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High pressure studies on the perovskite-type compound BaBiO3
Physica 139 & 140B (1986) 349-352 North-Holland, Amsterdam
HIGH PRESSURE STUDIES ON THE PEROVSKITE-TYPE COMPOUND BaBiO 3 H. S U G I U R A and T. YAMADAYA Physics Department, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama 236, Japan
BaBiO 3 has a pressure-induced phase transition at 8-10 GPa, where the lattice changes from nearly cubic to nearly tetragonal, and a new Raman band stretching mode appears. A similar change in the Raman spectrum is observed in a Sb substituted compound BaSb xBi~ xO3. These observations are considered to indicate that the separation of the valence state is accelerated by compression and Sb substitution, and that the mixed valence state, Bi 3~ and Bi 5+, is attained in the high pressure phase of BaBiO 3.
1. Introduction Experimental approaches to determine the valence state of Bi ions in BaBiO 3 have been performed by using several methods [1-4]. It is, however, difficult to find a consensus among the results. Such results probably indicate the fact that the valence of Bi ions is not so strictly fixed as a mixed state of 3+ and 5+ or a uniform valence of 4+. Crystallographic studies surely suggest the existence of two distinguishable sites for Bi ions [1, 2], but the difference in valence between the two sites is small [5]. It is possible to disturb the valence state of Bi by a change in the external conditions; it is, for example, an effect of compression under high pressure. The results of high pressure studies are often compared with the effect of substitution for ions with smaller ions. In this study Bi ions were substituted with Sb ions in order to make a comparison with the effect of high pressure. The valence states of Bi and Sb ions in BaSb0.sBi0 50 3 are Bi 3+ and Sb 5+ [6]. The evidence, which is also a crystallographic matter, is fairly clear contrary to the case of BaBiO 3. It is, therefore, considered that the effect of Sb substitution is not only the volume shrinkage but also the stabilization of the valence of the rest of the Bi ions. When the valence state changes into another type, or a different kind of ions exists, the order of octahedra will be affected, and a new order will be expected to appear. The stretching mode of
the octahedra should reflect such an aspect. Therefore the Raman spectra were measured with attention to the volume changes with compression and Sb substitution.
2. Experiments BaBiO 3 powder was prepared by the same method used in the previous work [7]. Sb substitution was done by a starting material of Sb20 3 with Sb contents from 0.1 to 0.5 in 0.1 steps. The powder samples were sintered at 800-900°C in air for Raman spectrum measurements, and the pellets were crushed for X-ray diffraction measurements. X-ray diffraction measurements were carried out on BaBiO 3 at high pressures up to about 2 0 G P a by using a diamond anvil cell ( D A C ) apparatus and a D e b y e - S c h e r r e r camera. The X-ray source was an Mo target operated at 50 kV and 10mV. The pressure medium was a 4:1 mixture of methanol and ethanol. The pressure was measured according to the ruby scale [8]. These were the same as the previous measurements [7]. The compression curve of BaBiO3 was reported in the range up to 8 GPa in previous work, and was extended to about 20 GPa in the present work. In measurements on an Sb substituted compound BaSbxBi I xO3 at atmospheric pressure, a diffractometer with Cu target was used. The crystal lattice was assumed to be cubic for the
0378-4363 / 86 / $03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
350
H. Sugiura and T. Yamadaya / High pressure studies on the perovskite-type compound BaBiO~
volume estimation in this compound and BaBiO 3 in the low pressure region. Raman spectra were measured by using a 1 m double monochromator (Ramanol U-1000, Jovin Yvon) and a photon counting system (R464S and C1230, Hamamatsu Photonics). The light source was the 488 nm line of an argon ion laser ( G L G 3200, NEC). The applied light power was about 50 mW. The gate time for photon counting was 20-30s. Raman spectra of BaBiO 3 were measured at high pressures by using D A C apparatus with a pressure medium of n-pentane.
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There was no drastic change in the X-ray diffraction patterns of BaBiO 3 at high pressures up to about 2 0 G P a . Although BaBiO 3 has a monoclinic perovskite-like structure, the deformation from a cubic form is very small and beyond the resolution of our high pressure measurement. Therefore the structure of BaBiO 3 was assumed to be cubic in order to calculate the volume up to 8 GPa. However, a split of only the {2~0} line of the pseudo-cubic cell was observed at high pressure above 8 GPa. The degree of the split is large compared with the monoclinic deformation from a cubic. This is a pressure induced phase transition, but it seems that the basic structure continues to be a perovskite related structure. In a rough estimation similar to the latter the low pressure phase was assumed to be cubic, the high pressure phase was close to a tetragonal structure. Therefore the volume above 8 GPa was estimated for a tetragonal structure. The compression curve is shown in fig. 1 together with the previous results [7]. The anomaly at about 4 GPa, which was previously reported, is a characteristic of the lattice which consists of rigid polyhedra. Although there is likely to be some change in detail in crystal structure, such a small change is beyond the resolution, and it is not expected that the anomaly will be related to the valence of the ion centered at the polyhedron. The difference in octahedral sites exists in the low
o
•
t
5
I
15 10 PRESSURE GPa
20
Fig. 1. Compression curve of BaBiOa. Open and closed squares show the changes of tetragonal c and a parameters with respect to the initial a value.
pressure phase, but it has almost no dependence on the compression up to 8 GPa. On the other hand, the compression above 8 G P a seems to be associated with the heterogeneous deformations of two kinds of octahedron. If the perovskite-like structure remains above 8 GPa, the difference in size or compressibility of the octahedra will have a significant role in compression above 8 GPa. Fig. 2 shows the change of the Raman spectrum of BaBiO 3 with increasing pressure. The observed band is a stretching mode of BiO 6 octahedra. The others were too weak to be investigated. The stretching mode frequency increased with pressure and vanished at 11 GPa. A new band appears around 9 GPa at the high energy side in the neighbourhood of the initial band. This phenomenon is considered to correspond to the phase transition observed in the X-ray diffraction measurements. If so the two bands are related to the same modes of the different phases. The change of the Raman spectrum observed in Sb substitution was similar to that observed in
351
H. Sugiura and T. Yamadaya / High pressure studies on the perovskite-type compound BaBiO 3 I
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peared as a shoulder, and grew with firing as shown in fig. 4. The initially observed band almost disappeared after 200 h, and three other strong Raman bands were observed at 101,346, and 530 cm-~. These new bands are related to a new order, which, although it required many hours to be completed, should be a 1 : 1 order of Bi 3÷ and Sb 5+. If the difference between the two sites for Bi in BaBiO 3 corresponds to the difference in valence state, the valence separation is larger in the high pressure phase than in the low pressure phase. According to this and the new Raman band of BaBiO 3 at high pressures above 9 GPa, the valence state of Bi ions in the high pressure phase of BaBiO 3 should be the mixed state, Bi 3+ and Bi ~+. The low pressure phase is clearly different from the high pressure phase in the Raman spectrum and probably in the compression mechanism. Therefore it is considered that the valence state of low pressure phase of BaBiO 3 is close not to the mixed state but to the uniform state.
Fig. 2. Raman spectra of BaBiO 3 under high pressure.
high pressure experiments. At first, as shown in fig. 3, the change of the stretching mode frequency with the increase of Sb contents is almost the same as the change with pressure in view of the volume compaction. The other change is the appearance of a new band with higher frequency. The intensity of this band depends on the sample treatment. The new band, the frequency of which was 700 cm-l, ap640
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Fig. 3. Stretching mode frequencies in BaSbx Bil_xO3. The solid line shows the change of this frequency in BaBiO~ as a function of compression.
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600 RAMAN
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Fig. 4. Dependence of thermal treatment (900°C in air) on the Raman spectrum of BaSbo ~Bi).~O~.
352
H. Sugiura and T. Yamadaya / High pressure studies on the perovskite-type compound BaBiOa
References [1] D.E. Cox and A.W. Sleight, Solid St. Commun. 19 (1976) 969. [2] D.E. Cox and A.W. Sleight, Acta. Cryst. B35 (1979) 1. [3] J.Th.W, de Hair and G. Blasse, Solid St. Commun. 12 (1973) 727. [4] A.F. Orchard and G. Thornton, J. Chem. Soc. Dalton Trans. (1977) 1238.
[5] L.F. Mattheiss and D.R. Hamann, Phys. Rev. B28 (1983) 4227. [6] G. Thornton and A.J. Jacobson, Acta. Cryst. B34 (1978) 351. [7] H. Sugiura and T. Yamadaya, Solid St. Commun. 49 (1984) 499. [8] G.J. Piermarini, S. Block, J.D. Barnett and R.A. Forman, J. Appl. Phys. 46 (1975) 2774.
HIGH PRESSURE STUDIES ON THE PEROVSKITE-TYPE COMPOUND BaBiO 3 H. S U G I U R A and T. YAMADAYA Physics Department, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama 236, Japan
BaBiO 3 has a pressure-induced phase transition at 8-10 GPa, where the lattice changes from nearly cubic to nearly tetragonal, and a new Raman band stretching mode appears. A similar change in the Raman spectrum is observed in a Sb substituted compound BaSb xBi~ xO3. These observations are considered to indicate that the separation of the valence state is accelerated by compression and Sb substitution, and that the mixed valence state, Bi 3~ and Bi 5+, is attained in the high pressure phase of BaBiO 3.
1. Introduction Experimental approaches to determine the valence state of Bi ions in BaBiO 3 have been performed by using several methods [1-4]. It is, however, difficult to find a consensus among the results. Such results probably indicate the fact that the valence of Bi ions is not so strictly fixed as a mixed state of 3+ and 5+ or a uniform valence of 4+. Crystallographic studies surely suggest the existence of two distinguishable sites for Bi ions [1, 2], but the difference in valence between the two sites is small [5]. It is possible to disturb the valence state of Bi by a change in the external conditions; it is, for example, an effect of compression under high pressure. The results of high pressure studies are often compared with the effect of substitution for ions with smaller ions. In this study Bi ions were substituted with Sb ions in order to make a comparison with the effect of high pressure. The valence states of Bi and Sb ions in BaSb0.sBi0 50 3 are Bi 3+ and Sb 5+ [6]. The evidence, which is also a crystallographic matter, is fairly clear contrary to the case of BaBiO 3. It is, therefore, considered that the effect of Sb substitution is not only the volume shrinkage but also the stabilization of the valence of the rest of the Bi ions. When the valence state changes into another type, or a different kind of ions exists, the order of octahedra will be affected, and a new order will be expected to appear. The stretching mode of
the octahedra should reflect such an aspect. Therefore the Raman spectra were measured with attention to the volume changes with compression and Sb substitution.
2. Experiments BaBiO 3 powder was prepared by the same method used in the previous work [7]. Sb substitution was done by a starting material of Sb20 3 with Sb contents from 0.1 to 0.5 in 0.1 steps. The powder samples were sintered at 800-900°C in air for Raman spectrum measurements, and the pellets were crushed for X-ray diffraction measurements. X-ray diffraction measurements were carried out on BaBiO 3 at high pressures up to about 2 0 G P a by using a diamond anvil cell ( D A C ) apparatus and a D e b y e - S c h e r r e r camera. The X-ray source was an Mo target operated at 50 kV and 10mV. The pressure medium was a 4:1 mixture of methanol and ethanol. The pressure was measured according to the ruby scale [8]. These were the same as the previous measurements [7]. The compression curve of BaBiO3 was reported in the range up to 8 GPa in previous work, and was extended to about 20 GPa in the present work. In measurements on an Sb substituted compound BaSbxBi I xO3 at atmospheric pressure, a diffractometer with Cu target was used. The crystal lattice was assumed to be cubic for the
0378-4363 / 86 / $03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
350
H. Sugiura and T. Yamadaya / High pressure studies on the perovskite-type compound BaBiO~
volume estimation in this compound and BaBiO 3 in the low pressure region. Raman spectra were measured by using a 1 m double monochromator (Ramanol U-1000, Jovin Yvon) and a photon counting system (R464S and C1230, Hamamatsu Photonics). The light source was the 488 nm line of an argon ion laser ( G L G 3200, NEC). The applied light power was about 50 mW. The gate time for photon counting was 20-30s. Raman spectra of BaBiO 3 were measured at high pressures by using D A C apparatus with a pressure medium of n-pentane.
r
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"
•
I
ig
"ulJflilt
3. Results and discussions
III
V/Vo
I
-9C
-88 0
°
IJl
-94
> .92
There was no drastic change in the X-ray diffraction patterns of BaBiO 3 at high pressures up to about 2 0 G P a . Although BaBiO 3 has a monoclinic perovskite-like structure, the deformation from a cubic form is very small and beyond the resolution of our high pressure measurement. Therefore the structure of BaBiO 3 was assumed to be cubic in order to calculate the volume up to 8 GPa. However, a split of only the {2~0} line of the pseudo-cubic cell was observed at high pressure above 8 GPa. The degree of the split is large compared with the monoclinic deformation from a cubic. This is a pressure induced phase transition, but it seems that the basic structure continues to be a perovskite related structure. In a rough estimation similar to the latter the low pressure phase was assumed to be cubic, the high pressure phase was close to a tetragonal structure. Therefore the volume above 8 GPa was estimated for a tetragonal structure. The compression curve is shown in fig. 1 together with the previous results [7]. The anomaly at about 4 GPa, which was previously reported, is a characteristic of the lattice which consists of rigid polyhedra. Although there is likely to be some change in detail in crystal structure, such a small change is beyond the resolution, and it is not expected that the anomaly will be related to the valence of the ion centered at the polyhedron. The difference in octahedral sites exists in the low
o
•
t
5
I
15 10 PRESSURE GPa
20
Fig. 1. Compression curve of BaBiOa. Open and closed squares show the changes of tetragonal c and a parameters with respect to the initial a value.
pressure phase, but it has almost no dependence on the compression up to 8 GPa. On the other hand, the compression above 8 G P a seems to be associated with the heterogeneous deformations of two kinds of octahedron. If the perovskite-like structure remains above 8 GPa, the difference in size or compressibility of the octahedra will have a significant role in compression above 8 GPa. Fig. 2 shows the change of the Raman spectrum of BaBiO 3 with increasing pressure. The observed band is a stretching mode of BiO 6 octahedra. The others were too weak to be investigated. The stretching mode frequency increased with pressure and vanished at 11 GPa. A new band appears around 9 GPa at the high energy side in the neighbourhood of the initial band. This phenomenon is considered to correspond to the phase transition observed in the X-ray diffraction measurements. If so the two bands are related to the same modes of the different phases. The change of the Raman spectrum observed in Sb substitution was similar to that observed in
351
H. Sugiura and T. Yamadaya / High pressure studies on the perovskite-type compound BaBiO 3 I
r
1
r
--
17.9
- -
E D
r0
-- ~
_
_
.
2
--
~
9.2 5.1 3.4 -
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5oo
6;0
8;0
7;0
RAMAN SHIFT / cm -1
peared as a shoulder, and grew with firing as shown in fig. 4. The initially observed band almost disappeared after 200 h, and three other strong Raman bands were observed at 101,346, and 530 cm-~. These new bands are related to a new order, which, although it required many hours to be completed, should be a 1 : 1 order of Bi 3÷ and Sb 5+. If the difference between the two sites for Bi in BaBiO 3 corresponds to the difference in valence state, the valence separation is larger in the high pressure phase than in the low pressure phase. According to this and the new Raman band of BaBiO 3 at high pressures above 9 GPa, the valence state of Bi ions in the high pressure phase of BaBiO 3 should be the mixed state, Bi 3+ and Bi ~+. The low pressure phase is clearly different from the high pressure phase in the Raman spectrum and probably in the compression mechanism. Therefore it is considered that the valence state of low pressure phase of BaBiO 3 is close not to the mixed state but to the uniform state.
Fig. 2. Raman spectra of BaBiO 3 under high pressure.
high pressure experiments. At first, as shown in fig. 3, the change of the stretching mode frequency with the increase of Sb contents is almost the same as the change with pressure in view of the volume compaction. The other change is the appearance of a new band with higher frequency. The intensity of this band depends on the sample treatment. The new band, the frequency of which was 700 cm-l, ap640
tq
[
I
I
i15
z
90
T
E u
6
620
I.--
LL 600 U3 Z < 580 :E <[
5O
rr
56o'
;9
68
;7
66
b5 64 ,93
VOLUME (V/Vo)
Fig. 3. Stretching mode frequencies in BaSbx Bil_xO3. The solid line shows the change of this frequency in BaBiO~ as a function of compression.
I
I
l
500
600 RAMAN
I
I
700 800 SHIFT ! c m "
Fig. 4. Dependence of thermal treatment (900°C in air) on the Raman spectrum of BaSbo ~Bi).~O~.
352
H. Sugiura and T. Yamadaya / High pressure studies on the perovskite-type compound BaBiOa
References [1] D.E. Cox and A.W. Sleight, Solid St. Commun. 19 (1976) 969. [2] D.E. Cox and A.W. Sleight, Acta. Cryst. B35 (1979) 1. [3] J.Th.W, de Hair and G. Blasse, Solid St. Commun. 12 (1973) 727. [4] A.F. Orchard and G. Thornton, J. Chem. Soc. Dalton Trans. (1977) 1238.
[5] L.F. Mattheiss and D.R. Hamann, Phys. Rev. B28 (1983) 4227. [6] G. Thornton and A.J. Jacobson, Acta. Cryst. B34 (1978) 351. [7] H. Sugiura and T. Yamadaya, Solid St. Commun. 49 (1984) 499. [8] G.J. Piermarini, S. Block, J.D. Barnett and R.A. Forman, J. Appl. Phys. 46 (1975) 2774.