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High pressure Raman studies on ABO3 perovskite type structure compound PbHfO3 to 40 GPa: Pressure-induced phase transitions
Pergamon
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HIGH PRESSURE RAMAN STUDIES ON ABO, PEROVSKITE TYPE STRUCTURE COMPOUND PbHfO, TO 40 GPa: PRESSURE-INDUCED PHASE TRANSITIONS A. JAYARAMAN,?
S. K. SHARMA,?
L. C. MING% and S. Y. WANG-f
tHawaii Institute of Geophysics and Planetology, and SDepartment of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, U.S.A. (Received I January 1994; accepted 2 May 1994)
Abstract-Lead hafnate (PbHfO,), an ABO, perovskite family antiferroelectric (AFE), has been investigated up to 40 GPa by high pressure Raman scattering in the diamond anvil cell. In the range 0 to 40 GPa three pressure-induced phase transitions are found to occur, near 2,7 and I5 GPa, as revealed by changes in the Raman spectra features at the above pressures. At pressures above 20 GPa the Raman peaks become very weak and the background scattering becomes very strong. Optical observations under a microscope show that the color of the samples changes from colorless to deep orange by about 40 GPa. We believe that this is connected with the downward motion of the d-band of the transition metal ion. The possible role of the electronic structure in the pressure-induced phase transitions is discussed. The phase transitions are subtle, and optical observations of the domain patterns (AFE domains) indicate that the high pressure phases II and III and possibly also IV are all AFE. X-Ray diffraction studies on PbHID, up to 52 GPa reported in the accompanying paper are in broad agreement with the high pressure Raman study. Keywords: A. oxides, C. high pressure, C. Raman spectroscopy, D. lattice dynamics, D. optical properties.
INTRODUCTION
Lead hafnate (PbHfO,) and lead zirconate (PbZrO,) belong to the ABO, perovskite family of oxides with a distorted cubic structure and are believed to be isomorphous [I]. They are antiferroelectric (AFE) at room temperature and become paraelectric near 500 K, when they undergo a temperature-induced phase change to the cubic perovskite structure [2]. Lead hafnate is reported to have a tetragonal structure at room temperature [2] although the superstructure lines seen in the X-ray diffraction pattern suggest orthorhombic symmetry (pseudotetragonal), as in the case of PbZrO, [3]. The temperature induced transitions in both PbHfO, and PbZrO, have been studied and characterized by X-ray diffraction and dielectric measurements [I, 21. There has been no earlier report of any Raman scattering experiments on PbHfOJ until we undertook a Raman study on the room temperature antiferroelectric phase and on the temperature-induced phase transitions at ambient pressure. The results of these studies, reported elsewhere [4], encouraged us to investigate the high pressure behavior of PbHfO, by Raman scattering and especially to explore pressure-induced phase transitions and their effects on the AFE behavior. Further, high-pressure
phase transitions in perovskite structure compounds are of interest to the earth’s mantle physics and chemistry. The results are presented and discussed in this paper. We have also conducted a detailed high pressure X-ray diffraction study on PbHfO, and these results are the subject of the accompanying paper [5]. EXPERIMENTS
Small single crystals of PbHfO, grown by J. P. Remeika of AT&T Bell Laboratories using the flux method were available to us. These were l-2 mm square pieces of 0.2-l mm in thickness. Some of the crystals were clear and transparent, others were brownish and somewhat opaque. The clear transparent ones showed domain structure between crossed Polaroids, due to antiferroelectric domains. Small crystal platelets were cleaved with a knife for high pressure experiments. Pressure was generated using a diamond cell of the Mao-Bell type, with a medium of a 4: I methanol-ethanol mixture. Pressure was calibrated by the well known ruby fluorescence technique [6]. In general, Raman spectra were recorded using a Spex Raman double monochromator, equipped with a RCA-C31034 PMT tube for photon counting.
1207
A. JAYARAMAN
1208
Raman data were acquired through a Datamate Computer. For Raman excitation the 488.0nm line of an argon ion laser was used at power levels of 20-30 mW on the sample. In some experiments spectra were taken with a Spex Triplemate spectrometer equipped with an OMA detector. All measurements were carried out at room temperature. RESLJLTS
The room temperature-ambient pressure Raman spectrum of PbHfO, is shown in Fig. I. The observed Raman mode frequencies and other relevant information are listed in Table I. Our samples were multidomain single crystals and hence it was not possible to get polarized Raman data to determine the symmetry of the modes. Therefore, only a qualitative assignment of the modes is given, following the earlier work on PbZrG, 171. When crystals were placed in the diamond cell, their orientation could not be controlled. Depending on the differences in the orientation in a particular experiment, the frequencies of some modes shifted, presumably due to quasi mode behavior, and the intensities also differed. Therefore, we had to perform many high pressure experiments to build up a consistent picture. In Fig. 2, we show the Raman spectrum of PbHQ recorded at several pressures. The Raman data from a large number of high pressure runs are plotted in Fig. 3, mode frequency vs pressure. The solid lines and the scatter in the data illustrate the aforementioned difficulties, inherent to this system. The discontinuities in the spectra suggest three pressure-induced phase transitions, at approximately 2 GPa, 7 GPa and 15 GPa. In spectra recorded above 15 GPa the background scattering dominated and only a few strong Raman
et al. Table I. Observed Raman peak frequencies in the orthorhombic room temperature phase of PbHQ W
(cm-‘)
I
593 (sh) 560 532 420 351 334 260 220 200 190 172 124 68 53 49 40
w w% VW s m S
m m m m 5 W
s S VS
du/dP (cm-’ GPa-I) 5.5 7.5 6.0 4.5 2.5 0 0 0 0 0 0 0 0.3 0.2 0 2.5
Band assignments Hf-0 stretch Hf-O stretch Hf-O stretch Torsional mode Torsional mode PbO group Hf-0 bending Hf-O bending Hf-0 bending External mode External mode External mode External mode External mode
vs-very strong, s-strong, m-medium, w-weak, VW-very weak, sh-shoulder and (b)-broad.
peaks were seen above the background level. At pressures higher than 20 GPa it was impossible to observe any distinct Raman peaks in the spectra taken with the double monochromator. Hence, all our Raman measurements with the latter instrument had to be terminated at 20GPa; however, with the Triplemate and OMA detector, a few broad Raman peaks were observed in spectra recorded above 20 GPa. The spectrum recorded at 37.5 GPa on one particular sample is shown in Fig. 4. An interesting finding is that if pressure is released from 15 GPa and below to ambient, the original Raman spectrum of the room temperature orthorhombic phase is obtained. On the other hand, when pressure is released from 18 GPa and higher to ambient, this is not the case, and the spectrum appears to resemble the intermediate temperature y phase. This behavior is shown in Fig. 4, below the spectrum for 37.5 GPa.
E
DISCUSSION t bar
100
200
300
400
500
600
600
Raman shift (cm-‘)
I. Raman spectrum of the otthothombic ambient pressure phase of PbHfO,, recorded with 488 nm excitation and Spex Raman double mon~hromator. was 30 mW.
The laser power
Ambient pressure powder X-ray diffraction studies on PbHfO, have established the structural aspects of the temperature-induced phase transitions [2]. The pressure and temperature dependence of the dielectric behavior were investigated by Samara [I] and confirmed the nature of the temperature ordering of the phases (c( --+ y -+ 6 ) as paraelectric (2) to antiferroelectric (y and S ) as the system was cooled. The transition temperatures are reported as 476 and 433 K with increasing temperature. Further, Samara [ 1,8] reported a pressure-induced antiferroelectric phase (/I) intruding the y-cx transition. The phase boundaries 6/y, r//I and /I/a were delineated by him up to f GPa and these were found to have positive
1209
High pressure Raman studies
I
97GPa
Raman shift (cm-‘)
Fig. 2. Raman spectra of PbHtG, recorded at four different pressures with the Spex Raman double monochromator, under 488 nm excitation, laser power 30mW. The low frequency part has been suppressed with a supernotch filter. The spectra marked 4.3 GPa is the phase II region and 11.5GPa is the phase III region.
&-
-
1 bar (pressurn released)
_ _ :*
.
..L
-I
--
,
D
5
!
I 10
I 15
Pressure @Pa)
Fig. 3. The pressure dependence of the Raman peak frequencies are plotted. The lines drawn through the data indicate the trend. The dotted lines represent the phase transition pressures as judged by discontinuous or distinct changes in the Raman spectral features. The line near 15 GPa drawn from the results based on the X-ray diffraction study [S].
200
490 999 999 1009 1, IO Raman shift (cm-‘)
Fig. 4. The spectra shown on top was recorded at 37.5 GPa using the Spex Triplemate equipped with an OMA detector. Some Raman peaks can be seen but they are weak and ride over a large background scattering. The bottom spectra is on pressure-related sample from 37.5 GPa. It is clear that the sample does not revert to the starting orthorhombic phase. The spectra shown resemble rather the so called ( y ) phase of PbHfO,.
A. JAYARAMAN et af.
12IO
slopes for dT/dP (the transition temperature increasing with pressure) in the range investigated. This appears to be consistent with the behavior expected for perovskite compounds, namely that pressure raises the AFE transition temperature, whereas it lowers the FE transition temperature [I]. This is because of the volume contraction associated with the AFE transition and volume expansion associated with the FE transition, from the thermodynamic point of view. Our ambient pressure Raman investigation [4] on PbHQ supports orthorhombic symmetry for the phase stable at room temperature (S) and further suggests tetragonal symmetry for the (y) phase stable between 438 K and 484 K. Furthermore, the number of observed Raman modes appears to be consistent with the orthorhombic space group (Dif:) Pnma [7] rather than the lower symmetry space group (Cp,)Pha2 with 8 molecules in the unit cell proposed for the isomorphous PbZrO, [3] from X-ray powder diffraction data. However. this question cannot be settied with certainty using the present Raman data, A thorough polarized Raman and infrared investigation would be needed on single domain crystals. Pressure-induced
phase transitions
From the data presented in Fig. 3 we believe that there are three pressure-induced phase transitions in PbHfO,, as shown by the dashed lines. The evidence for the first transition near 2 GPa are the discontinuities associated with the Raman modes near 260 and i60cm-‘. For the second transition, near 7 GPa, discontinuous changes occur for several vibrational modes. Optical observation of the sample between crossed poiaroids indicates striking changes in the domain pattern at both the transitions. We believe that the material is still antiferroelectric, and the structural changes at these transitions are subtle. At pressures higher than 15 GPa the Raman spectrum becomes so weak that it is no longer a useful guide to detect phase changes. We believe that there is definitely a phase transition near IS GPa for the following reason: samples pressurized to 15 GPa and below promptly revert to the original phase (as judged by the Raman spectra) when pressure is released to ambient, whereas this is not the case when samples were pressurized to I8 GPa and higher. We believe this difference in the sample’s reversal characteristics is strong evidence for the occurrence of another phase transition near IS GPa. In fact solid proof for this phase transition comes from the high pressure X-ray diffraction study [5], the results of which are reported in the accompanying paper. Briefly stated, the X-ray diffraction results support a
transition from orthorhombic to possibly tetragonai symmetry near 7GPa, because the superstructure peaks observed in the powder diffraction pattern disappear above this pressure. As for the transition near I5 GPa several powder X-ray diffraction peaks show splitting, and the pattern can be indexed on the basis of an orthorhombic lattice. Thus, the results of Raman and X-ray studies support each other. No distinct changes are observed near 2 GPa in the X-ray diffraction pattern, whereas the Raman results suggest a phase change. We believe that the 2GPa transition is a very subtle one and X-ray powder diffraction is not sensitive enough to reflect the change. Comparison
with PbTiO,
Lead titanate is tetragonal (C:,-P4mm) at room temperature and is a ferroelectric material [9]. It undergoes a temperature induced phase transition at 766 K to the cubic perovskite structure (0 i-Pm3m), which is paraelectric. Lead titanate has been investigated by high pressure Raman spectroscopy [IO, Ii] and has been shown to transfo~ to the paraeiectric cubic perovskite phase near I2 GPa at room temperature, on intersecting the tetragonai to cubic phase boundary. The latter has a negative dT/dP. In the case of PbTi03 the A,(TO) and the E(T0) modes soften with pressure and go to zero frequency near 12 GPa at room temperature. The first-order Raman spectrum disappears at this pressure, as expected for the cubic phase. In this case of PbHfO,, the PE to AFE boundary has a positive slope [I] and hence a pressure-induced transition to the cubic perovskite structure is not expected at compression at room temperature. Perhaps this explains the absence of pressure-induced soft mode behavior similar to that of PbTiO, [lo. I I]. Further, there is no evidence from high pressure X-ray studies on PbHfO, for a cubic perovskite phase up to 50 GPa. Optical absorption
at high pressure
Lead hafnate appears slightly yellow in color near I5 GPa and becomes deep orange by 35 GPa. The change is progressive. We have made a very rough measurement of the optical absorption characteristics of the sample as a function of pressure and estimate the shift in the absorption edge to be a red shift of approximately 25meVGPa-‘. We believe that this shift is due to the downward motion of the 5d band of the transition metal ion. A red shift of 50 + 5 meVGPa_’ was obtained in the case of PbMoO, which yielded a deformation potential of -3.5 eV for PbMoO, [12] with 5, = 64GPa. In the case of PbHfO, (with a bulk modulus of _ I50 GPa)
High pressure Raman studies the shift translates into a deformation -4eV. Electronic sitions
structure,
potential of
bonding and phase tran-
From the X-ray diffraction study, pressure-induced phase changes are found to occur at 8, IS and 45GPa and these are from orthorhombic (I} + tetragonal --+ orthorhombic (II) to phase IV. The pressure-volume data on PbHlD, indicate that there is practically no volume change (A I’) associated with these phase transitions. In fact the pressure volume behavior can be fitted with the Birch-Murnaghan equation of state from 0 to 31 GPa, yielding a value of 169 GPa for the bulk modulus B,. This is far from 142 GPa, obtained by fitting up to only 8 GPa for the orthorhombic phase I of PbHfO,. Therefore, it appears that the observed pressure-induced phase transitions are rather subtle. For these transitions we would like to suggest polyhedral tilting as the mechanism, involving HfO, octohedra, the principal structural building block in the system. Phase transitions caused by tilting of polyhedral units are well known [12]. Although they change the symmetry properties of the system, their effect on volume is negligible. For the pressure-induced phase transition near 45 GPa, we would like to refer to an earlier suggestion [ 131and propose that the driving force is rooted in the changes to the electronic structure associated with the d-states of the transition metal ions with pressure. In this connection a well known effect of application of pressure is the lowering of the d-state towards the valence band because of an increase in the strength of the crystal field splitting. In the present example this is the possible lowering of the d-state of Pb-ions toward the valence band derived from the oxygen p-states. This proposed lowering of the d-state is well supported by the optical absorption characteristics at high pressures, namely the observed red shift. The result would be an increase in p-d mixing. Our view is that p-d interaction is a dominant factor in determining the structure and if the external parameters (pressure and temperature) induce slight changes in the p-d bonding, the rest of the structure will follow. A quantitative analysis would have to include higher order perturbation of the octahedral p-d bond, allowing for slight changes in bond lengths or angles, in the spirit of the “chemical grip” model proposed by Harrison [141. A fundamental question concerns the differences in behavior between PbTiO,, PbZrO, and PbHfO,. Although the former is a ferroelectric compound with a Curie temperature of 760 K, the latter two
1211
are antiferroelectric with T,, of 507 and 484 K for the transition to the paraelectric phase. In seeking an explanation of the observed differences in behavior based on the electronic structure the following has been suggested [IS]. In perovskite structured materials with Ti, Zr or Hf as the B ion, the diffuseness of the outer d-orbitals, 3d, 4d and Sd, respectively, increases as one goes down in the series. Consequently the d,-p, overlap and the covalency of the M-O bond increases. In the framework of the theory of phase transitions based on the pseudo Jahn-Teller effect (PJTE) [l&16], the overlap of the orbitals and the covalency of the M-O bond would weaken the energetics of the FE-AFE ordering and hence the ordering temperature should decrease, as one moves from Ti to Hf. This is in broad agreement with the empirical facts. By the same argument, application of high pressure should increase the said overlap, and therefore, the ordering temperature may be expected to decrease with pressure; however, this does not seem to be the case [I]. Our observations of a domain structure seem to indicate that the AFE ordering continues at least up to 35 GPa in our room temperature experiments. It must be remembered, however, that the theory based on the adiabatic potential and PJTE views the transition as of order-disorder type, whereas the FE-AFE transitions are regarded as displacive type PI. Whatever the deeper theoretical concepts are concerning these transitions, it is important to build an experimental base. In this regard high-pressure Raman and X-ray studies on PbZrO, would be valuable, for they can bring out the systematics of the series PbTiO,, PbZrO, and PbHfO,.
Acknowledgements-This work is supported in part by the National Science Foundation and by the Materials and Applied Science Group at the University of Hawaii. This is School of Ocean and Earth Science and Technology Contribution 3462.
REFERENCES 1. Samara G. A., P&s. Rev. B9, 3777 (1970). 2. Shirane G. and Pepinsky R.. f%ys. Rev. 91, 812 (1953). 3. Jona F., Shirane G., Mazz C. F. and Pepinsky R., P1z.v~ Rev. 105, 849 (19.57). 4. Sharma S. K., Jayaraman A., Chowdhury P. C. and Wang S. Y., J. Raman Spectr. 25, 331 (1994). 5. Ming L. C., Jayaraman A., Shieh S. R., Kim Y. H. and
Manghnani
M. H., J. Phys. Chem.
So/ids 55,
1213
(1994).
Barnett J. D., Block S. and Piermarini G. J., Ret?. Sci. Instrum. 44, 1 (1973). 7. Pasti A. E. and Condrate Sr, R. A., in Adr~ances in Raman Spec~oscopy (Edited by J. P. Mathieu), Vol. 1, p. 196. Heydon, London (1972).
6.
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A. JAYARAMAN
8. Samara G., Phys. Letf. 3OA, 446 (1969). 9. Burns G. and Scott B. A., Phys. Rev. Lett. 25, 167 ( 1970); Phys. Rev. B7,3088 (1973)(and references cited therein). 10. Sanjurjo J. A., Lopez-Cruz E. and Burns G., Phys. Rev. B1, 7260 (1983). I I. Jayaraman A., Remeika J. P. and Katiyar R. S., Mar. Res. Symp. Proc. 22, 165 (1986). 12. Hazen R. M. and Firger L. W., Comparatiue Crystal Chemisrry, pp. 197-214. Wiley, New York (1982).
et al.
13. Jayaraman A., Batlogg B. and VanUitert L. G.. Phys. Rev. B31, 5423 (1985).
14. Harrison W. A., Electronic Structure and the Properties of Solids, p. 459. Freeman, San Francisco (1980). 15. Bersuker I. B., Jahn-Teller Efict and Vibronic Interactions in Modern Chemistry, p. 234. Plenum Press, New York (1984). 16. Bersuker 1. B. and Polinger V. Z. Vibronic Interactions in Molecules and Crystals, p. 344-379. Springer Verlag, New York (1983).
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0022~3697#NooO&4
HIGH PRESSURE RAMAN STUDIES ON ABO, PEROVSKITE TYPE STRUCTURE COMPOUND PbHfO, TO 40 GPa: PRESSURE-INDUCED PHASE TRANSITIONS A. JAYARAMAN,?
S. K. SHARMA,?
L. C. MING% and S. Y. WANG-f
tHawaii Institute of Geophysics and Planetology, and SDepartment of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, U.S.A. (Received I January 1994; accepted 2 May 1994)
Abstract-Lead hafnate (PbHfO,), an ABO, perovskite family antiferroelectric (AFE), has been investigated up to 40 GPa by high pressure Raman scattering in the diamond anvil cell. In the range 0 to 40 GPa three pressure-induced phase transitions are found to occur, near 2,7 and I5 GPa, as revealed by changes in the Raman spectra features at the above pressures. At pressures above 20 GPa the Raman peaks become very weak and the background scattering becomes very strong. Optical observations under a microscope show that the color of the samples changes from colorless to deep orange by about 40 GPa. We believe that this is connected with the downward motion of the d-band of the transition metal ion. The possible role of the electronic structure in the pressure-induced phase transitions is discussed. The phase transitions are subtle, and optical observations of the domain patterns (AFE domains) indicate that the high pressure phases II and III and possibly also IV are all AFE. X-Ray diffraction studies on PbHID, up to 52 GPa reported in the accompanying paper are in broad agreement with the high pressure Raman study. Keywords: A. oxides, C. high pressure, C. Raman spectroscopy, D. lattice dynamics, D. optical properties.
INTRODUCTION
Lead hafnate (PbHfO,) and lead zirconate (PbZrO,) belong to the ABO, perovskite family of oxides with a distorted cubic structure and are believed to be isomorphous [I]. They are antiferroelectric (AFE) at room temperature and become paraelectric near 500 K, when they undergo a temperature-induced phase change to the cubic perovskite structure [2]. Lead hafnate is reported to have a tetragonal structure at room temperature [2] although the superstructure lines seen in the X-ray diffraction pattern suggest orthorhombic symmetry (pseudotetragonal), as in the case of PbZrO, [3]. The temperature induced transitions in both PbHfO, and PbZrO, have been studied and characterized by X-ray diffraction and dielectric measurements [I, 21. There has been no earlier report of any Raman scattering experiments on PbHfOJ until we undertook a Raman study on the room temperature antiferroelectric phase and on the temperature-induced phase transitions at ambient pressure. The results of these studies, reported elsewhere [4], encouraged us to investigate the high pressure behavior of PbHfO, by Raman scattering and especially to explore pressure-induced phase transitions and their effects on the AFE behavior. Further, high-pressure
phase transitions in perovskite structure compounds are of interest to the earth’s mantle physics and chemistry. The results are presented and discussed in this paper. We have also conducted a detailed high pressure X-ray diffraction study on PbHfO, and these results are the subject of the accompanying paper [5]. EXPERIMENTS
Small single crystals of PbHfO, grown by J. P. Remeika of AT&T Bell Laboratories using the flux method were available to us. These were l-2 mm square pieces of 0.2-l mm in thickness. Some of the crystals were clear and transparent, others were brownish and somewhat opaque. The clear transparent ones showed domain structure between crossed Polaroids, due to antiferroelectric domains. Small crystal platelets were cleaved with a knife for high pressure experiments. Pressure was generated using a diamond cell of the Mao-Bell type, with a medium of a 4: I methanol-ethanol mixture. Pressure was calibrated by the well known ruby fluorescence technique [6]. In general, Raman spectra were recorded using a Spex Raman double monochromator, equipped with a RCA-C31034 PMT tube for photon counting.
1207
A. JAYARAMAN
1208
Raman data were acquired through a Datamate Computer. For Raman excitation the 488.0nm line of an argon ion laser was used at power levels of 20-30 mW on the sample. In some experiments spectra were taken with a Spex Triplemate spectrometer equipped with an OMA detector. All measurements were carried out at room temperature. RESLJLTS
The room temperature-ambient pressure Raman spectrum of PbHfO, is shown in Fig. I. The observed Raman mode frequencies and other relevant information are listed in Table I. Our samples were multidomain single crystals and hence it was not possible to get polarized Raman data to determine the symmetry of the modes. Therefore, only a qualitative assignment of the modes is given, following the earlier work on PbZrG, 171. When crystals were placed in the diamond cell, their orientation could not be controlled. Depending on the differences in the orientation in a particular experiment, the frequencies of some modes shifted, presumably due to quasi mode behavior, and the intensities also differed. Therefore, we had to perform many high pressure experiments to build up a consistent picture. In Fig. 2, we show the Raman spectrum of PbHQ recorded at several pressures. The Raman data from a large number of high pressure runs are plotted in Fig. 3, mode frequency vs pressure. The solid lines and the scatter in the data illustrate the aforementioned difficulties, inherent to this system. The discontinuities in the spectra suggest three pressure-induced phase transitions, at approximately 2 GPa, 7 GPa and 15 GPa. In spectra recorded above 15 GPa the background scattering dominated and only a few strong Raman
et al. Table I. Observed Raman peak frequencies in the orthorhombic room temperature phase of PbHQ W
(cm-‘)
I
593 (sh) 560 532 420 351 334 260 220 200 190 172 124 68 53 49 40
w w% VW s m S
m m m m 5 W
s S VS
du/dP (cm-’ GPa-I) 5.5 7.5 6.0 4.5 2.5 0 0 0 0 0 0 0 0.3 0.2 0 2.5
Band assignments Hf-0 stretch Hf-O stretch Hf-O stretch Torsional mode Torsional mode PbO group Hf-0 bending Hf-O bending Hf-0 bending External mode External mode External mode External mode External mode
vs-very strong, s-strong, m-medium, w-weak, VW-very weak, sh-shoulder and (b)-broad.
peaks were seen above the background level. At pressures higher than 20 GPa it was impossible to observe any distinct Raman peaks in the spectra taken with the double monochromator. Hence, all our Raman measurements with the latter instrument had to be terminated at 20GPa; however, with the Triplemate and OMA detector, a few broad Raman peaks were observed in spectra recorded above 20 GPa. The spectrum recorded at 37.5 GPa on one particular sample is shown in Fig. 4. An interesting finding is that if pressure is released from 15 GPa and below to ambient, the original Raman spectrum of the room temperature orthorhombic phase is obtained. On the other hand, when pressure is released from 18 GPa and higher to ambient, this is not the case, and the spectrum appears to resemble the intermediate temperature y phase. This behavior is shown in Fig. 4, below the spectrum for 37.5 GPa.
E
DISCUSSION t bar
100
200
300
400
500
600
600
Raman shift (cm-‘)
I. Raman spectrum of the otthothombic ambient pressure phase of PbHfO,, recorded with 488 nm excitation and Spex Raman double mon~hromator. was 30 mW.
The laser power
Ambient pressure powder X-ray diffraction studies on PbHfO, have established the structural aspects of the temperature-induced phase transitions [2]. The pressure and temperature dependence of the dielectric behavior were investigated by Samara [I] and confirmed the nature of the temperature ordering of the phases (c( --+ y -+ 6 ) as paraelectric (2) to antiferroelectric (y and S ) as the system was cooled. The transition temperatures are reported as 476 and 433 K with increasing temperature. Further, Samara [ 1,8] reported a pressure-induced antiferroelectric phase (/I) intruding the y-cx transition. The phase boundaries 6/y, r//I and /I/a were delineated by him up to f GPa and these were found to have positive
1209
High pressure Raman studies
I
97GPa
Raman shift (cm-‘)
Fig. 2. Raman spectra of PbHtG, recorded at four different pressures with the Spex Raman double monochromator, under 488 nm excitation, laser power 30mW. The low frequency part has been suppressed with a supernotch filter. The spectra marked 4.3 GPa is the phase II region and 11.5GPa is the phase III region.
&-
-
1 bar (pressurn released)
_ _ :*
.
..L
-I
--
,
D
5
!
I 10
I 15
Pressure @Pa)
Fig. 3. The pressure dependence of the Raman peak frequencies are plotted. The lines drawn through the data indicate the trend. The dotted lines represent the phase transition pressures as judged by discontinuous or distinct changes in the Raman spectral features. The line near 15 GPa drawn from the results based on the X-ray diffraction study [S].
200
490 999 999 1009 1, IO Raman shift (cm-‘)
Fig. 4. The spectra shown on top was recorded at 37.5 GPa using the Spex Triplemate equipped with an OMA detector. Some Raman peaks can be seen but they are weak and ride over a large background scattering. The bottom spectra is on pressure-related sample from 37.5 GPa. It is clear that the sample does not revert to the starting orthorhombic phase. The spectra shown resemble rather the so called ( y ) phase of PbHfO,.
A. JAYARAMAN et af.
12IO
slopes for dT/dP (the transition temperature increasing with pressure) in the range investigated. This appears to be consistent with the behavior expected for perovskite compounds, namely that pressure raises the AFE transition temperature, whereas it lowers the FE transition temperature [I]. This is because of the volume contraction associated with the AFE transition and volume expansion associated with the FE transition, from the thermodynamic point of view. Our ambient pressure Raman investigation [4] on PbHQ supports orthorhombic symmetry for the phase stable at room temperature (S) and further suggests tetragonal symmetry for the (y) phase stable between 438 K and 484 K. Furthermore, the number of observed Raman modes appears to be consistent with the orthorhombic space group (Dif:) Pnma [7] rather than the lower symmetry space group (Cp,)Pha2 with 8 molecules in the unit cell proposed for the isomorphous PbZrO, [3] from X-ray powder diffraction data. However. this question cannot be settied with certainty using the present Raman data, A thorough polarized Raman and infrared investigation would be needed on single domain crystals. Pressure-induced
phase transitions
From the data presented in Fig. 3 we believe that there are three pressure-induced phase transitions in PbHfO,, as shown by the dashed lines. The evidence for the first transition near 2 GPa are the discontinuities associated with the Raman modes near 260 and i60cm-‘. For the second transition, near 7 GPa, discontinuous changes occur for several vibrational modes. Optical observation of the sample between crossed poiaroids indicates striking changes in the domain pattern at both the transitions. We believe that the material is still antiferroelectric, and the structural changes at these transitions are subtle. At pressures higher than 15 GPa the Raman spectrum becomes so weak that it is no longer a useful guide to detect phase changes. We believe that there is definitely a phase transition near IS GPa for the following reason: samples pressurized to 15 GPa and below promptly revert to the original phase (as judged by the Raman spectra) when pressure is released to ambient, whereas this is not the case when samples were pressurized to I8 GPa and higher. We believe this difference in the sample’s reversal characteristics is strong evidence for the occurrence of another phase transition near IS GPa. In fact solid proof for this phase transition comes from the high pressure X-ray diffraction study [5], the results of which are reported in the accompanying paper. Briefly stated, the X-ray diffraction results support a
transition from orthorhombic to possibly tetragonai symmetry near 7GPa, because the superstructure peaks observed in the powder diffraction pattern disappear above this pressure. As for the transition near I5 GPa several powder X-ray diffraction peaks show splitting, and the pattern can be indexed on the basis of an orthorhombic lattice. Thus, the results of Raman and X-ray studies support each other. No distinct changes are observed near 2 GPa in the X-ray diffraction pattern, whereas the Raman results suggest a phase change. We believe that the 2GPa transition is a very subtle one and X-ray powder diffraction is not sensitive enough to reflect the change. Comparison
with PbTiO,
Lead titanate is tetragonal (C:,-P4mm) at room temperature and is a ferroelectric material [9]. It undergoes a temperature induced phase transition at 766 K to the cubic perovskite structure (0 i-Pm3m), which is paraelectric. Lead titanate has been investigated by high pressure Raman spectroscopy [IO, Ii] and has been shown to transfo~ to the paraeiectric cubic perovskite phase near I2 GPa at room temperature, on intersecting the tetragonai to cubic phase boundary. The latter has a negative dT/dP. In the case of PbTi03 the A,(TO) and the E(T0) modes soften with pressure and go to zero frequency near 12 GPa at room temperature. The first-order Raman spectrum disappears at this pressure, as expected for the cubic phase. In this case of PbHfO,, the PE to AFE boundary has a positive slope [I] and hence a pressure-induced transition to the cubic perovskite structure is not expected at compression at room temperature. Perhaps this explains the absence of pressure-induced soft mode behavior similar to that of PbTiO, [lo. I I]. Further, there is no evidence from high pressure X-ray studies on PbHfO, for a cubic perovskite phase up to 50 GPa. Optical absorption
at high pressure
Lead hafnate appears slightly yellow in color near I5 GPa and becomes deep orange by 35 GPa. The change is progressive. We have made a very rough measurement of the optical absorption characteristics of the sample as a function of pressure and estimate the shift in the absorption edge to be a red shift of approximately 25meVGPa-‘. We believe that this shift is due to the downward motion of the 5d band of the transition metal ion. A red shift of 50 + 5 meVGPa_’ was obtained in the case of PbMoO, which yielded a deformation potential of -3.5 eV for PbMoO, [12] with 5, = 64GPa. In the case of PbHfO, (with a bulk modulus of _ I50 GPa)
High pressure Raman studies the shift translates into a deformation -4eV. Electronic sitions
structure,
potential of
bonding and phase tran-
From the X-ray diffraction study, pressure-induced phase changes are found to occur at 8, IS and 45GPa and these are from orthorhombic (I} + tetragonal --+ orthorhombic (II) to phase IV. The pressure-volume data on PbHlD, indicate that there is practically no volume change (A I’) associated with these phase transitions. In fact the pressure volume behavior can be fitted with the Birch-Murnaghan equation of state from 0 to 31 GPa, yielding a value of 169 GPa for the bulk modulus B,. This is far from 142 GPa, obtained by fitting up to only 8 GPa for the orthorhombic phase I of PbHfO,. Therefore, it appears that the observed pressure-induced phase transitions are rather subtle. For these transitions we would like to suggest polyhedral tilting as the mechanism, involving HfO, octohedra, the principal structural building block in the system. Phase transitions caused by tilting of polyhedral units are well known [12]. Although they change the symmetry properties of the system, their effect on volume is negligible. For the pressure-induced phase transition near 45 GPa, we would like to refer to an earlier suggestion [ 131and propose that the driving force is rooted in the changes to the electronic structure associated with the d-states of the transition metal ions with pressure. In this connection a well known effect of application of pressure is the lowering of the d-state towards the valence band because of an increase in the strength of the crystal field splitting. In the present example this is the possible lowering of the d-state of Pb-ions toward the valence band derived from the oxygen p-states. This proposed lowering of the d-state is well supported by the optical absorption characteristics at high pressures, namely the observed red shift. The result would be an increase in p-d mixing. Our view is that p-d interaction is a dominant factor in determining the structure and if the external parameters (pressure and temperature) induce slight changes in the p-d bonding, the rest of the structure will follow. A quantitative analysis would have to include higher order perturbation of the octahedral p-d bond, allowing for slight changes in bond lengths or angles, in the spirit of the “chemical grip” model proposed by Harrison [141. A fundamental question concerns the differences in behavior between PbTiO,, PbZrO, and PbHfO,. Although the former is a ferroelectric compound with a Curie temperature of 760 K, the latter two
1211
are antiferroelectric with T,, of 507 and 484 K for the transition to the paraelectric phase. In seeking an explanation of the observed differences in behavior based on the electronic structure the following has been suggested [IS]. In perovskite structured materials with Ti, Zr or Hf as the B ion, the diffuseness of the outer d-orbitals, 3d, 4d and Sd, respectively, increases as one goes down in the series. Consequently the d,-p, overlap and the covalency of the M-O bond increases. In the framework of the theory of phase transitions based on the pseudo Jahn-Teller effect (PJTE) [l&16], the overlap of the orbitals and the covalency of the M-O bond would weaken the energetics of the FE-AFE ordering and hence the ordering temperature should decrease, as one moves from Ti to Hf. This is in broad agreement with the empirical facts. By the same argument, application of high pressure should increase the said overlap, and therefore, the ordering temperature may be expected to decrease with pressure; however, this does not seem to be the case [I]. Our observations of a domain structure seem to indicate that the AFE ordering continues at least up to 35 GPa in our room temperature experiments. It must be remembered, however, that the theory based on the adiabatic potential and PJTE views the transition as of order-disorder type, whereas the FE-AFE transitions are regarded as displacive type PI. Whatever the deeper theoretical concepts are concerning these transitions, it is important to build an experimental base. In this regard high-pressure Raman and X-ray studies on PbZrO, would be valuable, for they can bring out the systematics of the series PbTiO,, PbZrO, and PbHfO,.
Acknowledgements-This work is supported in part by the National Science Foundation and by the Materials and Applied Science Group at the University of Hawaii. This is School of Ocean and Earth Science and Technology Contribution 3462.
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