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28th International Conference on Low Temperature Physics (LT28)
IOP Conf. Series: Journal of Physics: Conf. Series
969 (2018)1234567890‘’“”012122

IOP Publishingdoi:10.1088/1742-6596/969/1/012122

Magnetic, thermodynamic and optical properties ofSb-substituted Ba2PrBiO6 double perovskite oxides
K. Onodera, T. Kogawa, M. Matsukawa, H. Taniguchi, K. Nishidate
Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
E-mail: matsukawa@iwate-u.ac.jp

A. Matsushita, M. Shimoda
National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan
Abstract. We demonstrated crystal structures, magnetic, thermodynamic and opticalproperties of the B-site substituted perovskite oxides Ba2 Pr(Bi1−x ,Sbx ) O6 (x=0, 0.1 and0.2). Polycrystalline samples of Sb-substituted Ba2 PrBiO6 were prepared with the conventional
solid-state reaction technique. The X-ray diﬀraction data revealed that the polycrystallinesamples are an almost single phase with a monoclinic structure (C2/m). Substitution ofsmaller Sb ion at Bi site causes a monotonic decrease in both the lattice parameters and volume.
Magnetization measurements at high temperatures above 200 K show that the eﬀective magnetic
moment is estimated to be around 3.15 µB , which is close to that for Pr3+ ion. The X-ray
photoemission spectroscopy analysis revealed that a prominent peak of Pr3+ is dominant witha smaller shoulder structure of Pr4+ . A Schottky-like anomaly observed in the low-temperaturespeciﬁc heat measurement is explained by low-lying splitting of Pr ions under the crystal ﬁeld
eﬀect. Optical spectra were measured using a diﬀuse-reﬂectance method. The band gaps were
estimated from the optical data to be 0.977 eV and 1.073 eV, at x=0 and 0.2, respectively. The
eﬀect of band gap opening due to Sb substitution is examined by using the density functional
theory.

1. Introduction
 
A large number of double perovskite oxides A2 B B O6 have been widely studied due totheir attractive physical properties and potential applications[1, 2]. Some of semiconducting 
A2 B B O6 compounds exhibit photocatalytic properties such as hydrogen generation by watersplitting and are taken as alternative materials for TiO2 oxide [3]. In particular, Ba2 PrBiO6compound has been shown to possess high photocatalytic activity, which is probably related
to the valence mixing[4, 5]. A previous study on the magnetic states of Ba2 PrBiO6 compoundsuggests an anomalous valence situation for Pr ions.[6] The complicated ground states of rareearth ions such as Pr ion under the crystal ﬁeld eﬀect remain open question, not only in thephysical properties of the double perovskite compound, but also in the view point of physics of4f electron systems.

There are signiﬁcant factors such as charge and size diﬀerences between B and B sites, todetermine the B-site ordering of the double perovskite oxide[7]. Increase in lattice strain and/or
increase in the electrostatic repulsion overcome the entropy contribution toward disordering, 3+  5+
B O6 composition, the B sites tend to odercausing the alternate arrangement. For A2+2 B

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28th International Conference on Low Temperature Physics (LT28)
IOP Conf. Series: Journal of Physics: Conf. Series
969 (2018)1234567890‘’“”012122

IOP Publishingdoi:10.1088/1742-6596/969/1/012122

with increasing the ion size diﬀerence rB = rB  − rB  . If rB > 0.2 Å, it is well known thatthe B sites of the compounds are almost ordered alternately in the all crystallographic axes[7].
3+ 5+In Ba2+2 Pr Bi O6 compound, the B-site ionic radius diﬀerence rB = 0.23 Å meets the above
condition, suggesting the B-site ordering. (rB  (Pr3+ )=0.99 Å and rB  (Bi5+ )=0.76 Å.[8] ) The
density functional theory based calculation reveals the band structure and its associated orbitalstates of this compound, and predicts the stability of crystal structures of B site ordered states.
In this paper, we investigate crystal structures, magnetic, thermodynamic and opticalproperties of the Ba2 Pr(Bi1−x ,Sbx ) O6 compounds (x=0, 0.1 and 0.2), to determine the physicalproperties of B-site substituted double perovskite oxide. For comparison, the physical properties
of the B site substituted Ba2 LaBiO6 compound without magnetic ions are examined.
4000
400
Intensity (arb. unit)

Intensity

3000
2500
2000
1500

16

1718

19 20
2θ (deg.)

2122

1000
500

250
200(b)
6.24050 60
2θ (deg.)abc/√270

10080 90V50(b)

333

6.16

331.5
331(Å3)332

Volume332.5
6.18

6.14

0

333.5

100

150 200
T(K)Pr 3d3/2

Intensity (arb. units)

20 30

6.22
lattice parameters (Å)

300

Sb0%
Sb10%
Sb20%

1500106.12(a)350(101)

χ-1 (molOe/emu)

(a)

3500

250

300

Pr 3d5/2

330.5
051015
Sb content (%)

970

330
20Figure 1. (a)X-ray diﬀraction patternsof the parent Ba2 PrBiO6 compound.
The (101) peak in the inset corresponds
to one of typical Miller indexes of Bsite ordered double-perovskite structure.
The calculated curve was obtained byusing the lattice parameters. (b)Thelattice parameters and volume as afunction of Sb content.

960

950
940
930
Binding Energy (eV)920

Figure 2. (a)Inverse magnetic susceptibilities (χ−1 ) for Ba2 Pr(Bi1−x ,Sbx )O6(x=0, 0.1 and 0.2). (b)the XPS spectrumof Pr 3d core level emission for the parent sample. The dashed line corresponds
to a peak position in the Pr3+ standard
Pr2 O3 spectrum. The arrow represents
the Pr4+ standard peak position.

2. Experiment
Polycrystalline samples of Sb-substituted Ba2 PrBiO6 were prepared with the conventional solidstate reaction technique. Stoichiometric mixtures of BaCO3 , Pr6 O11 , Bi2 O3 , and Sb2 O3 powders
were ground, sintered in air at 800-900 ◦ C for 2 days with intermediate grindings, and pressed intopellets. The pellet samples were annealed in air at 1000 ◦ C for 4 days. The X-ray diﬀraction data
revealed that the polycrystalline samples are an almost single phase with a monoclinic structure228th International Conference on Low Temperature Physics (LT28)
IOP Conf. Series: Journal of Physics: Conf. Series
969 (2018)1234567890‘’“”012122

IOP Publishingdoi:10.1088/1742-6596/969/1/012122(C2/m). For the parent Ba2 PrBiO6 , the lattice parameters are a = 6.2038Å, b = 6.1689Å,c = 8.7011Åand β = 89.7303◦ , which are in fair agreement with those reported by a previous
study[6]. The emergence of (101) reﬂection indicates B-cation ordering which is characteristicof B-site ordered double-perovskite structure. Substitution of smaller Sb5+ (0.60 Å) ion atBi5+ (0.76 Å) site causes a monotonic decrease in both the lattice parameters and volume asshown in Fig.1 (b). The dc magnetization was conducted at a magnetic ﬁeld of 1 T under thezero-ﬁeld cooling process using a superconducting quantum interference device magnetometer(MPMS,Quantum Design). X-ray photoemission spectroscopy (XPS) measurements for Pr 3d
core-level emission were performed in a VG ESCALAB MkII spectrometer by using MgKα Xray irradiation (1254 eV). Speciﬁc heat measurements were conducted by a thermal relaxation
method using a physical property measuring system (PPMS,Quantum Design). Optical spectra
were measured by using a diﬀuse-reﬂectance method with a spectrophotometer (Hitachi U-3500)
and BaSO4 was used as the reference material. Optical band gaps for the powder samples wereevaluated from reﬂectance spectral data using the conventional Kubelka-Munk functions[6, 9]. We calculated the band structure of Ba2 PrBiO6 by using the density functional theory as
implemented in the code VASP.
3. Results and Discussion
The inverse magnetic susceptibilities (χ−1 )for Ba2 Pr(Bi1−x ,Sbx )O6 (x=0, 0.1 and 0.2) are shownin Fig.2(a) as a function of temperature at a magnetic ﬁeld of 1 T. In the Ba2 LaBiO6 compound,no magnetization signal was observed because of the absence of magnetic ions. From themagnetization data at high temperatures above 200 K, we estimate the eﬀective magnetic
moment according to the Curie-Weiss law. The value of eﬀective magnetic moment μef f isevaluated by using the following formula,
C = N μef2f μB2 /3kB
where C, N , and μB denote the Curie constant, the number of magnetic atom per mol, and theBohr magneton, respectively. Performing the calculation, we obtained that μef f = 3.15 μB and3.12 μB for the parent and Sb20% samples, respectively. These values are close to the eﬀective
magnetic moment for the Pr3+ free ion (3.57μB ). However, these ﬁndings do not exclude thepresence of Pr4+ [5]. The XPS spectrum of Pr 3d core level for the parent sample is shown as afunction of the binding energy in Fig 2 (b). The XPS proﬁle strongly suggests that a prominent
peak of Pr3+ is dominant at the speciﬁed binding energy values around 933 eV, followed bya smaller shoulder structure of Pr4+ around 935 eV. The dashed line corresponds to a peak
position in the Pr3+ standard Pr2 O3 spectrum. The arrow points to the Pr4+ standard PrO2spectrum.
In Fig. 3(a), speciﬁc heats of the parent Ba2 PrBiO6 and its related Ba2 LaBiO6 compounds
are shown as a function of temperature between 2 and 300 K. In addition, we plot in Fig. 3(b)low-temperature speciﬁc heat data of both the samples, C/T (Pr) and C/T (La). Applying theDebye T 3 law to the low temperature data, we estimate the Debye temperature, θ=112 K for
Ba2 LaBiO6 . We assume that the lattice component in C/T of the parent sample is almost
equivalent to the values of C/T of the nonmagnetic Ba2 LaBiO6 compound since both samples
share the same crystal structure. Thus, speciﬁc heat diﬀerences dC/T between both compounds
are taken as non lattice components of the parent sample. A Schottky-type speciﬁc heat withtwo energy levels [10] is given by
CSch = kB (x/2)2 /cosh2 (x/2) with x = E/kB T.
When the crystal ﬁeld splitting between the ground and low-lying excited states of this compoundis set to be E =1.5 meV, we obtain a good agreement between the calculated curve and3

28th International Conference on Low Temperature Physics (LT28)
IOP Conf. Series: Journal of Physics: Conf. Series
969 (2018)1234567890‘’“”012122

IOP Publishingdoi:10.1088/1742-6596/969/1/012122

experimental data. Accordingly, we attribute the observed broad peak in C/T to the Schottkytype speciﬁc heat anomaly.

Reflectance (arb. units)

1500(a)C (J/Kmol)

400C(La)
C(Pr)

300
200

0.60.401000 1500 2000
wave length (nm)2500

100150 200
T (K)250(b)KM ph1.2(b)

300ε )1/2 (a.u.)

50

10.8
C/T (Pr)
C/T (La)
dC/T
C/T sch

0.6
0.4
0.2
05101520T(K)

25

432

Sb0%
Sb10%
Sb20%
B'=La(αC/T (J/K2mol)

500

501.4

0

Sb0%
Sb10%
Sb20%
B'=La

0.2

1000(a)

0.8

100.5

11.522.5
photon energy (eV)330

Figure 3.
(a)Speciﬁc heats ofthe parent Ba2 PrBiO6 and its related Ba2 LaBiO6 compounds, C(Pr) andC(La). (b) Low temperature speciﬁcheats of both samples, C/T (Pr) andC/T (La). Speciﬁc heat diﬀerences dC/T
in both compounds are also plotted.
Dashed curves represent the calculated
Schottky heat anomaly with E =1.5
meV.

Figure 4. Optical measurements on
Ba2 Pr(Bi1−x ,Sbx )O6 (x=0, 0.1 and 0.2)
powder samples. (a)Diﬀuse reﬂectancespectra as a function of wave length(b) Kubelka-Munk conversion of diﬀuse
reﬂectance spectra as a function ofphoton energy. For the x=0 and x=0.2samples, we evaluate Eg = 0.977 ± 0.023
eV and 1.073 ± 0.014 eV, respectively.
Straight lines denote the extrapolations
to estimate the band gaps using theleast squares method. For comparison,
we exhibit the optical data of theBa2 LaBiO6 compound, giving Eg =1.767 ± 0.028 eV.

For Ba2 Pr(Bi1−x ,Sbx )O6 (x=0, 0.1 and 0.2) powder samples, the diﬀuse reﬂectance spectra
are shown in Fig.4 (a) as a function of wave length. First, the observed reﬂectance data for thepowder samples are transformed to the absorption coeﬃcient αKM by using the conventional
Kubelka-Munk function. Figure 4 (b) shows the square root of the absorption coeﬃcient(αKM εp )1/2 as a function of photon energy εp . For the Kubelka-Munk conversion data near
the band edge, we extrapolate the tangent line to the εp axis and evaluate the optical band gaps
from the intersection according to the equation of(αKM εp )1/2 ∝ (εp − Eg )428th International Conference on Low Temperature Physics (LT28)
IOP Conf. Series: Journal of Physics: Conf. Series
969 (2018)1234567890‘’“”012122

IOP Publishingdoi:10.1088/1742-6596/969/1/012122, where αKM , εp , and Eg are the absorption coeﬃcient, photon energy, and band gap [4, 5]. We
estimate the values of band gaps to be Eg = 0.977 ± 0.023 eV at x = 0, 1.003 ± 0.025 eV at
x = 0.1 and 1.073 ± 0.014 eV at x = 0.2. For comparison, we also exhibit the optical data of theBa2 LaBiO6 compound, resulting in Eg = 1.767 ± 0.028 eV. All band gaps are evaluated withinerrors of plus or minus several percent.
The eﬀect of the band gap opening by the atomic substitution is supported by our ﬁrstprinciples electric structure calculation. We have calculated the band structure of Ba2 PrBiO6by using the density functional theory as implemented in the code VASP. The estimated bandgap energy of the parent Ba2 PrBiO6 is around 0.5 eV as shown in Fig5 (a) , leading to anunderestimation in comparison to the experimental value. However, the band gap of Ba2 PrSbO6
becomes 4.1 eV when we substitute Sb for Bi (Fig5 (b)). In addition, we obtain Eg =2.5 eV for theBa2 LaBiO6 where B site is replaced by La (not shown here). These results probably predict thequalitative trends than the absolute values. The detailed analysis will be published elsewhere.

Energy (eV)(a) Ba2PrBiO6

DOS (a.u.)

Energy (eV)(b) Ba2PrSbO6

DOS (a.u.)(a.u.)
DOSFigure 5. Energy band structures and total density of states (DOS) calculated by the density
functional theory for (a) Ba2 PrBiO6 and (b) Ba2 PrSbO6 compounds. The band gaps are
estimated to to be ∼ 0.5 eV and 4.1 eV, for the parent Ba2 PrBiO6 and Sb substituted Ba2 PrSbO6compounds, respectively.

528th International Conference on Low Temperature Physics (LT28)
IOP Conf. Series: Journal of Physics: Conf. Series
969 (2018)1234567890‘’“”012122

IOP Publishingdoi:10.1088/1742-6596/969/1/012122

4. Summary
We demonstrated crystal structures, magnetic, thermodynamic and optical properties of theB-site substituted perovskite oxides Ba2 Pr(Bi1−x ,Sbx ) O6 (x=0, 0.1 and 0.2). X-ray diﬀraction
data point to the B-site ordered double-perovskite structure, which is consist with previous
predictions based on the charge and ionic size diﬀerences. The magnetization and X-ray
photoemission spectroscopy measurements considerably indicate the existence of Pr3+ , but do
not exclude the presence of Pr4+ . The low temperature speciﬁc heat of the parent compound iswell described by the Schottky heat anomaly due to crystal ﬁeld splitting of Pr ion with E =1.5
meV. The values of the band gap are evaluated to be 0.977±0.023 eV and 1.073±0.014 eV at theparent and Sb20% substituted samples, by using the conventional Kubelka-Munk function. Theeﬀect of band gap opening due to Sb substitution was examined by using the density functional
theory.
5. Acknowledgment
The authors are grateful for Dr. K. Nonaka for his assistance in our experiments.
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Cer. Soc. Jpn. 118 91[5] Matsushita A, Nakane T, Naka T, Isago H, Yamada Y and Yamada Yuh 2012 Jpn. J. Appl. Phys. 51 121802[6] Harrison W T A, Reis K P, Jacobson A J, Schneemeyer L F and Waszczak J V 1995 Chem. Mater. 7 2161
[7] Vasala S and Karppinen M 2015 Prog. Solid State Chem. 43 1
[8] Shannon R 1976 Acta. Crystallogr. A32 751[9] Tang J W, Zou Z G and Ye J H 2007 J. Phys. Chem. C111 12779[10] Kittel C 1976 Introduction to Solid State Physics sixth edition (New York: Wiley)6