between ∆E0 and the phase-relatedparameters, ∆E0 was set to -5.2 eV based on the average value of the first run of fitting for all

55

Figure 4-4:

Crystal structure of SrTiO3 used in the FEFF calculation.

The Ti ion

displacement with zTi=0.001 Å is indicated by an arrow.

Table 4-3: Main assumed parameters used in EXAFS data fitting with the tetragonal model.
Parameter

description

initial valuezTi

Ti ion displacement along [001] direction

0 Å

eta

Volumetric expansion coefficient for SS

0

etaMS

Volumetric expansion coefficient for MS

0

2
σO

MSRD of 6 O in the first shell

0.003

2
σSr

MSRD of 8 Sr in the second shell

0.003

2
σTi

MSRD of 6 Ti in the third shell

0.003

2
σMS

MSRD for the MS paths

0.003

56

data. Furthermore, the value of the amplitude reduction factor S20 due to many-body effects
was set to an estimated value of 0.95. The volumetric expansion coefficient eta was used
due to the thermal expansion with the increasing temperature and the vibration of scattering
atoms in the direction perpendicular to the chemical bonds. The actual distances from thecentral absorbing Ti atom to the scattering atom were the calculated atomic distance of theinitial crystal structure and the deviation in the calculated atomic distance, which was causedby the Ti displacement and the volumetric expansion coefficient eta.
The k range used for the forward Fourier transformation is 2.0–14.5 Å−1 . The curve fitting
was performed using the Fourier transform of the k 2 χ(k) profile in the r space of 0.8–4.1 Å.

4.4

Temperature dependence of Ti K-edge EXAFS spectrum

Experimental k 2 -weighted EXAFS profiles k 2 χ(k) of SrTiO3 at 22–300 K at the Ti K-edge
are shown in Figure 4-5 (a). All the obtained EXAFS k 2 χ(k) signals have a good signal-to-noise
ratio. The signal stops at 15.5 Å−1 because of the appearance of a small Cr K-edge, whichis caused by a small Cr3+ contamination. The wave group from 3–9 Å−1 represents theinterference of the outgoing photoelectron waves from the Ti atom and the scattering waves
from the first shell of six O atoms. The wave group from 9 Å−1 to the end of this data
represents the interference of the outgoing photoelectron waves from the Ti atom and thescattering waves from the second shell of eight Sr atoms and the third shell of six Ti atoms.
Thus, each wave group contains the information about a specific structure. A clear trend ofthese oscillations is the decreasing intensity with the increasing temperature, which is due tothe enhanced thermal vibration at high temperatures.
Figure 4-5 (b) displays the magnitude of the Fourier-transformed k 2 χ(k) profiles |FT

k 2 χ(k)|. The peak in the range of 1.0–2.0 Å−1 corresponds to the six O atoms in the
TiO6 octahedral coordination. The two peaks in the range of 2.5–4.0 Å−1 , which are notwell-separated, correspond to the eight Sr atoms in the corner of the cubic SrTiO3 and thesix Ti atoms at the center of the neighboring cubic SrTiO3 units. It is noticeable that theintensity of all the peaks decrease with the increasing temperature. However, the decrease in57

Figure 4-5: (a) k 2 -weighted experimental EXAFS profiles k 2 χ(k) of SrTiO3 at the Ti K-edge,and (b) their Fourier transforms |FT k 2 χ(k)| from 22 K to 300 K.

58

the amplitude of the first peak is less than that of the second and third peaks, which can be
explained by the bonding nature of Ti ions with the surrounding coordination. The chemicalbond between Ti and O is a covalent bond, and Ti-Sr or Ti-Ti belongs to the ionic bond.
Therefore, the TiO6 octahedra can be considered to be relatively more rigid than the Ti-Ti
and Ti-Sr sublattice [68].
Based on FEFF calculation, the obtained paths show that the first shell signal contains
a single scattering from O coordination. The second peak contains a single scattering from
Sr coordination and the triangle multiple scattering Ti0 -O-O-Ti0 (Ti0 is the absorber). The
third peak contains a single scattering from Ti0 -Ti and various linear multiple scattering
contributions with different Ti0 –O–Ti0 chains.
Because the purpose of this work is to investigate the Ti ion motion, we mainly focus
on the first shell. During the fitting process, the tails of the paths belonging to the second
and third shells sometimes contribute to the first shell; thus, for obtaining a more accurate
value, the second and third shells were also fitted. Analyzing the second and third peaks ismore difficult because of the overlap of the contributions from outer coordination shells andstrong multiple-scattering effects. The fitting of the data at 22 K using the tetragonal modelis shown in Figure 4-6. The solid black line represents the experimental EXAFS profile, andthe dashed red line represents the fitted profiles. The same parameter setting was used for
all the data at different temperatures ranging from 22 K to 300 K. Each fitted profile agreed
well with the corresponding experimental EXAFS profile. The misfit in the range of 0–0.9 Å
was caused by the low-frequency background signal.
The effect of Ti ion displacement zTi along z-axis on temperature dependence is shown in
Figure 4-7. Obvious Ti ion displacements are present over the entire temperature range. So
the distortion of TiO6 is confirmed by the results by using EXAFS. The origin of PL property
can be explained with using the same electronic principle discussed in the last section with
self-activated titanate phosphors.
The observed Ti ion displacements are contradictory to the centrosymmetric cubic andtetragonal structures obtained by XRD. As mentioned earlier, for a specific material, the59Figure 4-6: Magnitude of Fourier transforms |FT k 2 χ(k)| of Ti K-edge EXAFS of SrTiO3 forexperimental data (solid black line) and fitted data (red dashed line)

Figure 4-7: Ti ion displacement zTi along the z-axis, as measured by Ti K-edge EXAFS.

60

macroscopic structure obtained by XRD and the microscopic structure that can be measured
by EXAFS are greatly different. The distinct Ti ion displacement was explained by the
dynamic disorder by using nuclear magnetic resonance spectroscopy [38], which has been
confirmed by many other reports as well [39, 40]. In addition, the measured Ti displacement
explains the existence of the pre-edge feature in the Ti K-edge of SrTiO3 . Based on Vedrinskii’sreport, the area of peak eg A in the pre-edge of Ti K-edge of SrTiO3 is proportional to thesquare of the off-center displacement d [55].
A=

γ 2
d
3

(4-1)

where γ is a constant [6]. If SrTiO3 is perfectly centrosymmetric, the zero value ofoff-center displacement d cannot explain the pre-edge peak. However, in our obtained data,
there are large error bars and irregular assumed values for zTi. This is caused by the high
2 . The MSRD defined by (r − r)2 indicatescorrelation between the parameter zTi and σO

relative vibration between the absorber and scatter, which must be within a physicallyreasonable range of values. During the fitting procedure, the Ti ion displacement caused an
uncertainty of statistical analysis. To obtain a physically reasonable value for each parameter,zTi was set to 0.06 Å; this value was not set arbitrarily. The dynamic disorder of Ti ions has an
2 with the temperature obtained
almost constant value, as indicated by EXAFS results [40]. σO
2 increased with the temperature, indicating that theafter fixing zTi is shown in Figure 4-8. σOvariance in the distance between the absorber Ti0 and the first shell O increased. The EXAFS
2equation indicates that χ(k) monotonically decreases with σ 2 ; therefore, the increasing σOwith temperature explains the decreasing intensity of the EXAFS profiles k 2 χ(k) and the firstshell of |FT k 2 χ(k)|.

4.5

UV dependence of Ti K-edge EXAFS spectrum

The experimental k 2 -weighted Ti K-edge EXAFS profiles k 2 χ(k) of SrTiO3 and themagnitude of their Fourier-transformed |FT k 2 χ(k)| at 22 K with and without UV irradiation
are shown in Figure 4-9. It is noticeable that the intensity of both EXAFS profiles and their
Fourier transforms are damped under UV irradiation. To clarify the reason for this damping,61

2 measured by Ti K-edge EXAFS with
Figure 4-8: MSRD values obtained for Ti-O bond σO

temperature dependence (black squares) and UV effect (red squares)
the fitting with the same parameter set was performed with UV irradiated data. The MSRD
values obtained for UV irradiated data are compared with those of non-UV-irradiated data
in Figure 4-8. It is apparent that the intensity damping in the EXAFS profiles with UV
effect was caused by the increasing MSRD factor, which indicates the increasing varianceof the distance between the Ti ion and O. However, one factor remains to be clarified. In
2 was used for all the six O
the current fitting model, because the same MSRD factor σO

coordinations, the direction responsible for the increasing variance of the distance is unclear.
To find the specific direction that is sensitive to the UV effect, the six degenerate O werereduced to two vertical O ( O11 and O12 )and four planar O2 (Figure 4-4). Because of therelatively compensating distance between Ti-O11 and Ti-O12, they share the same MSRD
2 . For the planar O2, σ 2 is used. The results of fitting for σ 2 and σ 2 are shown
factor σO1O2
O1O2
2 and σ 2 exhibit an increasing trend within Figure 4-10. For the temperature effect, both σO1
O2
2 is large, which may be caused by thetemperature. For the UV effect, the error bar of σO1

correlation with other parameters during the fitting process. The significant increase in thevalue of σ 2 for Ti-O11 is obvious, which indicates the relative movement between the Ti ion

62Figure 4-9: (a) k 2 -weighted experimental EXAFS profiles k 2 χ(k) of SrTiO3 at the Ti K-edge,and (b) their Fourier transforms |FT k 2 χ(k)| at 22 K with and without UV irradiation.

63

Figure 4-10: MSRD values obtained for Ti-O11 bond σ 2 (Ti-O1) and (b) Ti-O2 bond σ 2 (Ti-O2)
measured by Ti K-edge EXAFS with temperature dependence (black markings) and UV effect(red markings)

64

2 does not exhibit a clear increase in its value, and its
and apical O. Under UV irradiation, σO2
2 . Thisvariation trend with temperature is almost the same as that of the six degenerate O σO
2 andillustrates the relatively stable bond length between Ti and planar O. The variation of σO1
2 under UV irradiation indicates the amplitude damping of the Ti K-edge EXAFS profiles
σO2

k 2 χ(k) of SrTiO3 , and its Fourier transforms are caused by the enhanced Ti ion vibration
along the z-axis.

4.6

Conclusion

The local structure of SrTiO3 based on the influence of temperature and UV irradiation
was investigated using Ti K-edge EXAFS. The tetragonal structure model of SrTiO3 with
Ti ion displacement along the z-axis was used in the data fitting procedure. The amplitude
of Ti K-edge EXAFS profiles k 2 χ(k) of SrTiO3 and the magnitude of its Fourier transforms|FT k 2 χ(k)| were damped with the increase in the temperature. The obtained increasing
2 illustrate that these dampings were caused by thesix degenerate MSRD of Ti-O bond σOenhanced thermal vibration. Clear Ti ion displacement was observed in SrTiO3 for the entire
temperature range, which explains the existence of the pre-edge peak of Ti K-edge XANESSrTiO3 . The amplitude of Ti K-edge EXAFS profiles k 2 χ(k) of SrTiO3 and the magnitude of
its Fourier transforms |FT k 2 χ(k)| were also damped under UV irradiation. By reducing thedegeneracy of six O in the TiO6 octahedron to two vertical oxygen and four planar oxygen,
2 and no increase in the value of MSRD σ 2
a significant increase in the value of MSRD σO1
O2

were obtained. This illustrates that the UV-induced damping of k 2 χ(k) and |FT k 2 χ(k)| iscaused by the enhanced Ti ion vibration along the z-axis.

655Conclusions
In this research, two types of self-activated titanate phosphors were investigated, namely,

fresnoite-type materials with TiO5 polyhedron and perovskite titanate SrTiO3 with TiO6octahedra. The electronic states of the fresnoite-type phosphors were analyzed by XANES
to investigate the in-gap states, which are essential for realizing the PL property. The local
structure of the TiO6 octahedra in SrTiO3 was analyzed by EXAFS to investigate the local
structure for creating in-gap states. The UV irradiation and the temperature effect wereconsidered in the experiments. The main results of this research are summarized below:
The Ti K-edge XANES of fresnoite self-activated titanate phosphors were analyzed under
UV irradiation. The features in the pre-edge region were examined in detail to study theelectronic states involved in generating the PL property. Theoretical calculations indicatedthat the observed large pre-edge peak corresponds to the electron transition from the Ti 1score level to the electronic states hybridized with Ti 3d, Ti 4p, and O 2p states. The electronic
states of the hump act as traps for the electrons excited by UV irradiation. The hybridization
between Ti 3d and Ti 4p originated from the TiO5 polyhedron. The electronic states of thelarge pre-edge peak provide in-gap states to emit visible light. It can thus be concluded that
the TiO5 polyhedra play an important role in self-activated titanate phosphors. An energy
diagram of self-activated titanate phosphors was proposed to explain the electron transitionin the PL process.
To investigate the relationship between the distortion of TiO6 octahedron and the PL
property, SrTiO3 was chosen as the target material to study the local structure. The Ti
K-edge EXAFS of SrTiO3 was investigated at 22–300 K under UV irradiation. The EXAFS
data were fitted well by using a tetragonal structure model with Ti ion displacement along
the z-axis. Clear Ti ion displacement with 0.06 Å was obtained in SrTiO3 over the entire
temperature range. It not only confirmed the distortion of TiO6 octahedron in SrTiO3 , butalso explained the existence of pre-edge peaks in Ti K-edge XANES SrTiO3 . The Ti ion
vibration along the z-axis in SrTiO3 was significantly enhanced by UV irradiation. The
increase in the intensity of Ti K pre-edge peak and the dielectric constant of SrTiO3 under66

UV irradiation is attributed to the enhanced Ti ion vibration.
In summary, the existence of TiO5 polyhedron is essential to realize the PL property in
non-ion-doped titanate compounds. The role of the TiO5 polyhedron is to create in-gap states,
which are required for emitting visible light. The TiO5 polyhedron is intrinsically contained infresnoite materials. The distortion of the TiO6 octahedron in SrTiO3 induces the PL property
in SrTiO3 . The relationship between the PL property and the local structure distortionin non-ion-doped titanates opens up a new approach to characterize the titanium–oxygen
polyhedron. Furthermore, the interaction between Ti ion vibration and UV-induced electronictransition in SrTiO3 must be investigated to understand the cause of enhanced Ti ion vibration
under UV irradiation, which will be the avenue of our future research.

67

Acknowledgment
My foremost and big “Thank you!” is to my mentor, Prof. Nobuo Nakajima, who was
greatly patient in teaching me the areas of both daily life in Japan and scientific activities inlab. His guidance not only propelled me to gain a rich knowledge in physics, but also exposed
to me the joy in scientific research, and has motivated me to take up research as a career.
I would like to express my sincere gratitude to Dr. Naoki Ishimatsu, who offered valuablecomments and suggestions for my research, as well as was concerned about my life. The mostimportant thing I learned from them is the rigorous academic attitude, which has not only
helped with my scientific research, but also enlightened me regarding my way of thinking andpersonal behavior in daily life. Their profound knowledge, keen scientific insight, and thespirit for exploring science will form an exemplary model for me that I will follow through mywhole life.
I thank Prof. Yoshihiro Kuroiwa who provided me an opportunity to make an oralpresentation in the Japan-Korea workshop, and provided many beneficial comments on myresearch. I thank all the staff of the Physical Science Office and the Student Support Officeof the Graduate School of Science for helping me with paper works, information sharing, andscholarship recommendation. I would like to acknowledge the great support for the XAFS
experiments from the beamline scientists in beamline 9A and 12C of Photon Factory. Their
excellent instrumental operation helped us greatly with our experiment. I would like to greatly
thank my colleague Doctor Andris Anspoks, who is from the Institute of Solid State Physics,
University of Latvia. He provided me with invaluable help in the EXAFS data analysis.
I would like to thank all members of our laboratory: Mr. S. Ono, Mr. K. Ohshiro, Mr.
Y. Noji, Mr. D. Miyashita, Mr. T. Toriyu, Mr. S. Kato, Ms. M. Kousa, and Mr. S. Iwasaki,
Mr. A. Kumagae, Mr. K. Kuramochi, Mr. S. Kanamori, Mr. K. Hiromori, Mr. K. Ishimoto,
Mr. Y. Kubo, Mr. H. Sakano, and Mr. Y. Endo. As an overseas student, with your help
and friendship, I quickly adapted to the environment of learning and living in Japan, andmaintained well with my research work. I greatly enjoyed getting along with you, and will
never forget the happy times we worked and played together. I must thank Doctor Cong Lu,

68

who is a previous overseas student and another member of our laboratory, and coincidentally
graduated from our lab by the time I arrived. He generously shared his experience withresearch in our lab and the life of staying in Japan.
I express my gratitude to the scholarship organizations JASSO and YAHATA for providing
scholarships for my science works. I thank my best friends Yanning Wang and Jianye Sunvery much. Exchanging scientific knowledge and experience with Yanning is something I enjoy
doing. Since the times of high school up to the present, Jianye and I have always shared ourthoughts about studying, growing, and the philosophy of human life. Every time I have been
confused and depressed, he listened to my thoughts and encouraged me greatly.
To my father Baoyun Fan, mother Jinhui Li, brother Dongcan Fan, sister-in-law ShuyuanSong, my cutest nephew Songjia Fan, and niece Jiani Fan: I love you all very much. Although
I was born in a farmer’s family without adequate income and my parents have not received
high levels of education, they have ever supported me with all their might and educated me
through all these years. My brother Dongcan gave up his educational opportunity very early
to make money to support our family. The birth of my nephew and niece has brought endlessjoy and pleasure to our family.
June 2020
Dongxiao F an

69

References[1] A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
[2] Y. Kuroiwa, S. Aoyagi, A. Sawada, J. Harada, E. Nishibori, M. Takata and M. Sakata,Physical Review Letters, 2001, 87, 217601.
[3] J. Ding, Y. Li, Q. Wu, Q. Long, Y. Wang and Y. Wang, RSC Advances, 2016, 6,
8605–8611.
[4] J. M. Edge, Y. Kedem, U. Aschauer, N. A. Spaldin and A. V. Balatsky, Physical Review
Letters, 2015, 115, 247002.
[5] C. C. Sun, Journal of Pharmaceutical Sciences, 2009, 98, 1671–1687.
[6] B. Ravel, E. Stern, R. Vedrinskii and V. Kraizman, Ferroelectrics, 1998, 206, 407–430.
[7] J. J. Manappallil, Basic dental materials, JP Medical Ltd, 2015.
[8] S. Calvin, XAFS for Everyone, CRC press, 2013.
[9] G. Bunker, Introduction to XAFS: a practical guide to X-ray absorption fine structure
spectroscopy, Cambridge University Press, 2010.
[10] A. J. McSloy, I. Trussov, A. Jarvis, D. J. Cooke, P. R. Slater and P. M. Panchmatia, The
Journal of Physical Chemistry C, 2018, 122, 1061–1069.
[11] J. Gopalakrishnan, K. Ramesha, K. K. Rangan and S. Pandey, Journal of Solid State
Chemistry, 1999, 148, 75–80.
[12] S. Sugano, Multiplets of transition-metal ions in crystals, Elsevier, 2012.
[13] P. Pizani, E. Leite, F. Pontes, E. Paris, J. Rangel, E. Lee, E. Longo, P. Delega and J. A.
Varela, Applied Physics Letters, 2000, 77, 824–826.
[14] S. Mochizuki, F. Fujishiro and S. Minami, Journal of Physics: Condensed Matter, 2005,
17, 923.
[15] S. Nozawa, T. Iwazumi and H. Osawa, Physical Review B, 2005, 72, 121101.
70

[16] Y. Li, M. Gecevicius and J. Qiu, Chemical Society Reviews, 2016, 45, 2090–2136.
[17] Y. Huang, T. Tsuboi and H. J. Seo, Ceramics International, 2013, 39, 861–864.
[18] X. Liu and J. Qiu, Chemical Society Reviews, 2015, 44, 8714–8746.
[19] D. Harrison, N. Melamed and E. Subbarao, Journal of the Electrochemical Society, 1963,
110, 23–28.
[20] E. Orhan, J. A. Varela, A. Zenatti, M. Gurgel, F. Pontes, E. Leite, E. Longo, P. Pizani,
A. Beltran and J. Andres, Physical Review B, 2005, 71, 085113.
[21] J. Woicik, E. L. Shirley, C. Hellberg, K. Andersen, S. Sambasivan, D. Fischer,
B. Chapman, E. Stern, P. Ryan, D. Ederer et al., Physical Review B, 2007, 75, 140103.
[22] G. Blasse, Journal of Inorganic and Nuclear Chemistry, 1979, 41, 639–641.
[23] Z.-y. Mao and Y.-c. Zhu, Materials Research Bulletin, 2014, 51, 197–201.
[24] D. Fan, N. Nakajima and S. Kato, Journal of Physics: Condensed Matter, 2020.
[25] T. Hasegawa, S. W. Kim, S. Kamei, T. Ishigaki, K. Uematsu, K. Ohmi, K. Toda and
M. Sato, Dalton Transactions, 2016, 45, 11554–11559.
[26] B. Bouma and G. Blasse, Journal of Physics and Chemistry of Solids, 1995, 56, 261–265.
[27] Q. Wu and Z. Hu, Journal of the American Ceramic Society, 2019, 102, 2727–2736.
[28] P. Fleury, J. Scott and J. Worlock, Physical Review Letters, 1968, 21, 16.
[29] H. Ohta, S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, Y. Nakanishi,
Y. Ikuhara, M. Hirano et al., Nature Materials, 2007, 6, 129–134.
[30] K. Ahadi, L. Galletti, Y. Li, S. Salmani-Rezaie, W. Wu and S. Stemmer, Science
Advances, 2019, 5, eaaw0120.
[31] J. Hemberger, M. Nicklas, R. Viana, P. Lunkenheimer, A. Loidl and R. Böhmer, Journalof Physics: Condensed Matter, 1996, 8, 4673.
[32] U. Aschauer and N. A. Spaldin, Journal of Physics: Condensed Matter, 2014, 26, 122203.
71

[33] V. Lemanov, E. Smirnova, P. Syrnikov and E. Tarakanov, Physical Review B, 1996, 54,3151.
[34] M. Itoh, R. Wang, Y. Inaguma, T. Yamaguchi, Y. Shan and T. Nakamura, PhysicalReview Letters, 1999, 82, 3540.
[35] P. Zubko, G. Catalan, A. Buckley, P. Welche and J. Scott, Physical Review Letters, 2007,
99, 167601.
[36] D. Lee, H. Lu, Y. Gu, S.-Y. Choi, S.-D. Li, S. Ryu, T. Paudel, K. Song, E. Mikheev,
S. Lee et al., Science, 2015, 349, 1314–1317.
[37] X. Li, T. Qiu, J. Zhang, E. Baldini, J. Lu, A. M. Rappe and K. A. Nelson, Science, 2019,
364, 1079–1082.
[38] B. Zalar, A. Lebar, J. Seliger, R. Blinc, V. V. Laguta and M. Itoh, Physical Review B,
2005, 71, 064107.
[39] A. I. Frenkel, D. Ehre, V. Lyahovitskaya, L. Kanner, E. Wachtel and I. Lubomirsky,
Physical Review Letters, 2007, 99, 215502.
[40] A. Kodre, I. Arčon, J. P. Gomilšek and B. Zalar, AIP Conference Proceedings, 2007, pp.
481–483.
[41] M. Takesada, T. Yagi, M. Itoh and S.-y. Koshihara, Journal of the Physical Society ofJapan, 2003, 72, 37–40.
[42] S. Nozawa, T. Iwazumi, H. Osawa and T. Uozumi, Applied Physics Express, 2013, 6,061501.
[43] P. B. MOORE and S. J. Louisnathan, Zeitschrift für Kristallographie-Crystalline
Materials, 1969, 130, 438–448.
[44] T. Höche, W. Neumann, S. Esmaeilzadeh, R. Uecker, M. Lentzen and C. Rüssel, Journalof Solid State Chemistry, 2002, 166, 15–23.
[45] P. Willmott, An introduction to synchrotron radiation: techniques and applications, John
Wiley & Sons, 2019.
72

[46] M. Benfatto and C. Meneghini, Synchrotron Radiation, Springer, 2015, pp. 213–240.
[47] M. Nomura and A. Koyama, Design and performance of a new XAFS beamline at thephoton factory; BL-12C, National lab. for high energy physics technical report, 1996.
[48] M. Newville, Reviews in Mineralogy and Geochemistry, 2014, 78, 33–74.
[49] O. Bunău and Y. Joly, Journal of Physics: Condensed Matter, 2009, 21, 345501.
[50] J. A. Van Bokhoven and C. Lamberti, X-ray absorption and X-ray emission spectroscopy:
theory and applications, John Wiley & Sons, 2016, vol. 1.
[51] J. J. Rehr, J. J. Kas, F. D. Vila, M. P. Prange and K. Jorissen, Physical Chemistry
Chemical Physics, 2010, 12, 5503–5513.
[52] B. Ravel and M. Newville, Journal of Synchrotron Radiation, 2005, 12, 537–541.
[53] M. Newville, Journal of Synchrotron Radiation, 2001, 8, 322–324.
[54] E. Stern, M. Newville, B. Ravel, Y. Yacoby and D. Haskel, Physica B, 1995, 208, 117–117.
[55] R. Vedrinskii, V. Kraizman, A. Novakovich, P. V. Demekhin and S. Urazhdin, Journalof Physics: Condensed Matter, 1998, 10, 9561.
[56] F. Farges, G. E. Brown, J. Rehr et al., Physical Review B, 1997, 56, 1809.
[57] C. Lu, N. Nakajima and H. Maruyama, Journal of Physics: Condensed Matter, 2016,
29, 045702.
[58] N. Nakajima, M. Deguchi, H. Maruyama, K. Ishiji and Y. Tezuka, Japanese Journal ofApplied Physics, 2010, 49, 09ME04.
[59] J.-H. Lee, W.-J. Lee, S.-H. Lee, S. M. Kim, S. Kim and H. M. Jang, Physical Chemistry
Chemical Physics, 2015, 17, 7857–7863.
[60] T. Learmonth, P. Glans, J. Guo, M. Greenblatt and K. Smith, Journal of Physics:
Condensed Matter, 2009, 22, 025504.
[61] M. Deguchi, N. Nakajima, K. Kawakami, N. Ishimatsu, H. Maruyama, C. Moriyoshi,
Y. Kuroiwa, S. Nozawa, K. Ishiji and T. Iwazumi, Physical Review B, 2008, 78, 073103.
73

[62] S. Kawakami, N. Nakajima, M. Nakatake, N. Kawamura, M. Mizumaki and
H. Maruyama, Japanese Journal of Applied Physics, 2015, 54, 10NC03.
[63] B. Ravel and E. Stern, Physica B: Condensed Matter, 1995, 208, 316–318.
[64] M. Newville, P. Lı̄viņš, s. Y. Yacoby, J. Rehr and E. Stern, Physical Review B, 1993, 47,
14126.
[65] Q. Hui, M. G. Tucker, M. T. Dove, S. A. Wells and D. A. Keen, Journal of Physics:
Condensed Matter, 2005, 17, S111.
[66] N. Sicron, B. Ravel, Y. Yacoby, E. A. Stern, F. Dogan and J. J. Rehr, Physical Review
B, 1994, 50, 13168.
[67] B. Ravel, N. Slcron, Y. Yacoby, E. A. Stern, F. Dogan and J. J. Rehr, Ferroelectrics,
1995, 164, 265–277.
[68] A. Anspoks, D. Bocharov, J. Purans, F. Rocca, A. Sarakovskis, V. Trepakov, A. Dejneka
and M. Itoh, Physica Scripta, 2014, 89, 044002.

74