Some structural and photophysical properties of two functional double oxides Bi2MTaO7 (M = Ga and In)

Article abstract of DOI:10.1016/j.jallcom.2004.01.059

Two new functional double oxides, Bi₂MTaO₇ (M = Ga and In), were synthesized by the conventional solid-state reaction method. They were characterized by the Rietveld structural analysis and UV-Vis diffuse reflectance spectroscopy. Bi

Full text of DOI:10.1016/j.jallcom.2004.01.059

Journal of Alloys and Compounds 377 (2004) 248–252
Some structural and photophysical properties of two
functional double oxides Bi2MTaO7 (M = Ga and In)
Junhu Wanga, Zhigang Zoub, Jinhua Yea,c,∗
a
Ecomaterials Center, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
Photoreaction Control Research Center, National Institute of Advanced Industrial Science and Technology (AIST),
b
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
PRESTO, Japan Science and Technology Agency, 4-1-8 Honchou Kawaguchi, Saitama, Japan
c
Received 25 April 2003; accepted 15 January 2004
Abstract
Two new functional double oxides, Bi2MTaO7 (M = Ga and In), were synthesized by the conventional solid-state reaction method. They
were characterized by the Rietveld structural analysis and UV-Vis diffuse reflectance spectroscopy. Bi2MTaO7 have distorted pyrochlore-type
structure. The lattice distortion of Bi2InTaO7 (0.067) is larger than that of Bi2GaTaO7 (0.039). The angle between the corner-linked MO6
octahedral in Bi2InTaO7 (143.4◦) is closer to 180◦ than that of Bi2GaTaO7 (128.3◦). The abilities of the H2 and O2 evolution over Bi2MTaO7
were evaluated from an aqueous CH3OH solution and an aqueous Ce(SO4)2 solution under UV light irradiation, respectively. The rates of
the H2 and O2 evolution over Bi2InTaO7 were clearly larger than that of Bi2GaTaO7. These results suggest that the abilities of the H2 and O2
evolution might be correlated with the lattice distortions and the angles between the corner-linked MO6 octahedral in Bi2MTaO7.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Bi2MTaO7; Pyrochlore-type structure; Photophysical property; UV light irradiation; H2 and O2 evolution
1. Introduction
under UV light irradiation, respectively. The results indi-
cated that the ability of the H2 evolution over Bi2InTaO7
is the largest among Bi2MTaO7 and Bi2MNbO7. Here,
syntheses, crystal structures and some photophysical prop-
erties of the two new functional double oxides, Bi2MTaO7
(M = Ga and In), are reported.
Functional Bi2MNbO7 (M = Al, Ga, In, and Fe) and
Bi2RNbO7 (R = rare earth ions) double oxides with a
distorted pyrochlore-type structure have been reported by
Zou et al. [1–3]. Crystal structures and some photophysical
properties, such as the abilities of the H2 and O2 evolution
over Bi2MNbO7 and Bi2RNbO7 under UV light irradiation,
are clearly changed with the change of M or R. This indi-
cates that the different M or R are possible to modify their
crystal and electronic structures, then lead to the difference
in the abilities of the H2 and O2 evolution under UV light
irradiation.
In this study, two new functional double oxides, Bi2
MTaO7 (M = Ga and In), were successfully synthesized by
substituting Nb by Ta in Bi2MNbO7. Bi2MTaO7 were char-
acterized by the Rietveld structural analysis and UV-Vis
diffuse reflectance spectroscopy. The abilities of the H2 and
O2 evolution over Bi2MTaO7 were evaluated from an aque-
ous CH3OH solution and an aqueous Ce(SO4)2 solution
2. Experimental
2.1. Sample preparation
The polycrystalline samples of Bi2MTaO7 were prepared
by the conventional solid-state reaction method. The starting
materials, Bi2(CO3)O2, M2O3 (M = Ga and In) and Ta2O5
with purity of high grade, were purchased from Wako Pure
Chem. M2O3 (M = Ga and In) and Ta2O5 were dried at
873 K before use. The stoichiometric amounts of the start-
ing materials were weighed accurately and mixed intimately.
The precursors were sintered in alumina crucible in air
and analyzed by powder X-ray diffraction (XRD) measure-
ment. As a result, Bi2InTaO7 was synthesized at 1323 K and
Bi2GaTaO7 was synthesized at 1353 K for 30 h, respectively.
∗
Corresponding author. Tel. +81-29-859-2646; fax: +81-29-859-2601.
E-mail address: Jinhua.Ye@nims.go.jp (J. Ye).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.01.059

J. Wang et al. / Journal of Alloys and Compounds 377 (2004) 248–252
249
2.2. Crystal structure
solution; cerium sulfate tetrahydrate, as being an oxidizing
reagent, can promote an O2 evolution reaction in aqueous
Ce(SO4)2 solution [5].
The crystal structure of Bi2MTaO7 were determined by
the Rietveld structure refinement. A conventional JDX-3500
diffractometer with Cu K␣ radiation (λ = 1.54178 Å) was
used for the measurements of their XRD patterns. The XRD
data were collected with a step scan procedure in 2θ =
5–100◦. The step interval was 0.024◦ and scan speed, 1◦ per
minute. Full-profile structure refinements of the collected
powder diffraction data for Bi2MTaO7 were performed by
the Rietveld program RIETAN 97␤ [4].
3. Results and discussion
3.1. Crystal structure
Fig. 1 shows the XRD patterns of Bi2MTaO7. Although
Bi2MTaO7 show the similar XRD patterns, the 2θ angles
of reflections are clearly different. Therefore, the lattice pa-
rameters of Bi2MTaO7 are different. Fig. 2 shows the result
of the Rietveld structure refinement for Bi2InTaO7. This re-
sult is similar to that of the Rietveld structure refinement for
Bi2GaTaO7. The final positional and isotropic thermal pa-
rameters are shown in Table 1. The positional and isotropic
thermal parameters of all atoms in Bi2MTaO7 were refined
separately. The refinements yielded R factors, RI = 5.45%,
Rp = 7.60%, Rwp = 12.55% for Bi2GaTaO7 and RI =
4.80%, Rp = 6.85%, Rwp = 11.42% for Bi2InTaO7 in space
group Fd3m.
2.3. Photophysical property
The UV-Vis diffuse reflectance spectra of Bi2MTaO7
were measured by an UV-Vis spectrometer (Shimadzu
UV-2500) with BaSO4 as a reference at room temperature
and were converted from reflection to absorbance by the
Kubella–Munk method.
The reactions of the H2 and O2 evolution over Bi2MTaO7
were carried out by a gas-closed circulation system and an
inner-irradiation type quartz cell with a 400 W high-pressure
Hg lamp (Riko-Kagaku Co. Ltd., Japan). The samples were
stirred by a magnetic stirrer and then suspended in an aque-
ous CH3OH solution for the H2 evolution and an aqueous
Ce(SO4)2 solution for the O2 evolution. The gases evolved
were measured with a thermal conductivity detector (TCD)
gas chromatograph (Shimadzu GC-8AIT), which was con-
nected to a gas circulating line.
222
(a)
400
440
622
840
662
620
531
444
311
111
331 511
800
844
Evaluation of the H2 and O2 evolution was performed
in an aqueous CH3OH solution (Powder sample: 2.0 g;
CH3OH: 50 cm3; pure H2O: 320 cm3; 0.2 wt.% Pt) and
an aqueous Ce(SO4)2 solution (Powder sample: 1.0 g;
Ce(SO4)2·4H2O: 1.0 mmol; pure H2O: 370 cm3), respec-
tively. The Pt metal powder was loaded on the surface of
the samples by photodeposition of H2PtCl6·6H2O, CH3OH,
and Ce(SO4)2·4H2O were used as sacrificial reagents. Un-
der light irradiation, methanol, as a reducing reagent, can
promote a H2 evolution reaction in an aqueous CH3OH
(b)
15
30
45
2θ (deg.)
60
75
90
Fig. 1. Powder X-ray diffraction patterns for Bi2GaTaO7 (a) and
Bi2InTaO7 (b) with a distorted pyrochlore-type structure.
Fig. 2. Result of the Rietveld structure refinement of the X-ray diffraction patterns for Bi2InTaO7 (Plus marks: observed; solid line: calculated; RI = 4.80%,
Rp = 6.85%, Rwp = 11.42%).

250
J. Wang et al. / Journal of Alloys and Compounds 377 (2004) 248–252
Table 1
Final structural parameters of Bi2MTaO7 (M = Ga and In) obtained by
the Rietveld structure refinement
Atom
x
y
z
Beq
Bi2GaTaO7
Bi
0.500
0.000
0.336(2)
0.375
0.000
0.500
0.000
0.125
0.375
0.000
0.500
0.000
1.5
0.375
0.000
3.5
0.6
Ga
O1
O2
Ta
1.0
0.6
Bi2InTaO7
Bi
In
O1
O2
Ta
0.500
0.000
0.308(2)
0.375
0.000
0.500
0.000
0.125
0.375
0.000
0.500
0.000
1.6
0.375
0.000
3.1
0.5
1.0
0.5
The outcomes of the final refinement indicate that
Bi2MTaO7 have distorted pyochlore-type structure, cubic
system with space group Fd3m and their lattice parameters
are a = 10.4478(3) Å for Bi2GaTaO7 and a = 10.7612(2) Å
for Bi2InTaO7. All of the reflection peaks can be success-
fully indexed based on the lattice parameter and the space
group (see Fig. 1). Although Bi2MTaO7 can crystallize
into the similar distorted pyrochlore-type structure (Cubic,
space group Fd3m), the lattice parameter of Bi2GaTaO7 is
clearly smaller than that of Bi2InTaO7 because M3+ (M
= Ga and In) ions have different effective ionic radii, i.e.,
Ga3+ (0.620 Å) <In3+ (0.800 Å) [6].
Fig. 3. Schematic structural diagram of Bi2MTaO7 (M = Ga and
In). Bi2MTaO7 consist of a three-dimensional network structure of
corner-linked MO6 (M = Ga, In and Ta) octahedral.
smaller than the In(Ta)–O1–In(Ta) bond angles (143.3◦) of
Bi2InTaO7. It is obvious that not only the lattice parame-
ters but also the M–O1, Bi–O1, Bi–O2 bond lengths and
M–O1–M bond angles are changed when Ga is substituted
by In in Bi2MTaO7.
The x structural parameters of O1 are 0.336(2) for
Bi2GaTaO7 and 0.308(2) for Bi2InTaO7 as listed in Table 1.
The x structural parameter is an index of the structure’s vari-
ation on the pyrochlore-type A2B2O7 double oxides (Cubic,
space group Fd3m) and is equal to 0.375 when the six O1
atoms are located on the position of the related fluorite-type
double oxides [7]. Therefore, information on the lattice
distortion can be obtained from the x structural parameters.
The lattice distortion of Bi2InTaO7 is larger than that of
Bi2GaTaO7 because deviation of the x structural parameter
compared with 0.375 for Bi2InTaO7 (0.067) is larger than
that for Bi2GaTaO7 (0.039). Charge separation is demanded
to prevent recombination of the photoinduced electrons and
holes in the reactions of the H2 and O2 evolution under
UV light irradiation. The lattice distortion is considered
beneficial to separate the photoinduced electrons and holes.
Therefore, the larger the lattice distortion is, the higher the
ability of the H2 and O2 evolution might be expected.
Fig. 3 shows the schematic structural diagram of
Bi2MTaO7. Bi2MTaO7 consist of a three-dimensional net-
work structure of corner-linked MO6 (M = Ga, In and
Ta) octahedral. The MO6 octahedral are connected into
chains by Bi ions. Selected bond lengths and angles are
listed in Table 2. The six Bi–O1 bond lengths are longer
than those of the two Bi–O2 bond lengths. The Ga(Ta)–O1
bond lengths (2.053 Å) of Bi2GaTaO7 are longer than
the In(Ta)–O1 bond lengths (2.004 Å) of Bi2InTaO7. The
Ga(Ta)–O1–Ga(Ta) bond angles (128.3◦) of Bi2GaTaO7 are
The angle between the corner-linked MO6 octahedral, i.e.,
the M–O1–M bond angle of Bi2InTaO7 (143.3◦) is closer
to 180◦ than that of Bi2GaTaO7 (128.3◦). Wiegel et al. [8]
pointed out that the closer the bond angle of M–O1–M is to
180◦, the more the excited state is delocalized. That is to say,
the photoinduced electrons and holes in Bi2InTaO7 should be
moved more easily than those in Bi2GaTaO7. The mobility
of the photoinduced electrons and holes affect the ability
of H2 and O2 evolution because it affects the probability
of electrons and holes to reach reaction sites on the sample
surface.
3.2. Some photophysical property
Fig. 4 shows the UV-Vis diffuse reflectance spectra of
Bi2MTaO7. Bi2MTaO7 show photo-absorption not only in
the UV light region but also in the visible light region (λ >
Table 2
Selected bond lengths (Å) and bond angles (◦) of Bi2MTaO7 (M = Ga
and In) obtained by the Rietveld structure refinement
Bi2GaTaO7
Bi2InTaO7
Bi–O1
Bi–O2
M–O1
Mean Bi–O
M–O1–M
2.521
2.262
2.053
2.391
128.3
2.805
2.330
2.004
2.567
143.4

J. Wang et al. / Journal of Alloys and Compounds 377 (2004) 248–252
251
(a) Bi2GaTaO7
0.12
(b) Bi2InTaO7
: Bi2InT aO7
: Bi2GaTaO7
(a)
0.10
0.08
0.06
0.04
0.02
0.00
(b)
250
300
350
400
450
500
Wave length / nm
Fig. 4. UV-Vis diffuse reflectance spectra of Bi2MTaO7 (M = Ga and In).
Fig. 6. Results of the O2 evolution over Bi2MTaO7 (M = Ga and In) from
an aqueous Ce(SO4)2 solution under UV light irradiation (Sample: 1.0 g;
Ce(SO4)2: 1 mmol; pure H2O: 370 cm3; light source: 400 W high-pressure
Hg lamp).
400 nm) though the photo-absorption in the visible light
region is weak. Compared with Bi2InTaO7, the onset of
Bi2GaTaO7 is shifted to shorter wavelength side, indicating
a wider energy gap than that of Bi2InTaO7. Their energy
gaps, estimated by plots of the square root of Kubelra–Munk
functions F(R) against photon energy [9], are 3.0(2) eV for
Bi2GaTaO7 and 2.9(1) eV for Bi2InTaO7. Figs. 5 and 6
shows the results of the H2 and O2 evolutions from an aque-
ous CH3OH solution and an aqueous Ce(SO4)2 solution un-
der UV light irradiation over Bi2MTaO7, respectively. Rates
of the H2 and O2 evolutions and some physical properties
were listed in Table 3. For a comparison, the results of the
H2 and O2 evolution on the reported Bi2MNbO7 (M = Ga
and In) double oxides were also included in Table 3 [2].
The rates of the H2 evolution were estimated to be
0.11 mmol h−1 for Bi2GaTaO7 and 0.99 mmol h−1 for
Bi2InTaO7 in the first 10 h. The rate of the H2 evolution for
Bi2InTaO7 is significantly higher than that of Bi2GaTaO7.
This is consistent with the expectation from their structural
feature as described above. The ability of the H2 evolution
for Bi2InTaO7 is larger than that of Bi2GaTaO7 since the lat-
tice distortion of Bi2InTaO7 is larger than that of Bi2GaTaO7
and the angle between the corner-linked MO6 octahedral for
Bi2InTaO7 is closer to 180 than that of Bi2GaTaO7. More-
over, the rate of the H2 evolution over Bi2InTaO7 is also
higher than that of the reported Bi2MNbO7 (M = Ga and In)
double oxides. The rates of the O2 evolution were estimated
to be 0.005 mmol h−1 for Bi2GaTaO7 and 0.01 mmol h−1
for Bi2InTaO7 in the first 10 h as listed in Table 3.
The effect of UV light irradiation was also observed by
light on/off shutter studies. The H2 evolution stopped when
the UV light irradiation was terminated. This phenomenon
showed that the reaction of the H2 evolution was induced
by the UV light irradiation. The similar rate was produced
after the system was evacuated. Meanwhile, the stabilities of
Bi2MTaO7 were checked by XRD analysis. Bi2MTaO7 are
stable under the UV light irradiation since no change can be
found in their XRD patterns before and after the reactions.
10
: Bi2InT aO7
: Bi2GaTaO7
Table 3
8
6
4
2
0
Some physical properties and the rates of the H2 and O2 evolution from
an aqueous CH3OH solution and an aqueous Ce(SO4)2 solution over
Bi2MTaO7 and the reported Bi2MNbO7 (M = Ga and In) double oxides
Double
oxide
Lattice
parameter
(Å)a
Band
gap
(eV)
Rate of gas
evolution
b
(mmol h−1
)
c
H2
O2
Bi2GaTaO7
Bi2InTaO7
Bi2GaNbO7
Bi2InNbO7
10.4478(3)
10.7612(2)
10.7342(2)
10.7793(2)
3.0
2.9
2.8
2.7
0.11
0.99
0.30
0.18
0.005
0.01
0.01
This study
This study
[2]
0.007
[2]
0
2
4
6
8
10
a
The cubic system with space group Fd3m was obtained by the
Time / h
Rietveld structure refinement.
b
Measured in an inner-irradiation quartz cell irradiated by a 400 W
Fig. 5. Results of the H2 evolution over Bi2MTaO7 (M = Ga and In)
from an aqueous CH3OH solution under UV light irradiation (Sample:
2.0 g; CH3OH: 50 cm3; pure H2O: 320 cm3; 0.2 wt.% Pt; light source:
400 W high-pressure Hg lamp).
high-pressure Hg lamp.
c
Pt was loaded on the surface of powder sample (Sample: 2.0 g;
CH3OH: 50 cm3; pure H2O: 320 cm3; 0.2 wt.% Pt).

252
J. Wang et al. / Journal of Alloys and Compounds 377 (2004) 248–252
4. Conclusion
double oxides. These results suggest that the abilities of
the H2 and O2 evolution over Bi2MTaO7 might be corre-
lated with the lattice distortions and the angles between the
corner-linked MO6 octahedral.
The Rietveld structural analysis indicate that Bi2MTaO7
(M = Ga and In) consist of a three-dimensional network
structure of corner-linked MO6 (M = Ga, In, and Ta)
octahedral. The lattice parameters, lattice distortions and
the angles between the corner-linked MO6 octahedral are
different due to the different effective ionic radii of M3+
(M = Ga and In).
References
[1] Z. Zou, J. Ye, H. Arakawa, Chem. Mater. 13 (2001) 1765.
[2] Z. Zou, J. Ye, H. Arakawa, Chem. Phys. Lett. 333 (2001) 57.
[3] Z. Zou, J. Ye, H. Arakawa, J. Phys. Chem. B 106 (2002) 517.
[4] F. Izumi, J. Cryst. Assoc. Jpn. 27 (1985) 23.
[5] A. Kudo, J. Ceram. Soc. Jpn. 109 (2001) 81.
[6] R.D. Shannon, Acta Cryst. A 32 (1976) 751.
[7] M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Prog. Solid
State Chem. 5 (1983) 55.
[8] M. Wiegel, W. Middel, G. Blasse, J. Mater. Chem. 5 (1995) 981.
[9] Y. Kim II, S. Atherton, E.S. Brigham, T.E. Mallouk, J. Phys. Chem.
219 (1993) 172.
Bi2MTaO7 show photo-absorption not only in the UV
light region but also in the visible light region (λ > 400 nm)
though the photo-absorption in the visible light region is
weak. The energy gaps of Bi2MTaO7 were estimated to be
3.0(2) eV for Bi2GaTaO7 and 2.9(1) eV for Bi2InTaO7. The
ability of the H2 evolution over Bi2InTaO7 from an aque-
ous CH3OH solution under UV light irradiation was clearly
larger than that of Bi2GaTaO7 and the reported Bi2MNbO7