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Magnetic States and Bandgaps of B-Site Substituted
Double-Perovskite Ba2Pr(Bi, Sb)O6
H. Taniguchi

1 , M. Matsukawa

1 , K. Onodera1 , K. Nishidate1 , and A. Matsushita2

1 Faculty of Science and Engineering, Iwate University, Morioka 020-8551, Japan
2 National Institute for Materials Science, Ibaraki 305-0047, JapanWe demonstrated crystal structures, magnetic, and optical properties of the B-site substituted double-perovskite Ba2 Pr(Bi1−x Sb x )O6 (x = 0, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5, and 1.0). The single-phase polycrystalline samples with the light Sb substitution are formed
in a monoclinic structure (C 2 /m). The heavily Sb-substituted samples crystallize in the rhombohedral system accompanied by
B-site disorder. Magnetization measurements show that the effective magnetic moments are located around 3.2 µ B for the lightly
Sb-substituted samples, indicating the valence mixing between Pr3+ and Pr4+ . The magnetic moments with the heavy Sb substitution
are close to the valence of Pr3+ . The magnitudes of bandgap energy for the two end-member samples were estimated from the opticalmeasurements to be E g = 0.977 eV at x = 0 and 2.395 eV at x = 1.0. The effect of the bandgap opening due to Sb substitution
is examined by using the density functional theory. The lightly and heavily Sb-doped compounds show typical absorption edges inindirect and direct semiconductors, respectively. These findings are qualitatively consistent with the calculated results.
Index Terms— Density functional theory, magnetic semiconductors.

I. I NTRODUCTIONA

LARGE number of double-perovskite oxides A2 B B O6

have been widely studied due to their attractive physicalproperties and potential applications [1], [2]. Some of thesemiconducting A2 B B O6 compounds exhibit photocatalytic
properties such as hydrogen generation by water splitting andare taken as alternative materials for TiO2 oxide [3], [4].
In particular, Ba2 PrBiO6 compound has been shown to possesshigh photocatalytic activity, which is probably related to thevalence mixing [5], [6]. A previous study on the magnetic
states of Ba2 PrBiO6 compound suggests an anomalous valence
situation for Pr ions [7]. The complicated ground states ofrare earth ions such as Pr ion under the crystal field effectremain an open question, not only in the physical propertiesof the double-perovskite compound but also in the viewpointof physics of 4 f electron systems.
There are significant factors such as charge and size differences between B and B sites to determine the B-site orderingof the double-perovskite oxide [8]. Increase in lattice strain
and/or increase in the electrostatic repulsion overcome theentropy contribution toward disordering, causing the alternate
3+ B5+ O composition, the B-sites
arrangement. For A2+
6
2 B
tend to order with increasing the ion size difference r B =r B  − r B  . If r B > 0.2 Å, it is well known that the Bsites of the compounds are almost ordered alternately in all
3+ 5+
the crystallographic axes [8]. In Ba2+
2 Pr Bi O6 compound,the B-site ionic radius difference r B = 0.23 Å meets theabove-mentioned condition, suggesting the B-site ordering(r B  (Pr3+ ) = 0.99 Å and r B  (Bi5+ ) = 0.76 Å) [9].
The density functional theory-based calculation reveals
the band structure and its associated orbital states of thisManuscript received June 5, 2018; accepted September 27, 2018.
Corresponding author: M. Matsukawa (e-mail: matsukawa@iwate-u.ac.jp).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2018.2874431

compound and predicts the stability of crystal structures of
B-site ordered states [10], [11].
In this paper, we report X-ray diffraction measurements,
magnetic susceptibilities, and diffuse reflectance spectra of
the Ba2 Pr(Bi1−x , Sbx ) O6 compounds, to determine crystal
structures, magnetic, and optical properties of B-site substituted double-perovskite oxide. In [13], we have focused on
the influence of light Sb substitution on crystal structures,
magnetic, and optical properties in Ba2 Pr(Bi1−x , Sbx ) O6 . It isof much interest to examine the B-site ordering of the doubleperovskite oxides by substituting the smaller Sb5+ ion for the
Bi5+ site, where the former and latter ions have the closed
shell of 4d 10 and 5d 10 .
II. E XPERIMENT
Polycrystalline samples of Sb-substituted Ba2 PrBiO6 wereprepared with the conventional solid-state reaction technique.
The stoichiometric mixtures of BaCO3 , Pr6 O11 , Bi2 O3 , andSb2 O3 powders were ground, sintered in air at 800 ◦ C–900 ◦ C
for 2 days with intermediate grindings, and pressed into
pellets. The pellet samples were annealed in air at 1000 ◦ C
for 4 days. We performed X-ray diffraction measurements atroom temperature with an Ultima IV diffractometer (Rigaku)
using Cu-Kα radiation.
The dc magnetization was conducted at a magnetic field of
1 T under the zero-field cooling process using a superconducting quantum interference device magnetometer (Magnetic
Property Measurement System, Quantum Design). Optical
spectra were measured by using a diffuse reflectance methodwith a spectrophotometer (Hitachi U-3500) and BaSO4 wasused as the reference material. The optical bandgaps for the
powder samples were evaluated from reflectance spectral datausing the conventional Kubelka–Munk functions [7], [12].
We calculated the band structures of monoclinic Ba2 PrBiO6
and rhombohedral Ba2 PrSbO6 by using the density functional

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Fig. 1. (color online) X-ray diffraction patterns for (a) Ba2 PrBi0.8 Sb0.2 O6and (b) Ba2 PrBi0.5 Sb0.5 O6 . Insets of (a) and (b) display the enlarged
diffraction profiles and the lattice parameters as a function of Sb content.
The calculated profiles based on the monoclinic and rhombohedral structure
models are shown in (a) and (b), respectively.

theory as implemented in the code Vienna Ab initio Simulation
Package (VASP).
III. R ESULTS AND D ISCUSSION
The X-ray diffraction patterns for Ba2 PrBi0.8 Sb0.2 O6 and
Ba2 PrBi0.5 Sb0.5 O6 are shown in Fig. 1(a) and (b), respectively.
The polycrystalline samples with the lower Sb contents areformed in almost single phases with a monoclinic structure(the space group C 2 /m) as shown in [7] and [13]. For theparent Ba2 PrBiO6 , the lattice parameters are a = 6.2038 Å,
b = 6.1689 Å, c = 8.7011 Å and β = 89.7303◦. The
emergence of (101) reflection [the inset of Fig. 1(a)] indicates
B-cation ordering, which is characteristic of the B-site ordered
double-perovskite structure. The substitution of the smaller
Sb5+ (0.60 Å) ion at the Bi5+ (0.76 Å) site causes a monotonic
decrease in the lattice parameters as displayed in the inset of
Fig. 1(b). The heavily Sb-substituted samples with x = 0.5
and 1.0 crystallize in a rhombohedral structure with the spacegroup R3. For the Ba2 PrSbO6 , the lattice parameters a =6.04671 Å and α = 60.1646◦ are estimated from the X-ray
diffraction data using RIETAN-FP program, which are ingood agreement with previous data [14], [15]. In Fig. 1(b),the corresponding (101) peak observed at the x = 0.2 sample

Fig. 2.
(color online) (a) Inverse magnetic susceptibilities (χ −1 ) for
Ba2 Pr(Bi1−x ,Sbx ) O6 (x = 0, 0.1 0.2, 0.3, 0.4, 0.5, and 1.0). (b) Effective
magnetic moment plotted as a function of Sb content. The value of μeff isevaluated using the Curie–Weiss law.

is strongly suppressed at x = 0.5, indicating the B-site
disordering.
The inverse magnetic susceptibilities (χ −1 ) for
Ba2 Pr(Bi1−x ,Sb x ) O6 (x = 0, 0.1 0.2, 0.3, 0.4, 0.5, and 1.0)
are shown in Fig. 2(a) as a function of temperature at amagnetic field of 1 T. All measured samples show no signatureof magnetic ordering over a wide range of temperatures. The
low-temperature magnetic behavior in the parent Ba2 PrBiO6
is well described by the crystal field effect of the Pr ion [6].
From the magnetization 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 μeff is evaluated by using the following formula:
C = Nμ2eff μ2B /3k B
where C, N, and μ B denote the Curie constant, the number ofmagnetic atom per mol, and the Bohr magneton, respectively.
Performing the calculation for the end member samples,
we obtained that μeff = 3.15 and 3.67 μ B in the x = 0 and
x = 1.0 cases, respectively. As shown in Fig. 2(b), the magnetic moment is almost stable at low-Sb content, then it showsa gradual increase upon increasing the Sb content and finallyreaches ∼3.6 μ B . Next, we try to estimate the ratio of Pr3+
and Pr4+ using the following:
μ2eff = yμ2eff (Pr3+ ) + (1 − y)μ2eff (Pr4+ )This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
TANIGUCHI et al.: MAGNETIC STATES AND BANDGAPS OF B-SITE SUBSTITUTED DOUBLE-PEROVSKITE BA2 PR(BI, SB)O6

Fig. 3. (color online) Optical properties of Ba2 Pr(Bi1−x , Sbx ) O6 (x = 0,0.2, 0.5, and 1.0). (a) Plots of (αKM ε p )1/2 versus ε p and (αKM ε p )2
versus ε p . Straight lines: extrapolations to estimate the bandgaps using theleast square method. (b) Bandgap energy versus Sb content. We evaluate
E g = 0.977 eV at x = 0 and 2.395 eV at x = 1.0, assuming indirect
and direct energy gaps, respectively.

where μeff (Pr3+ ) = 3.58 μ B and μeff (Pr4+ ) = 2.54 μ B . For
the parent sample, the ratio of Pr3+ and Pr4+ (y:1− y) is 0.55:
0.45. When Sb content is increased up to 0.5, the ratio of Pr3+(∼0.75) becomes more dominant than that of Pr4+ . For theend member x = 1.0, we expect that Pr ions almost exist
as the trivalent state. The X-ray photoemission spectroscopy
analysis of the parent sample [13] revealed that a prominent
peak of Pr3+ is dominant with a smaller shoulder structureof Pr4+ , which is consistent with the mixed valence stateof Pr ion.
We demonstrate the optical properties for Ba2 Pr(Bi1−x ,Sb x )O6 (x = 0, 0.2, 0.5, and 1.0) powder samples measured by
the diffuse reflectance method. First, the observed reflectancedata for the powder samples are transformed to the absorption
coefficient αKM by using the conventional Kubelka–Munkfunction. Next, for the Kubelka–Munk conversion data near theband edge, we extrapolate the tangent line to the ε p -axis andevaluate the optical bandgaps from the intersection according
to the equation of(αKM ε p )n ∝ (ε p − E g )where αKM , ε p , and E g are the absorption coefficient, the photon energy, and the bandgap energy [5], [6]. Here, the power
exponent n on the left-hand side of the above-mentioned

3

Fig. 4. Energy band structures and total density of states calculated by thedensity functional theory for (a) Ba2 PrBiO6 and (b) Ba2 PrSbO6 compounds.
Bandgaps are estimated to be ∼ 0.62 and 4.42 eV, for the parent monoclinicand rhombohedral compounds, respectively.

formula depends on the types of photon transition of opticalabsorption. For direct and indirect bandgaps, we assume thatn = 2 and n = 1/2 [5], [6]. The optical measurements
determine whether the gap is direct or indirect. Fig. 3(a)shows the absorption coefficient as a function of photon
energy. For the x = 0 and x = 0.2 samples, we adopt(αKM ε p )1/2 versus ε p plot. The square of the absorption
coefficient for the x = 0.5 and x = 1.0 samples, (αKM ε p )2 ,is plotted as a function of ε p . We estimate E g = 0.977 eV
at x = 0 and 2.395 eV at x = 1.0, assuming indirect anddirect photon transitions, respectively. Straight lines denote theextrapolations to estimate the bandgaps using the least square
method. The magnitude of the energy bandgap is substantiallyenhanced with increasing the Sb content. When the Sb content
exceeds 50%, the nature of photon absorption is changed fromthe indirect to direct transitions.
Finally, the effect of the bandgap opening by the atomic substitution is examined by our first-principles electric structure
calculation. In Fig. 4, we have calculated the band structureof monoclinic Ba2 PrBiO6 and rhombohedral Ba2 PrSbO6 byusing the density functional theory as implemented in the code
VASP [10], [11], [16]. For monoclinic Ba2 PrBiO6 , the energy
gap is determined to be an indirect transition type because thevalence and conduction band edges are located at different
points of X and . The magnitude of its bandgap is evaluatedThis article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.
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to be about 0.62 eV at the parent Ba2 PrBiO6 , which is quite
smaller than the observed value (0.98 eV). The underestimation of the calculated bandgap in the parent monoclinic
compound is probably related to strongly electron correlation
not to consider in our research. On the other hand, in the caseof rhombohedral Ba2 PrSbO6 [Fig. 4(b)], we clearly predict a
direct transition type of bandgap with E g = 4.4 eV that is not
so far from the observed value (=∼2.4 eV). The discrepancy
in the magnitude of bandgap seems to be influenced by
the B-site disordering of the highly Sb-substituted samples.
A previous study [12] on BaBiO perovskite oxide predicts
that the 6 s orbitals of the Bi atom occupy a large portion ofboth valence and conduction bands, forming a narrow bandgapof this compound. The B-site Sb substitution for Bi causes a
reduction of the Bi orbitals and then results in the enlarged
bandgaps. We believe that these findings are closely related toits variation from the indirect to direct transitions.
IV. C ONCLUSION
In this paper, we synthesized the single-phase powder samples of Ba2 Pr(Bi1−x Sbx ) O6 by means of the Sb substitution
for the Bi sites. The X-ray diffraction data revealed thatthe polycrystalline samples with low Sb contents crystallize
in a monoclinic structure. For the heavily Sb substitution,
the rhombohedral structure is stable, but the (101) reflection
is strongly suppressed, indicating the B-site disordering.
Furthermore, we investigated the magnetic and optical properties of the B-site substituted perovskite oxides. The effective magnetic moment for the lightly Sb-substituted samples,
3.2 μ B , indicates the valence mixing between Pr3+ and Pr4+ .
For the heavy Sb substitution, the value of the magnetic
moment is close to the valence of Pr3+ . The optical spectra
were measured using a diffuse reflectance method. For thetwo end-member samples, we evaluated from the plots of theabsorption coefficient versus the photon energy, the indirect
and direct energy gap energies, E g = 0.977 eV at x = 0 and
2.395 eV at x = 1.0. Finally, the effect of Sb substitution
on the energy bandgap was examined by using the density
functional theory. The Sb substitution for the Bi sites causes
a substantial reduction of the Bi orbitals and then results inthe enhanced bandgap. We believe that these findings areclosely related to its variation from the indirect to directtransitions.

ACKNOWLEDGMENT
This work was supported by a research grant from The
Mazda Foundation.
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