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Influence of Bi3+ ion displacement on the soft mode phase transition in Bi2Ti4O11
Physics Letters A 175 (1993) 246—251 North-Holland
PHYSICS LETTERS A
Influence of Bi3~ion displacement on the soft mode phase transition in Bi
2Ti4O11
*
Guangtin Zou, Jinfang Meng, Qiliang Cui, Yongnian Zhao and Dongmei Li State Key Laboratory ofSuperhard Materials, Jilin University. Changchun 130023, China Received 25 November 1992; revised manuscript received 15 January 1993; accepted for publication 12 February 1993 Communicated by J. Flouquet
The Raman spectra of Bi2Ti4O11 at non-hydrostatic pressure and Bi2Ti38Te02O1, at hydrostatic pressure 3~ionasdisplacement. well as at high temperature reveal that the soft mode phase transition in Bi2Ti4O11 can be connected with the unstable Bi
1. Introduction
2. Experimental
The existence of Bi 2Ti4O11 was demonstrated by Subbarao [1], who reported its X-ray powder diffraction pattern. Since then single crystals of Bi2Ti4O11 have been grown by hydrothermal crystallization and from oxidic melts [2]. Its space group at ambient pressure and temperature is C~h[91.The Raman spectra at high temperature indicated that the phase transition at 250°C was by softt) driven with increasing ening of the [4]. soft In mode cm study, the Raman temperature our (38 previous spectra of Bi 2Ti4O11 at hydrostatic pressure showed that a pressure-induced phase transition occurring at 3.7 GPa could also be connected with softening of the soft optical mode [5]. The purpose of the present paper is to investigate the mechanism of the pressure-induced soft mode phase transition of Bi2Ti4O11 by the change ofthe pressure-transmitting medium and the discussion of the Raman spectra of Bi2Ti38Te02O11 at hydrostatic pressure as well as high temperature. As far as we know, this is the first investigation of the effect of the pressure-transmitting medium on a soft mode phase transition.
The methods of preparation and instrumentation used were in agreement with those of ref. [1]. The powder samples of Bi2Ti38Te0,2011 have been synthesized from a blended mixture of the chemical reagents Bi203 (99.5%), Ti02 (99.9%) and Te02
~
¶4—~9L11111IThh1
4~~9 * Supported by the National Natural Science Foundation of
China,
246
Fig. 1. Schematic diagram of the measuring system for Raman spectra and pressure distribution in the sample cell. 1: Ar~laser, 2: filter, 3: camera, 4: eyepiece, 5: camera display screen, 6: spectrometer, 7: Datemate unit, 8: plotter, 9: diamond anvil cell.
0375-9601/93/s 06.00 C 1993 Elsevier SciencePublishers B.V. All rights reserved.
Volume 175, number 3,4
12 April 1993
PHYSICS LETTERS A
(99.99%) in the mole volume ratio TiOz:TeOz: Bi203 = 3.8 : 0.2 : 1, by a solid state reaction in a covered crucible at a temperature of 800-l 100” C for 8 h. The Raman spectra and the powder X-ray diffraction patterns of BiZTi3.8Te0.20LLresembled sufficiently those of Bi,Ti,O,,. Hence, it was reasonably considered that its lower temperature phase has still a centrosymmetric structure with a C$, space group. The backscattering Raman spectra at room temperature and Nigh pressure were measured by use of a miniature diamond anvil cell (DAC) and a SPEX 1403 Ramalog system. A 5 14.5 nm line from an argon-ion laser was used as a light source at an output power of 300 mW. Two kinds of pressure-transmitting media were used in the present study: The first was a fluid mixture of methanol, ethanol and water in the volume ratio 16: 3: 1, which is a hydrostatic pressure-transmitting medium below 14.6 GPa. The second was the sample itself, which is a non-hydrostatic pressure-transmitting medium. The pressure was calibrated by the well-known ruby fluorescence method. In this paper the Raman spectrum and pressure measurements at hydrostatic pressure, nonhydrostatic pressure conditions and high temperature were completed using the spectral system shown in fig. 1.
15
65
20
BAYAN SHIFT
68
116
(cm-‘)
Fig. 2. Raman spectra of two low frequency phonon modes in Bi2Ti,01, at ambient temperature and various pressures. (a) Hydrostatic pressure; (b) non-hydrostatic pressure.
3. Res&s and discussion Two low frequency Raman spectra of Bi2Ti40LLat hydrostatic, non-hydrostatic pressure and ambient temperature are shown in fig. 2. The pressure dependence of the low frequency Raman spectra (below 265 cm-‘) at non-hydrostatic pressure is shown in fig. 3. It should be noted that the marked pressures in tig. 2h all correspond to those at the centre of the sample cell. The pressure dependence of the linewidths of the soft mode is shown in fig. 4. The experimental results show that the dependence of some properties of the soft mode on pressure is obviously affected by the pressure-transmitting medium. From hiydrostatic pressure to non-hydrostatic pressure the linewidths of the soft mode (38 cm-’ ) are widened, the phase transition pressure has a wider range (4-5.5 GPa) and the soft mode frequency at the phase transition increased from 20 to 34 cm-‘.
In fig. 3 the pressure dependences of the modes corresponding to 64, 128 and 192 cm-’ are the same as those of BiZTi401, at hydrostatic pressure. The low frequency Raman spectra of Bi2Ti3.8Te0.2011and the pressure as well as the temperature dependence of the corresponding phonon modes are shown in figs. 5a, 5b and 6a, 6b, respectively. It is interesting to note that the partial substitution of the Te“+ ion for the Ti4+ ion in Bi,Ti,O,i causes the phase transition pressure or the temperature of the soft mode to increase from 3.7 to 4 GPa or 240 to 26O”C, respectively. The Raman spectra of the doped system BizTi,_,Te,Or, (x=0, 0.1, 0.2, .... 4) at ambient temperature and pressure have revealed that in the range x= O-4 the lowest frequency mode ( 38 cm-’ ) does not “soften” with the Te4+ ion concentration x whereas in BiZ_-xNdxTidO1lthe substitution of the 247
Volume 175, number 3,4
PHYSICS LETTERS A
16
220
~
I~ ~
A
A A
A AA
A A
A
(b)
A
A
6
120
~
60
40
12 April 1993
-
-
.
14
9
AAA
—
A~
468 PRESSURE (GPa)
~
—
1D
Fig. 4. Pressure dependence of the linewidth of the soft mode in 0 2 0
I
2
10
PRESSURE (GPa) Fig. 3. Pressure dependence of the Raman frequencies (below 265 cm—’) in Bi2Ti4O,, at non-hydrostatic pressure.
3~ion for the Bi3~ion causes a “softening” of the Nd mode up to x = 0.3 [6]. These facts show that soft the soft mode in Bi 2Ti4O,, might 3~and Bi3~ ions. originate a vibration between Hence, it isBifurther proposed that the pressureinduced phase transition occurring in Bi2Ti4O,,Due can 3~ion displacement. be the related to theof unstable Bi to structure Bi 3’~ion obviously 2Ti4O,, each Bi coordination deviates from the centre of its oxygen polyhedron [3]. With the increase of the external pressure or temperature it is impossible for the Bi3~ ions to stay at their original positions so that they 3’~ might move in one direction. Aof displacement Bi ions will cause a polarization its oxygen of coordination polyhedron. As a result the structure of the sample is changed. On the other hand, because of the layer—lattice structure of 211, the Bi3~ions in the unit cell are easily affected by the hydrostatic pressure-transmitting medium, so that the displacement of the Bi3~ions with pressure may be syn248
Bi pressure. 2Ti4O,,. (a) Hydrostatic pressure; (b) non-hydrostatic
chronous. So the sensitivity of the soft mode to pressure will increase and the phase transition pressure decrease. The better the pressure-transmitting properties of the the phase pressure will medium, decrease.the As more it is well knowntransition that the fluid mixture of methanol, ethanol and water in the volume ratio the 16:3: 1 is transition the best pressure-transmitting medium, phase pressure and the corresponding soft mode frequency of Bi 2Ti4O,, will reach aThis minimum of 3.7 GPa 20 cm’, respectively. is different from theand situation ofthe nonsoft mode phase transition of some materials, in which the phase transition pressure increases with improvement ofthe pressure-transmittingproperties of the medium, such as Bi 203 [7]. A pressure distribution the samplewhich cell under non-hydrostatic pressure in conditions, was measured with the microspectrum system of fig. 1, is shown in fig. 7. It shows that the pressure at the centre of the sample cell is 5 GPa when the pressure on the edges is about 4 GPa. In the sample cell this large pressure gradient, which increases with increasing pressure, will produce a large shearing stress acting on the Bi3” ions. Now the displacement of the
Volume 175, numb r 3,4
PHYSICS LETTERSA
12 April 1993
250
(b)
(a)
P(GPa)
~I
~‘11~ik~~ ~IT
___
200 ‘~
~
4.05
III
0 0
0 Dm0 00~ 00
0
00
Oo
0o~00~ 0 00
~150
0
0
0
U
3.65
-~
____________
LU100
‘!~
50
15
00
4.58
___
8& ‘I
5—
00 0
140
285
8.o
:
::
:::::::‘0:~~::
~.o
~.o
PRESSURE (GPo)
RAMAN SHIFT (cm~°) Fig. 5. (a) Ranian spectra of low frequency phonon modes in Bi 2Ti38Te020,, at various pressures. (b) Pressure dependence of the Raman frequencies in Bi2Ti38Te0•20,, at hydrostatic pressure.
3~ions may be asynchronous with increasing presBi sure. In fact, the soft mode frequencymeasured is an average one. I~ence,the sensitivity of the soft mode to the external pressure will decrease.This is the rcason why the l~newidthof the soft mode is widened and the phase transition pressure is in a wider range. On the other hand, an alternative picture of the transition, in which an electronic instability at the Bi3~ion is ofprimary importance, seems to us to be more attractive as it provides at least a qualitative picture ofthe optical properties ofthe material, which are extremely difficult to explain solely in terms of the soft mode ~nodel.We shall attempt to explain the mechanism of~the Bi3~ion displacement by the lone electron pair model of the ferroelastic distortion. 4~has an unfilled 3d shell, the transfer of Since Tifrom the Bi3~lone electron pair to the electrons empty 3d orbital of the Ti4~ion via the oxygen ions is possible witJh increasing external pressure or temperature. The oxygen ions can be considered as “bridges” in a Bi—O—Ti system with an effective
transfer ofcharge to and from Bi—O and Ti—O bonds. In Bi 4~, sincefilled, the fullcannot 3d’°orbitals in Te which2Ti3,8Te0•20,, are completely accommodate more electrons, the process of electron transfer is Severely restricted. The charge transfer channels are, therefore, blocked preventing the Bi3~ions from “throwing” their lone pairs. This is connected with the Bi3~ion displacement. The more the number of Te4~ions increases, the more the number of Bi3~ ions moving in one direction decreases. As a result the sensitivity of the soft mode to pressure or temperature is lowered. So the result of partial substitution ofTe4~ions for Ti4~ions is to make the phase transition pressure or temperature of Bi 2Ti4O,, increase. In Bi2Te4O,,(38 wecm-’) did observe thatsoften the lowest Raman frequency does not with the increase of the external pressure up to 8 GPa or of temperature up to 600°C.According to the above discussion we consider that the Ti4~ion concentration (x) dependence ofthe phase transition pressure or temperature is very similar to the dependence of 249
Volume 175, number 3,4
PHYSICS LETTERS A
900
12 April 1993
-___________________________________________________
700
~
285
~ 500
r—~
•
•
•
-
-
~300
-
:
--~-----~---~--
‘—~
‘-
-
~iiii~iIii~iiiiiit:______ ~
100
~*
_____________________
________________
16
607. 5
1000
ItAlIAN SHIFT (cm°)
100
200 TEMPERATURE
300
400
(~(•~}
Fig. 6. (a) Raman spectra of low frequency phonon modes in Bi
2Ti33Te02O,, at various temperatures. (b) Temperature dependence of
the Raman frequencies in Bi2Ti38Te0.2011.
5 -
~
-....
-
--
--.
- -
..
-
I
100
-.
I
-50
0
50
100
11 ~ ~) Fig. 7. Diagram of the measurement of the pressure distribution in the sample cell. the phase transition pressure on the pressure-transmitting medium. 3 + ions will reach However, value the displacement Bi a saturation at the phaseof transition pressure. In 250
fact, the effect ofthe lone electron pair is to produce an aspherical ion which is then displaced from its central position in its potential well. The centre of this potential well, which is produced by the surrounding eight oxygen ions, is constrained by3~ion symmetry to remain undisplaced. Thus theexperience Bi displacement increases, the Bi3~ ionsas will a steepening repulsive potential which will hinder any further displacement and, therefore, produce saturation. As the external pressure remains unchanged, the saturation number of Bi3~ions increases with the improvement of the pressure-transmitting media. Because of the two causes referred to above the displacement of the Bi3 + ions will reach a maximum at the phase transition pressure. This is in agreement with the result that the soft mode frequency in Bi 2Ti4O,, will reach a minimum of 20 cm’ (P~=3.7 GPa) with increasing hydrostaticofthe pressure. In summary, the improvement properties of the pressure-transmitting medium and the partial
Volume 175,
number 3,4
PHYSICS LETTERS A
substitution of Te4~ions for Ti4~ions all can cause the phase transition pressure of Bi 2Ti4O,, to increase. The ptesent results show the importance of the 3~ soft optic modeinand in particular thetransition, resulting displacement driving the phase Bi also suggest that the true order parameter of the but system may be electronic in origin and associated with the ability of the Bi3~ion to “throw” a lone electron pair and hence displace it from its original position.
12 April 1993
References
(1] E.C. Subbarao, J. Am. Ceram. Soc. 45 (1962) 564. [2] L.V. M.V.Petshkova, BarsukovaS.P. et al., J. Cryst. Dokl. Growth 13/14 (1972) [3] Dmitriva, Akad. Nauk SSSR530. 216 (1974) 544. [4] K. Hisao and K. Toda, SolidQ. State (1977) 247. [5] J. Meng, 0. Zou, Y. Zhao, CuiCommun. and D. Li,24Phys. Lett. A 163 (1992) 135. [6] Zou Guangtian, Meng Jinfang, Cui Qiiang, Zhao Yongnian and Li Dongmei, Chin. Sd. Bull. 4 (1992)321. [7] Zhenxian Liu, Zou Guangtian and Wang Lizhong, Chin. J. High Press. Phys. 4 (1990) 81.
251
PHYSICS LETTERS A
Influence of Bi3~ion displacement on the soft mode phase transition in Bi
2Ti4O11
*
Guangtin Zou, Jinfang Meng, Qiliang Cui, Yongnian Zhao and Dongmei Li State Key Laboratory ofSuperhard Materials, Jilin University. Changchun 130023, China Received 25 November 1992; revised manuscript received 15 January 1993; accepted for publication 12 February 1993 Communicated by J. Flouquet
The Raman spectra of Bi2Ti4O11 at non-hydrostatic pressure and Bi2Ti38Te02O1, at hydrostatic pressure 3~ionasdisplacement. well as at high temperature reveal that the soft mode phase transition in Bi2Ti4O11 can be connected with the unstable Bi
1. Introduction
2. Experimental
The existence of Bi 2Ti4O11 was demonstrated by Subbarao [1], who reported its X-ray powder diffraction pattern. Since then single crystals of Bi2Ti4O11 have been grown by hydrothermal crystallization and from oxidic melts [2]. Its space group at ambient pressure and temperature is C~h[91.The Raman spectra at high temperature indicated that the phase transition at 250°C was by softt) driven with increasing ening of the [4]. soft In mode cm study, the Raman temperature our (38 previous spectra of Bi 2Ti4O11 at hydrostatic pressure showed that a pressure-induced phase transition occurring at 3.7 GPa could also be connected with softening of the soft optical mode [5]. The purpose of the present paper is to investigate the mechanism of the pressure-induced soft mode phase transition of Bi2Ti4O11 by the change ofthe pressure-transmitting medium and the discussion of the Raman spectra of Bi2Ti38Te02O11 at hydrostatic pressure as well as high temperature. As far as we know, this is the first investigation of the effect of the pressure-transmitting medium on a soft mode phase transition.
The methods of preparation and instrumentation used were in agreement with those of ref. [1]. The powder samples of Bi2Ti38Te0,2011 have been synthesized from a blended mixture of the chemical reagents Bi203 (99.5%), Ti02 (99.9%) and Te02
~
¶4—~9L11111IThh1
4~~9 * Supported by the National Natural Science Foundation of
China,
246
Fig. 1. Schematic diagram of the measuring system for Raman spectra and pressure distribution in the sample cell. 1: Ar~laser, 2: filter, 3: camera, 4: eyepiece, 5: camera display screen, 6: spectrometer, 7: Datemate unit, 8: plotter, 9: diamond anvil cell.
0375-9601/93/s 06.00 C 1993 Elsevier SciencePublishers B.V. All rights reserved.
Volume 175, number 3,4
12 April 1993
PHYSICS LETTERS A
(99.99%) in the mole volume ratio TiOz:TeOz: Bi203 = 3.8 : 0.2 : 1, by a solid state reaction in a covered crucible at a temperature of 800-l 100” C for 8 h. The Raman spectra and the powder X-ray diffraction patterns of BiZTi3.8Te0.20LLresembled sufficiently those of Bi,Ti,O,,. Hence, it was reasonably considered that its lower temperature phase has still a centrosymmetric structure with a C$, space group. The backscattering Raman spectra at room temperature and Nigh pressure were measured by use of a miniature diamond anvil cell (DAC) and a SPEX 1403 Ramalog system. A 5 14.5 nm line from an argon-ion laser was used as a light source at an output power of 300 mW. Two kinds of pressure-transmitting media were used in the present study: The first was a fluid mixture of methanol, ethanol and water in the volume ratio 16: 3: 1, which is a hydrostatic pressure-transmitting medium below 14.6 GPa. The second was the sample itself, which is a non-hydrostatic pressure-transmitting medium. The pressure was calibrated by the well-known ruby fluorescence method. In this paper the Raman spectrum and pressure measurements at hydrostatic pressure, nonhydrostatic pressure conditions and high temperature were completed using the spectral system shown in fig. 1.
15
65
20
BAYAN SHIFT
68
116
(cm-‘)
Fig. 2. Raman spectra of two low frequency phonon modes in Bi2Ti,01, at ambient temperature and various pressures. (a) Hydrostatic pressure; (b) non-hydrostatic pressure.
3. Res&s and discussion Two low frequency Raman spectra of Bi2Ti40LLat hydrostatic, non-hydrostatic pressure and ambient temperature are shown in fig. 2. The pressure dependence of the low frequency Raman spectra (below 265 cm-‘) at non-hydrostatic pressure is shown in fig. 3. It should be noted that the marked pressures in tig. 2h all correspond to those at the centre of the sample cell. The pressure dependence of the linewidths of the soft mode is shown in fig. 4. The experimental results show that the dependence of some properties of the soft mode on pressure is obviously affected by the pressure-transmitting medium. From hiydrostatic pressure to non-hydrostatic pressure the linewidths of the soft mode (38 cm-’ ) are widened, the phase transition pressure has a wider range (4-5.5 GPa) and the soft mode frequency at the phase transition increased from 20 to 34 cm-‘.
In fig. 3 the pressure dependences of the modes corresponding to 64, 128 and 192 cm-’ are the same as those of BiZTi401, at hydrostatic pressure. The low frequency Raman spectra of Bi2Ti3.8Te0.2011and the pressure as well as the temperature dependence of the corresponding phonon modes are shown in figs. 5a, 5b and 6a, 6b, respectively. It is interesting to note that the partial substitution of the Te“+ ion for the Ti4+ ion in Bi,Ti,O,i causes the phase transition pressure or the temperature of the soft mode to increase from 3.7 to 4 GPa or 240 to 26O”C, respectively. The Raman spectra of the doped system BizTi,_,Te,Or, (x=0, 0.1, 0.2, .... 4) at ambient temperature and pressure have revealed that in the range x= O-4 the lowest frequency mode ( 38 cm-’ ) does not “soften” with the Te4+ ion concentration x whereas in BiZ_-xNdxTidO1lthe substitution of the 247
Volume 175, number 3,4
PHYSICS LETTERS A
16
220
~
I~ ~
A
A A
A AA
A A
A
(b)
A
A
6
120
~
60
40
12 April 1993
-
-
.
14
9
AAA
—
A~
468 PRESSURE (GPa)
~
—
1D
Fig. 4. Pressure dependence of the linewidth of the soft mode in 0 2 0
I
2
10
PRESSURE (GPa) Fig. 3. Pressure dependence of the Raman frequencies (below 265 cm—’) in Bi2Ti4O,, at non-hydrostatic pressure.
3~ion for the Bi3~ion causes a “softening” of the Nd mode up to x = 0.3 [6]. These facts show that soft the soft mode in Bi 2Ti4O,, might 3~and Bi3~ ions. originate a vibration between Hence, it isBifurther proposed that the pressureinduced phase transition occurring in Bi2Ti4O,,Due can 3~ion displacement. be the related to theof unstable Bi to structure Bi 3’~ion obviously 2Ti4O,, each Bi coordination deviates from the centre of its oxygen polyhedron [3]. With the increase of the external pressure or temperature it is impossible for the Bi3~ ions to stay at their original positions so that they 3’~ might move in one direction. Aof displacement Bi ions will cause a polarization its oxygen of coordination polyhedron. As a result the structure of the sample is changed. On the other hand, because of the layer—lattice structure of 211, the Bi3~ions in the unit cell are easily affected by the hydrostatic pressure-transmitting medium, so that the displacement of the Bi3~ions with pressure may be syn248
Bi pressure. 2Ti4O,,. (a) Hydrostatic pressure; (b) non-hydrostatic
chronous. So the sensitivity of the soft mode to pressure will increase and the phase transition pressure decrease. The better the pressure-transmitting properties of the the phase pressure will medium, decrease.the As more it is well knowntransition that the fluid mixture of methanol, ethanol and water in the volume ratio the 16:3: 1 is transition the best pressure-transmitting medium, phase pressure and the corresponding soft mode frequency of Bi 2Ti4O,, will reach aThis minimum of 3.7 GPa 20 cm’, respectively. is different from theand situation ofthe nonsoft mode phase transition of some materials, in which the phase transition pressure increases with improvement ofthe pressure-transmittingproperties of the medium, such as Bi 203 [7]. A pressure distribution the samplewhich cell under non-hydrostatic pressure in conditions, was measured with the microspectrum system of fig. 1, is shown in fig. 7. It shows that the pressure at the centre of the sample cell is 5 GPa when the pressure on the edges is about 4 GPa. In the sample cell this large pressure gradient, which increases with increasing pressure, will produce a large shearing stress acting on the Bi3” ions. Now the displacement of the
Volume 175, numb r 3,4
PHYSICS LETTERSA
12 April 1993
250
(b)
(a)
P(GPa)
~I
~‘11~ik~~ ~IT
___
200 ‘~
~
4.05
III
0 0
0 Dm0 00~ 00
0
00
Oo
0o~00~ 0 00
~150
0
0
0
U
3.65
-~
____________
LU100
‘!~
50
15
00
4.58
___
8& ‘I
5—
00 0
140
285
8.o
:
::
:::::::‘0:~~::
~.o
~.o
PRESSURE (GPo)
RAMAN SHIFT (cm~°) Fig. 5. (a) Ranian spectra of low frequency phonon modes in Bi 2Ti38Te020,, at various pressures. (b) Pressure dependence of the Raman frequencies in Bi2Ti38Te0•20,, at hydrostatic pressure.
3~ions may be asynchronous with increasing presBi sure. In fact, the soft mode frequencymeasured is an average one. I~ence,the sensitivity of the soft mode to the external pressure will decrease.This is the rcason why the l~newidthof the soft mode is widened and the phase transition pressure is in a wider range. On the other hand, an alternative picture of the transition, in which an electronic instability at the Bi3~ion is ofprimary importance, seems to us to be more attractive as it provides at least a qualitative picture ofthe optical properties ofthe material, which are extremely difficult to explain solely in terms of the soft mode ~nodel.We shall attempt to explain the mechanism of~the Bi3~ion displacement by the lone electron pair model of the ferroelastic distortion. 4~has an unfilled 3d shell, the transfer of Since Tifrom the Bi3~lone electron pair to the electrons empty 3d orbital of the Ti4~ion via the oxygen ions is possible witJh increasing external pressure or temperature. The oxygen ions can be considered as “bridges” in a Bi—O—Ti system with an effective
transfer ofcharge to and from Bi—O and Ti—O bonds. In Bi 4~, sincefilled, the fullcannot 3d’°orbitals in Te which2Ti3,8Te0•20,, are completely accommodate more electrons, the process of electron transfer is Severely restricted. The charge transfer channels are, therefore, blocked preventing the Bi3~ions from “throwing” their lone pairs. This is connected with the Bi3~ion displacement. The more the number of Te4~ions increases, the more the number of Bi3~ ions moving in one direction decreases. As a result the sensitivity of the soft mode to pressure or temperature is lowered. So the result of partial substitution ofTe4~ions for Ti4~ions is to make the phase transition pressure or temperature of Bi 2Ti4O,, increase. In Bi2Te4O,,(38 wecm-’) did observe thatsoften the lowest Raman frequency does not with the increase of the external pressure up to 8 GPa or of temperature up to 600°C.According to the above discussion we consider that the Ti4~ion concentration (x) dependence ofthe phase transition pressure or temperature is very similar to the dependence of 249
Volume 175, number 3,4
PHYSICS LETTERS A
900
12 April 1993
-___________________________________________________
700
~
285
~ 500
r—~
•
•
•
-
-
~300
-
:
--~-----~---~--
‘—~
‘-
-
~iiii~iIii~iiiiiit:______ ~
100
~*
_____________________
________________
16
607. 5
1000
ItAlIAN SHIFT (cm°)
100
200 TEMPERATURE
300
400
(~(•~}
Fig. 6. (a) Raman spectra of low frequency phonon modes in Bi
2Ti33Te02O,, at various temperatures. (b) Temperature dependence of
the Raman frequencies in Bi2Ti38Te0.2011.
5 -
~
-....
-
--
--.
- -
..
-
I
100
-.
I
-50
0
50
100
11 ~ ~) Fig. 7. Diagram of the measurement of the pressure distribution in the sample cell. the phase transition pressure on the pressure-transmitting medium. 3 + ions will reach However, value the displacement Bi a saturation at the phaseof transition pressure. In 250
fact, the effect ofthe lone electron pair is to produce an aspherical ion which is then displaced from its central position in its potential well. The centre of this potential well, which is produced by the surrounding eight oxygen ions, is constrained by3~ion symmetry to remain undisplaced. Thus theexperience Bi displacement increases, the Bi3~ ionsas will a steepening repulsive potential which will hinder any further displacement and, therefore, produce saturation. As the external pressure remains unchanged, the saturation number of Bi3~ions increases with the improvement of the pressure-transmitting media. Because of the two causes referred to above the displacement of the Bi3 + ions will reach a maximum at the phase transition pressure. This is in agreement with the result that the soft mode frequency in Bi 2Ti4O,, will reach a minimum of 20 cm’ (P~=3.7 GPa) with increasing hydrostaticofthe pressure. In summary, the improvement properties of the pressure-transmitting medium and the partial
Volume 175,
number 3,4
PHYSICS LETTERS A
substitution of Te4~ions for Ti4~ions all can cause the phase transition pressure of Bi 2Ti4O,, to increase. The ptesent results show the importance of the 3~ soft optic modeinand in particular thetransition, resulting displacement driving the phase Bi also suggest that the true order parameter of the but system may be electronic in origin and associated with the ability of the Bi3~ion to “throw” a lone electron pair and hence displace it from its original position.
12 April 1993
References
(1] E.C. Subbarao, J. Am. Ceram. Soc. 45 (1962) 564. [2] L.V. M.V.Petshkova, BarsukovaS.P. et al., J. Cryst. Dokl. Growth 13/14 (1972) [3] Dmitriva, Akad. Nauk SSSR530. 216 (1974) 544. [4] K. Hisao and K. Toda, SolidQ. State (1977) 247. [5] J. Meng, 0. Zou, Y. Zhao, CuiCommun. and D. Li,24Phys. Lett. A 163 (1992) 135. [6] Zou Guangtian, Meng Jinfang, Cui Qiiang, Zhao Yongnian and Li Dongmei, Chin. Sd. Bull. 4 (1992)321. [7] Zhenxian Liu, Zou Guangtian and Wang Lizhong, Chin. J. High Press. Phys. 4 (1990) 81.
251