Solid State Sciences 107 (2020) 106352

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Solid State Sciences
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Structural, physical and photocatalytic properties of mixed-valence
double-perovskite Ba2Pr(Bi,Sb)O6 semiconductor synthesized by citratepyrolysis technique
Arisa Sato a, Michaki Matsukawa a, *, Haruka Taniguchi a, Shunsuke Tsuji a, Kazume Nishidate a,
Sumio Aisawa a, Akiyuki Matsushita b, Kun Zhang bab

Faculty of Science and Engineering, Iwate University, Morioka, 020-8551, Japan
National Institute for Materials Science, Ibaraki, 305-0047, JapanA B S T R A C T

We demonstrated crystal structures, magnetic, optical, and photocatalytic properties of the B-site substituted double perovskite Ba2Pr(Bi1–xSbx)O6 (x ¼ 0, 0.1, 0.2, 0.5
and 1.0) synthesized by the citrate pyrolysis method. The single-phase polycrystalline samples with the light Sb substitution crystallized in a monoclinic structure(I2=m). Magnetization measurements on all the samples showed that the effective magnetic moments are concentrated around 3 μB , indicating the valence mixing
states between Pr3þ and Pr4þ. The magnitudes of band gap energy for the two end member samples were estimated from the optical measurements to be Eg ¼ 1.06 eV
at x ¼ 0 and 2.71 eV at x ¼ 1.0. The Ba2Pr(Bi1–xSbx)O6 (x ¼ 0, and 0.1) powders obtained by the present technique exhibited enhanced photocatalytic activities whencompared to the same compounds prepared by the conventional solid state method. Our findings suggest that the higher photocatalytic activities strongly depend on
powder preparation methods as well as the band gaps and photogenerated charge separation.

1. Introduction
A large number of double perovskite oxides A2B0 B00 O6 have been
widely studied due to their attractive physical properties and potential
applications [1]. For example, the A2FeMoO6 compound shows negative
tunneling magnetoresistance effect at room temperature, which is ofgreat interest with a wide range of applications in magnetic devices [2].
Furthermore, multiferroic double perovskite oxides with a coupling
between spontaneous ferroelectric polarization and ferromagnetic
ordering are promising materials from view points of physics and its
applications [3]. Some of semiconducting A2B0 B00 O6 compounds exhibit
photocatalytic properties such as hydrogen generation by water splitting
and are taken as alternative materials for TiO2 oxide [4,5]. In particular,
Ba2PrBiO6 has been shown to possess high photocatalytic activity, which
is probably related to the valence mixing [6,7]. A previous study on themagnetic states of the Ba2PrBiO6 compound suggests an anomalous
valence situation for Pr ions [8]. The complicated ground states of rareearth ions such as the Pr ion under the crystal field effect remain open
question, not only in the physical properties of the double perovskite
compound, but also from the view point of physics of the 4f electron
systems.
There are significant factors such as charge and size differences be­

tween B0 and B00 sites, to determine the B-site ordering of the double
perovskite oxide [1]. Increase in lattice strain and/or increase in theelectrostatic repulsion overcome the entropy contribution toward dis­’3þ ’’5þ
B O6
ordering, causing the alternate arrangement. For A2þ2 B
composition, the B sites tend to order with increasing the ion size dif­ference △rB ¼ rB’ rB’’ . If △rB > 0:2 Å, it is well known that the B sitesof the compounds are almost ordered alternately in the all crystallo­3þ 5þ
graphic axes [1]. In the Ba2þ
2 Pr Bi O6 compound, the B-site ionicradius difference △rB ¼ 0:23 Å meets the above condition, suggesting
the B-site ordering. (rB’ (Pr3þ) ¼ 0.99 Å and rB’’ (Bi5þ) ¼ 0.76 Å [9].) In
this paper, we report x-ray diffraction measurements, magnetic sus­
ceptibilities, diffuse reflectance spectra, and 2-propanol (IPA) degrada­tion for the Ba2Pr(Bi1–x,Sbx) O6 compounds, to determine the crystal
structures, magnetic, optical, and photocatalytic properties of B-site
substituted double perovskite oxides. A citrate pyrolysis technique issimilar to nitrate combustion synthesis methods [10] and an uniqueroute to prepare reactive precursor mixtures through an ignition processof concentrated aqueous solution including metallic ions of stoichio­
metric composition. For YBa2Cu4O8 with Tc ¼ 80 K and its related
cuprate oxide compounds, it has been reported that high-quality sin­
gle-phase polycrystalline materials are synthesized under ambientpressure of oxygen gas at lower annealed temperatures [11,12]. We* Corresponding author.
E-mail address: matsukawa@iwate-u.ac.jp (M. Matsukawa).
https://doi.org/10.1016/j.solidstatesciences.2020.106352
Received 10 May 2020; Received in revised form 3 July 2020; Accepted 3 July 2020
Available online 14 July 2020
1293-2558/© 2020 Elsevier Masson SAS. All rights reserved.

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Solid State Sciences 107 (2020) 106352

expect that this procedure enables to synthesize highly homogeneousand fine powders. As for one approach to developing efficient visible
light driven photocatalysts, it is desirable to adjust the band gaps of theirsemiconductors to utilize a wide range of visible light [13,14]. Ac­
cording to this approach, they are controlled by the effect of Sb substi­
tution on the parent material. In addition to it, another approach forhigh performance is to enhance the photogenerated charge separation in
the photocatalystic materials, to avoid charge recombination between
electron and hole [13,15]. In our research, the valence mixing states
between Pr3þ and Pr4þ are closely related to the photo induced charge
separation.

were annealed in air at 900–1000 � C for 48–96 h, in order to synthesize
the Ba2PrBiO6 double perovskite phase. For scanning electron micro­
scope measurements, the Ba2PrBiO6 polycrystalline film on Ag substrate
was fabricated from the single-phase powders by an electrophoretic
deposition technique. The electrophoretic deposition was conducted in
the acetone and iodine bath under the application of electric voltage up
to 300 V for 120 s [16]. We set Pt and Ag plates as anode and cathodeelectrodes, respectively. We performed X-ray diffraction measurementson the produced samples at room temperature with an Ultima IV
diffractometer (Rigaku) using Cu-Kα radiation. The lattice parameters
were estimated from the x-ray diffraction data using the RIETAN-FP
program. The Brunauer-Emmett-Teller (BET) surface area of the pow­
der samples was evaluated using a surface area analyzer (BELSORP-mini
II, Microtrac).
Optical spectra were measured by a diffuse reflectance method using
a spectrophotometer (Hitachi U-3500) and BaSO4 was used as thereference material. The optical band gaps for the powder samples were
evaluated from reflectance spectral data using the conventional
Kubelka-Munk functions [6,18]. The dc magnetization was measured ata magnetic field of 0.1 T under the field cooling process using a super­
conducting quantum interference device magnetometer (MPMS,

2. Experiment
Polycrystalline samples of Sb-substituted Ba2PrBiO6 were synthe­
sized by using the citrate pyrolysis technique. In the first step, stoi­
chiometric mixtures of high purity BaCO3, Pr6O11, Bi2O3, and Sb weredissolved in a nitric acid solution at 70–80 � C. After adding citric acidand neutralizing it by aqueous ammonia, we then obtained the porous
products through the self-ignition process using halogen lamp stirrer. In
the next step, the precursors were ground and resultant fine powders

Fig. 1. (color online) X-ray diffraction patterns for the two end-member citrate samples, (a) Ba2PrBiO6 and (b) Ba2PrSbO6. In addition, the data for the intermediate
sample Ba2Pr(Bi0.5,Sb0.5)O6 are given in (c). The calculated profiles based on the monoclinic and cubic structure models are shown in (a) and (b), respectively. The
insets of (a) and (b) display the enlarged diffraction profiles and the lattice parameters as a function of Sb content. In the inset of (b), for monoclinic and cubic
pffiffiffi
pffiffiffi
structures, c* ¼ c= 2 and a* ¼ a= 2, respectively. In (c). the red and blue plots are responsible for the monoclinic and cubic structure models, respectively. (d)Crystal structures for the two end-member samples, Ba2PrBiO6 and Ba2PrSbO6. The left and right pictures represent the ideal monoclinic and cubic structures,
respectively. Bi(or Sb)O6 and PrO6 octahedra are connected by corner-sharing with each other. (For interpretation of the references to color in this figure legend, thereader is referred to the Web version of this article.)2

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Solid State Sciences 107 (2020) 106352

Quantum Design). We conducted the gaseous 2-propanol (IPA) degra­
dation experiment, to evaluate photocatalytic activities of the powder
samples (in detail, refer to Refs. [6,19]). The powder (about 1 g) was
placed on the bottom of a small glass cell and its cell was set in a 0.5-L
glass reactor vessel. After removing impurities from the vessel throughfilling a dry air, the dilute IPA gas (5 cc) was injected into its vessel usinga syringe. We started illuminating the visible light after the IPA con­centration remained constant, which implied that the IPA gas finishedabsorbing on the surface of particles. The visible light was obtainedusing a 300W Xe lamp equipped with UV and IR filtering functions(Cermax LX300F, Excelitas Technologies). The illuminating spectra of
the Xe lamp are limited in the visible wavelength range between 390 and
780 nm (see supplementary data Fig. S1). It is well known that the IPA
gas under photocatalytic reaction is finally decomposed into CO2 [19].
Accordingly, the CO2 concentration was measured as a function ofirradiation time using a gas chromatography system (GC-2014, Shimazu
Co.)
3. Results and discussion
The X-ray diffraction patterns for the two end-member oxides,
Ba2PrBiO6 and Ba2PrSbO6, are shown in Fig. 1 (a) and (b), respectively.
For the parent Ba2PrBiO6 with a monoclinic structure (the space groupI2=m), the lattice parameters are estimated to be a ¼ 6:2038 Å, b ¼
6:1689 Å, c ¼ 8:7011 Å and β ¼ 89:7303∘ from the x-ray diffraction datausing RIETAN-FP program, which are in good agreement with previous
data [8,17]. The emergence of (101) reflection (the inset of Fig. 1 (a))
indicates B-cation ordering which is characteristic of the ordered
double-perovskite structure. In particular, the tiny profiles around the(101) peak are well described by the least squared method based on
RIETAN-FP when we set B site ordering to be �70 %. This estimatedratio is nearly in good agreement with the published value ( �75 %) [8].
The polycrystalline samples for x < 0:5 are formed in almost single
phases of the monoclinic structure, while the x ¼ 1:0 sample crystallizes
in a cubic structure with the space group Fm3m. Substitution of thesmaller Sb5þ (0.60 Å) ion at the Bi5þ (0.76 Å) site causes a monotonic
decrease in the lattice parameters as displayed in the inset of Fig. 1 (b).
Upon further increasing Sb content, the x-ray diffraction patterns for x¼ 0.5 sample exhibit a two-phase mixture of monoclinic and cubic
structures (Fig. 1(c)). The double main peaks responsible for the
monoclinic and cubic phases are almost comparable and they are in theratio of 56 % (I2=m) to 44 % (Fm3m). For Ba2PrSbO6, we obtain thelattice parameters a ¼ 8:6042 Å and α ¼ 90∘ . Our result is qualitatively
different from the rhombohedoral structure as previously reported [20,21]. The differences in crystal structures are attributed to the tolerance
factors which are associated with the mixed valence state of Pr ions. We
will discuss this interesting issue following the magnetic data. The
crystal structures for the two end-member samples, Ba2PrBiO6 and
Ba2PrSbO6 are shown in Fig. 1(d), corresponding to the monoclinic andcubic structures, respectively. The Bi(or Sb)O6 and PrO6 octahedra areconnected by corner-sharing with each other. However, the corre­
sponding (101) peak observed at the low Sb substituted sample isstrongly suppressed at x ¼ 1.0, indicating the B-site disordering.
The microstructures of the parent samples prepared by the citratepyrolysis and solid-state reaction methods are shown in Fig. 2. (For the
Sb substituted samples with x ¼ 0.1 and 1.0, see supplementary data
Fig. S2) The particles of the former sample have an average size in therange of about 0.2–0.5 μm. On the other hand, the particle diameters of
the latter sample are on a micron order scale and about one-order grater
than those of the former. The citrate pyrolysis process provides homo­
geneously dispersed grains with sub micron size in comparison to theconventional solid-state procedure. We suspect that the substitution of
Sb ion at Bi sites promotes the refinement of powder samples as shown in
Fig. S2. This finding is probably related to the larger surface area for the
Sb substituted samples.

Fig. 2. (color online) SEM images of Ba2PrBiO6 powders. (a) citrate pyrolysis
and (b) solid-state reaction techniques. (For Ba2Pr(Bi1–x,Sbx)O6 with x ¼ 0.1,
and 1.0, see supplementary data Fig. S2).

The inverse magnetic susceptibility data (χ 1 ) for the Ba2Pr(Bi1–x,
Sbx) O6 compounds (x ¼ 0, 0.5 and 1.0) are shown in Fig. 3 as a functionof temperature at a magnetic field of 0.1 T. All the measured samples
exhibit no signature of magnetic ordering over a wide range of tem­
peratures. From the magnetization data at high temperatures above 200
K, we estimate the effective magnetic moment according to the CurieWeiss law, χ ¼ C=ðTΘÞ. Here, C and Θ are the Curie constant andCurie temperature. The value of effective magnetic moment μeff isevaluated by using the following formula,.
C ¼ N μeff2 μB2 3kB
where N and μB denote the number of magnetic atom per mol and the
Bohr magneton, respectively. Performing the calculation for the parent
and lightly Sb substituted samples, we obtain that μeff ¼ 3.07 μB and 3.05
μB in x ¼ 0 and x ¼ 0.1 cases, respectively. As shown in the inset of Fig. 3,
the magnetic moment is almost stable against the Sb substitution. Next,
we try to estimate the ratio of the Pr3þ and Pr4þ ions using the equation��
μ2eff ¼ yμ2eff Pr3þ þ ð1 yÞμ2eff Pr4þ
where μeff (Pr3þ) ¼ 3.58 μB and μeff (Pr4þ) ¼ 2.54 μB . For the parent
sample, we obtain that the ratio of Pr3þ and Pr4þ ions is 0.47 : 0.53. For
x ¼ 0.1, Pr3þ: Pr4þ ¼ 0.45 : 0.55. For all the samples, the magnetic data
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Solid State Sciences 107 (2020) 106352

compounds Ba2PrMO6[21]. On the other hand, for (Pr3þ, M5þ ¼ Sb5þ)5þ
and (Pr4þ, M4þ ¼ Sb3þ
0:5 Sb0:5 ), we have t ¼ 0:9697 and 0.9831, respec­tively. It is true that the former value is below the threshold value trhom ¼
� 0:974 for the phase boundary between rhombohedral and cubic
structures, but the latter increases above this critical value. This esti­
mation strongly supports the stability of cubic structure in the Pr4þ
dominant Ba2PrSbO6 compound. Our magnetic data for the end member
Ba2PrSbO6 showed that Pr4þ ions occupy the majority of 60% accom­
panied by the minority (40%) of Pr3þ. Furthermore, the x-ray data forthis compound also revealed the presence of the cubic crystal structure.
Our experimental data support the close relationship between thevalence state of Pr4þ and the occurrence of cubic phase through thetolerance factor. For the intermediate composition Ba2PrBi0.5Sb0.5O6,
we identified the monoclinic and cubic phases with Ba2PrBiO6 and
Ba2PrSbO6, respectively. The coexistence of these two phases is also
explained on the basis of the above scenario concerning the two end
member compositions.
We demonstrate the optical properties for the Ba2Pr(Bi1–x,Sbx)O6 (x¼ 0, 0.1, 0.5, and 1.0) powder samples measured by the diffuse reflec­
tance method. First, the observed reflectance data for the powder sam­ples are transformed to the absorption coefficient αKM by using theconventional Kubelka-Munk function. Next, for the Kubelka-Munkconversion data near the band edge, we extrapolate the tangent line tothe εp axis and evaluate the optical band gaps from the intersection
according to the equation of�n�
αεp ∝ εp Eg ;Fig. 3. (color online) Inverse magnetic susceptibility vs temperature. (χ 1 ) for
Ba2Pr(Bi1–x,Sbx) O6(x ¼ 0, 0.5, and 1.0). The inset denotes the effective mag­netic moment plotted as a function of Sb content. The value of μeff is evaluatedusing the Curie-Weiss law. The lower and upper limits represent μeff (Pr4þ) ¼
2.54 μB and μeff (Pr3þ) ¼ 3.58 μB , respectively. (For interpretation of the ref­erences to color in this figure legend, the reader is referred to the Web versionof this article.)

where α, εp , and Eg are the absorption coefficient, the photon energy,
and the band gap energy [6,7]. Here, it should be noted that the power
exponent n on the left hand side of the above formula decides the typesof photon transition of optical absorption. For direct and indirect
semiconductors, we assume that n ¼ 2 and n ¼ 1=2, respectively [6,7].
The optical measurements determine whether the energy gap is direct orindirect. Fig. 4 shows the absorption coefficient as a function of photon

strongly suggest the coexistence between trivalent and tetravalent statesof Pr ions. The X-ray photoemission spectroscopy analysis of the solidstate parent sample [17] revealed that a prominent peak of Pr3þ isvisible accompanied by a smaller shoulder structure of Pr4þ, which isconsistent with the mixed valence state of the Pr ion.
The magnetic data suggest that about half of Pr ions are oxidized tothe tetravalent state over the whole range of Sb substitution. Accord­
ingly, for the end members with x ¼ 0 and 1.0, we assume that4þ 3þ 5þ
Ba2þ
2 Pr M0:5 M0:5 O6 (M ¼ Bi or Sb) is probably realized under theenergy. For the x ¼ 0 and 0.1 samples, we adopt ðαKM εp Þ1=2 vs εp plot.
In Fig. 4 (b), the square of the absorption coefficient for the x ¼ 0.5
and 1.0 samples, ðαKM εp Þ2 , is plotted as a function of εp . We estimate Eg¼ 1.06 eV at x ¼ 0 and 2.71 eV at x ¼ 1.0, assuming indirect and direct
photon transitions, respectively. The magnitude of the energy band gap
is substantially enhanced with increasing Sb content. When the Sb
content exceeds 50 %, the nature of photon absorption is changed fromthe indirect to direct transitions. We give some comments on the opticalproperty for the Sb50% substituted sample. As discussed above, thecrystal structure of the present sample is explained by a two-phasemixture of monoclinic and cubic structures. The corresponding optical
data are fitted using both indirect and direct transition models in thephoton energy region between 1 and 2 eV. The former energy gap isestimated to be �1.12 eV, which is similar to the values of the mono­clinic single phase x ¼ 0 and 0.1 samples. However, the latter (1.43 eV)is quite different from 2.71 eV at the cubic single phase x ¼ 1.0 sample.
This reason has not been made clear but it has some relationship withchemical interaction between the two-phases such as partial substitutionof the Bi ions at Sb sites of the cubic phase.
Next, we conducted the gaseous IPA degradation experiment under avisible light irradiation, to evaluate photocatalytic activities of thepowder samples. Fig. 5 (a) shows the concentration of evolved CO2 as afunction of visible light irradiation time for the Sb-substituted Ba2Pr­
BiO6. For comparison, the data for the solid state samples are presented.
Here, we note that the concentration is normalized by the surface area ofthe powder samples listed in Table 1. The CO2 concentrations for boththe parent and x ¼ 0:1 citrate samples show a rapid rise at the initial 20
min under the visible light irradiation and then increase gradually withincreasing irradiation time. The stable photocatalytic efficiency after 20
min is related to the absorption process of photogenerated CO2 on thesurface of samples, which contributes to reduction of CO2 concentration.

3þ 5þ
charge neutrality in contrast to Ba2þ
2 Pr M O6. First of all, we consider
B site disordering of the parent sample on the basis of the B-site ionicradius difference △rB as mentioned in the part of Introduction. For the4þ 3þ 5þ
Ba2þ
2 Pr Bi0:5 Bi0:5 O6 composition, △rB was decreased from 0.23 Å at
the trivalent state of Pr ion to 0.045 Å at the tetravalent state, where rB’

95þ(Pr4þ) ¼ 0.85 Å and rB’’ (Bi3þ
0:5 Bi0:5 ) ¼ 0.895 Å . In addition to the smaller
ion radius difference, the difference in the B’ and B’’ cation oxidation
states △ZB becomes zero in the case of Pr4þ. For Pr3þ and Bi5þ, △ZB ¼
2. Considering the electrostatic repulsion between B site cations, when
△ZB is large, the ordered arrangement of B site cations gives loweredenergy compared to the disordered ones [1]. From the view point ofthese two factors, we expect that the Pr4þ based phase favors the B-site
disordering, which is consistent with the small (101) peak in the X-ray
diffraction data in Fig. 1(a). In the parent compound, the mixed valence
state of Pr ions is closely related to the partial disordering of B-site.
Next, we discuss the tolerance factors of the two end member sam­ples because they are associated with the crystal structures through theionic radii of Pr, Bi and Sb. The tolerance factor of double perovskite
compounds Ba2PrMO6 is given by the following equation,rBa þ rO�
t ¼ pffiffi �Mþ rO
2 rPr þr2where rBa , rO , rPr , and rM are the ionic radii of the respective ions. For
5þ
(Pr3þ, M5þ ¼ Bi5þ) and (Pr4þ, M4þ ¼ Bi3þ
0:5 Bi0:5 ), we obtain t ¼ 0:9356
and 0.9366, respectively. These values remain below the threshold value
tmono ¼� 0:96 for the phase boundary between monoclinic and rhom­bohedral structures in the phase diagram of double perovskite
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Solid State Sciences 107 (2020) 106352

Fig. 4. (color online) Optical properties of Ba2Pr(Bi1–x,Sbx) O6(x ¼ 0, 0.1, 0.5,
and 1.0). (a) Plots of ðαKM εp Þ1=2 vs εp . For the x ¼ 0 and 0.1 samples, Eg isestimated from an intersection point of base line and straight line by extrapo­

Fig. 5. (color online)Photocatalytic activities vs visible light irradiation timefor Sb-substituted Ba2PrBiO6. (a) citrate pyrolysis samples, and (b) solid-state
samples. The gaseous concentration is normalized by the surface area of thepowder samples listed in Table 1. (For interpretation of the references to color
in this figure legend, the reader is referred to the Web version of this article.)2

lation. In the inset, the x ¼ 0:1 data are shown. (b) ðαKM εp Þ vs εp for the x ¼ 0:5
and x ¼ 1:0 samples are plotted as a function of εp . The inset denotes plots ofðαKM εp Þ1=2 vs εp for x ¼ 0:5. The corresponding optical data are fitted using
both indirect and direct transition models in the photon energy region between
1 and 2 eV. The former and latter band gap energies are estimated to be e1.12
eV and 1.43 eV, respectively. (For interpretation of the references to color inthis figure legend, the reader is referred to the Web version of this article.)in the citrate samples are attributed to their morphology, where fineparticles are homogeneously distributed with a sub micron order. From
the data listed in Table 1, we conclude that the parent and low Sb
substituted samples exhibit good catalytic performance among the same
synthesis based material members.
Finally, we discuss the charge separation model for higher activitiesof the parent Ba2PrBiO6 compound, as shown in Fig. 6. As far as weknow, there are two approaches such as modification of the band gaps
and improvement of the photogenerated charge separation in the pho­tocatalystic materials, for completing high performance [13–15,23]. We
carried out the former approach by larger Sb substitution at Bi site in the
double perovskite parent compounds as mentioned before. Here, we
focus on the valence mixing states between Pr3þ and Pr4þ from the view
point of charge separation. As shown in Fig. 6, it is assumed that theband gap at the cluster region occupied by the Pr4þO6 octahedra isnarrower than that at its neighboring region occupied by the Pr3þO6octahedra. For the Pr4þ ions, the valence of their surrounding Bi ions is
expected to change from 5þ to 3þ under electric neutral condition.
Accordingly, the band gap becomes narrower in the presence of thePr4þO6 octahedra because 6s electrons are added to empty orbitals of

On the other hand, for the Sb50% and Sb100% substituted samples wedetect no clear evolution. We suspect that the band gap opening due tothe heavy Sb substitution suppresses the formation of electron-holepairs, causing a decrease of the photocatalytic reaction processes. In
our previous study [22], the effect of the band gap opening due to theatomic substitution was examined by using first-principles electric
structure calculation. The Sb substitution at Bi site gives rise to a
reduction of the Bi-orbitals and then results in the enlarged band gap.
The collapse of photocatalytic activity is probably originated from thelarger band gap, which is closely related to the removal of Bi orbitals in
the electric structures. The photocatalytic performance strongly depends
on the Sb substitution, accompanied by an increase of the band gap
energies. The photocatalytic data for the solid state samples with thesame composition also show similar behaviors as the citrate samples.
However, the evolved CO2 values of the x ¼ 0 and x ¼ 0.1 citrate samples
are more than twice as large as the data of the corresponding solid state
samples, as shown in Fig. 5 (b). The enhanced photocatalytic properties
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Solid State Sciences 107 (2020) 106352

Table 1
Physical and photocatalytic properties for Ba2Pr(Bi1–x,Sbx) O6 compounds (x ¼ 0, 0.1, 0.2, 0.5 and 1.0). Synthetic method, crystal symmetry, effective magnetic
moment, energy gap, surface area, and gas evolution of CO2 are listed as a function of Sb content. The value of evolved CO2 is evaluated after a visible light irradiationof 120 min. In details, see the text (* see the caption of Fig. 4(b)).
Sb content x

Synthetic method

Crystal symmetry

μeff

BET

(eV)

(m /g)(ppm/m2)

1.06
1.13
1.43/1.12*

1.31
2.88
2.72

111
102
<5

citrate pyrolysis
citrate pyrolysis
citrate pyrolysis

monoclinic
monoclinic
mon./cubic

3.08
3.05
3.07

1.0

citrate pyrolysis

cubic

3.0

0.0
0.1
0.2
1.0

solid-state reaction
solid-state reaction
solid-state reaction
solid-state reaction

monoclinic
monoclinic
monoclinic
rohmbohedral

3.15a
3.15a
3.12a
–

2.71

1.1a
1.17a
1.24a
2.22a

2

CO2

(μB )
0.0
0.1
0.5

a

Eg

–

<5

1.23
1.58
1.49
–

42
33
22
e5

See ref. [22].

The magnitudes of band gap energy for the two end member samples
were estimated from the optical measurements to be Eg ¼ 1.06 eV at x ¼
0 and 2.71 eV at x ¼ 1.0. The magnitude of the energy band gap issubstantially enhanced with increasing Sb content. When the Sb content
exceeds 50 %, the nature of photon absorption is changed from the in­direct to direct transitions. The photocatalytic performance strongly
depends on the Sb substitution accompanied by an increase of the band
gap energies. The photocatalytic activities of the citrate powder samples
with the light Sb substitution are considerably enhanced in comparison
with those values of the samples prepared by the conventional solid statemethod. We believe that our charge separation model qualitatively de­scribes one of the possible reasons for the efficient performance of theparent Ba2PrBiO6 compound in the presence of trivalent and tetravalentstates of the Pr ions. The mixed valence state of Pr ions is not only related
to B site partial disordering in the monoclinic phase, but also has close
relationship with occurrence of the cubic phase in the phase diagram ofthe double perovskite compounds Ba2PrSbO6.
We tried the two approaches to controlling the band gaps and real­izing the photogenerated charge separation in the photocatalystic ma­
terials through larger Sb substitution and the valence mixing of the Pr
ions. Our findings suggest that the higher photocatalytic activities
strongly depend on the powder preparation method as well as themodification of band gaps and the photo-induced charge separation.

Fig. 6. (color online) Charge separation model for the coexistence between
trivalent and tetravalent states of Pr ions. The band gap at the cluster region
occupied by the Pr4þO6 octahedra is assumed to be narrower than that at its
neighboring region occupied by the Pr3þO6 octahedra. (see the text). (Forinterpretation of the references to color in this figure legend, the reader isreferred to the Web version of this article.)

Bi5þ contributing to a majority of valence and conduction bands [18].
The ptotoexcited electrons from the lower valence to upper conduction
band do not recombine with holes, but transfer to its neighboring lower
conduction band, resulting in charge separation. It has been reported
that the Pr based compound with different valence states exhibits thehighest efficiency among Ba2ReBiO6 series (Re ¼ rare earth ions such as
La, Pr, Ce, Nd, Sm, Eu, Gd, and Dy [6]. In previous studies on the crystal
structure and magnetic properties of Ba2ReBiO6, the Pr ion only is shown
to exist in trivalent and tetravalent states (except for Ce) [21]. Therefore,
we believe that our proposed model qualitatively describes one of thepossible reasons for the efficient performance of the parent Ba2PrBiO6compound.

Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
This work was supported in part by MEXT Grands-in-Aid for Scien­tific Research (JPSJ KAKENHI Grants No. JP19K04995), Iketani Scienceand Technology Foundation, and The Mazda Foundation. We thank Mr.
K. Sasaki and M. Ito for the SEM measurement.

4. Summary

Appendix A. Supplementary data

We investigated the crystal structures, magnetic, and photocatalytic
properties of the B-site substituted double perovskite Ba2Pr(Bi1–xSbx)O6(x ¼ 0, 0.1, 0.2, 0.5 and 1.0) synthesized by the citrate pyrolysis pre­cursor method. For comparison, the photocatalytic data for the solidstate sample were examined. The present synthesis procedure enabled
to prepare highly homogeneous and fine powders. The single-phase poly
crystalline samples with the light Sb substitution crystallized in a
monoclinic structure (I2=m). Magnetization measurements showed thatthe effective magnetic moments are located around 3 μB , indicating thevalence mixing states between Pr3þ and Pr4þ.

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.solidstatesciences.2020.106352.
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