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Structure and dynamics of different phases of the superprotonic conductor CsHSO4
PHYSICA Physica B 213&214(1995) 1034 1036
ELSEVIER
Structure and dynamics of different phases of the superprotonic conductor CsHSO4 A.V. Belushkin a'*, M.A. Adams b, S. Hull b, A.I. Kolesnikov c, L.A. Shuvalov d Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Russian Federation b DRAL, ISIS Facili~, Chilton, Didcot, Oxon OXl l OQX, UK Institute ~?fSolid State Physics, RAS, 142432 Chernogolovka, Russian Federation d Institute of Co,stallography, RAS, 117333 Moscow, Russian Federation
Abstract
Caesium hydrogen sulphate (CsHSO4) shows a rich variety of phases in the pressure temperature phase diagram. The crystal structures of these phases have been studied by the neutron-diffraction method. The lattice dynamics of several crystallographic phases have also been studied by the inelastic neutron scattering. It is shown that anharmonicity plays an important role in the dynamic properties of phases occurring at ambient pressure. High pressure reduces the effects of anharmonicity and the crystal phase which appears at a pressure of 1.7 G P a exhibits harmonic behaviour.
1. Introduction
Caesium hydrogen sulphate (CsHSO4) shows a rich variety of phase transitions in its P T phase diagram as shown in Fig. 1 [1,2]. The structure and mechanism of the phase transitions at ambient pressure have been studied previously (see Refs. [-3,4] and references thereint. As a hydrogenated crystal of CsHSO4 is grown, it crystallizes into monoclinic phase IlI (according to the notations accepted in Ref. [2]) with space group P21/c. When heated above 349 K, phase III transforms into phase II. This phase is also monoclinic with space group P21/c. It differs from phase IlI by its lattice parameters, smaller unit cell volume and by the organization of hydrogen bonds. When phase II is obtained, it can be cooled down to liquid-helium temperatures. At 414 K
* Corresponding author.
phase II transforms into the superprotonic phase I. This phase is tetragonal with space group I4x/amd. The deuterated samples of CsHSO4 are similar to the hydrogenated samples in all respects. The only substantial difference is the absence of phase III in the deuterated crystals. A theoretical consideration [5] of the phase transitions predicts the P - T diagram shown in Fig. 2. The theory correctly explains the direct transition from the tetragonal high-temperature I4x/amd phase to the monoclinic low-temperature P21/c phase. This fact demonstrates that the predictions of theory can help interpret highpressure diffraction experiments. The present paper concerns structural phase transformations due to the application of high pressure and the dynamical properties of three different crystal phases which can exist at low temperature. The experiments were performed on the POLARIS neutron powder diffractometer [6] and the TFXA high-resolution inelastic
0921-4526/95/$09.50 i' 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 3 5 7 6
A.V. Belushkin et al./Physica B 213&214 (1995) 1034 1036
'*L
,
440 400 36O 320 280
v,y.
Temperature t ~ ~
VI~V~
14Jamd ~ ~
P2jc
0.4 0.8 1.2 1.6 6Po
,
P4n2 1~2d
,
I
monoclinic
,
,
P41212 Pnma Pnna Pnn2 62221 P212121 P21/c P21
D4
DI~6 h
D6h CI~)
/ i
I
D4
c
\
monoclinie
P21/c
D5
i
P211m
monoclini¢
1~211C
(phase VII) / phase VI most
probably'Pnma
P21/m- - P21 ~ Pressure or
i
orthorhombic
? /
P21/m
Fig. 1. Pressure temperature phase diagram for CsHSO4 as defined in Ref. [2].
tetragonal
1035
I
P'2tc
Fig. 2. Theoretically predicted possible symmetries of CsHSO,~ phases on a P T diagram. neutron scattering spectrometer [7] at the ISIS neutron facitility, DRAL, UK.
Fig. 3. The scheme of phase transitions proposed on the basis of the analysis of both experimental and theoretical results. the experiment, but it is impossible to distinguish between these two groups because they have the same selection rules. We cannot, therefore, be certain about the exact symmetry of phases IV and V, but assuming in addition that the phase transition from phase I to phase 1V goes through the intermediate phase VI (see Fig. 1) the most probable space group for phase IV is P21/m. Then the symmetry of phase VI should be Pnma (see Fig. 2). For phase V both symmetries P2~/m, P2~ are possible. The P T phase diagram proposed on the basis both the experimental and theoretical results is shown in Fig. 3. The lattice parameters for phases IV and V obtained on the basis of the analysis described above are: Phase IV: a = 10.752(10)A, b=4.593(6)A, c=9.861(18) A, [~ = 113.42(11)~', V = 446.9A 3, most o probable space group P21/m. Phase V: a = 10.574(4)A, b = 4.225(3) A, c = 9.920(8)A, [~ = 94.70(6)°, V = 441.7A 3, most probable space groups P2~/m or P21.
3. Inelastic scattering 2. Neutron diffraction The neutron-diffraction experiments were performed at ambient temperature on a deuterated sample of CsDSO4. A test at ambient pressure confirmed that the sample was in phase II. To obtain the high-pressure phases the sample was mixed with a pressure-transmitting fluid (Fluorinert FC-75) to ensure hydrostaticity and inserted into a pressure cell of the McWhan design. Pressure of 1.25 GPa was applied to obtain phase IV and 2 G P a to obtain phase V [2]. Clear changes in the diffraction patterns were observed which reflected the phase transformations. An autoindexing program [8] and a Pawley-type refinement program [9] were used to define the space groups of high-pressure phases. Two monoclinic space groups P21 and P21/m gave a very good description of
A detailed description of the experimental procedure and data analysis is published elsewhere [10]. Here we will briefly outline only those results obtained in the study of anharmonic effects. High-resolution inelastic neutron scattering spectra were obtained for phases II, llI and V of CsHSO4. Phase V was produced at room temperature from phase III by the application of a hydrostatic pressure of 1.7 GPa. All experiments were performed at a temperature of 30 K. The results are shown by solid lines in Fig. 4. The dashed lines show the multiphonon contribution in harmonic approximation as estimated from the experimentally obtained scattering law. It was found [10] that phase V shows harmonic behaviour due to the effect of high pressure. Analysis of the experimental data for phases II and III, together with the
1036
A.V. Belushkin et al./Physica B 213&214 (1995) 1034-1036
30
,
~
25
E
"10 ~t. =[z~~ . ~ _
o
o
~
'
,
4. Conclusions
i
E
~
o
" ,~ p h a s e V
=
+
+
CsHS04
We performed neutron diffraction on CsDSO~ and inelastic neutron scattering experiments on CsHSO4 to study the structures of high-pressure phases and the lattice dynamics of three different low-temperature crystalline phases of CsHSO4. The symmetry and lattice parameters of the high-pressure phases were obtained. The inelastic neutron scattering study showed that in the two crystal phases which are realised at low temperature and ambient pressure, the proton dynamics are strongly anharmonic. The high pressure phase on the other hand shows harmonic behaviour.
°
~" ~
41~
o
25 |
~
phase
20 -
~
'
III
A 25 "
20
~f2
~
,,
The authors wish to thank the Daresbury and Rutherford Appleton Laboratory for access to the ISIS facilities. We also express our gratitude to Dr. V.S. Shakhmatov for many useful discussions.
u phase II
j~
~
Eo References
0
50
100 150 Energy transfer (meV)
200
250
Fig. 4. The inelastic neutron scattering spectra from phases [!, Ill and V of CsHSO¢. The solid lines with error bars show the experimental data and the broken lines correspond to the estimated multiphonon contribution.
multiphonon calculation results, revealed that the proton vibrations in these phases are anharmonic. In phase 11 the anharmonicity can be explained using the simple model of an anharmonic oscillator for the proton vibrations. The estimated anharmonicity parameter was found to be 0.0472. In phase IIl the anharmonic effects cannot be explained in the same way as they are connected with the creation of a bound two-phonon state [11].
[1] A.I. Baranov, L.A. Shuvalov and N.M. Shchagina, JETP Lett. 36 (1982) 459. [2] E.G. Ponyatovskii, V.I. Rashchupkin, V.V. Sinitsyn, A.I. Baranov, L.A. Shuvalov and N.M, Shchagina, JETP Lett. 41 (1985) 139, [3] A.V. Belushkin, C.J. Carlile, W.I.F. David, R.M. Ibberson, L.A. Shuvalov and W. Zajac, Physica B 174 (1991) 268. [4] A.V. Belushkin, C.J. Carlile and L.A. Shuvalov, J. Phys.: Condens. Matter 4 (1992) 389. [5] V.S. Shakhmatov. Kristallografiya 38 (1993) 176. [6] S. Hull, R.I. Smith, W.I.F. David, A.C. Hannon, J. Mayers and R. Cywinski, Physiea B 180&181 (1992) 1000. [7] J. Penfold and J. Tomkinson, RAL report, RAL-86-019, Chilton (1986). [8] V.B. Zlokazov, J. Appl. Crystallogr. 25 (1992) 69. [9] W.I.F. David, R.M. Ibberson and J.C. Matthewman, RAL report, RAL-92-032, Chilton (1992). [10] A.V. Belushkin, M.A. Adams, A.1. Kolesnikov and L.A. Shuvalov. J. Phys.: Condens. Matter 6 (1994) 5823. [11] V.M. Agranovich, in: Spectroscopy and Excitation Dynamics of Condensed Molecular Systems, eds, V.M. Agranovich and R.M. Hochstrasser (North-Holland, Amsterdam, 1983) p. 83.
ELSEVIER
Structure and dynamics of different phases of the superprotonic conductor CsHSO4 A.V. Belushkin a'*, M.A. Adams b, S. Hull b, A.I. Kolesnikov c, L.A. Shuvalov d Frank Laboratory of Neutron Physics, JINR, 141980 Dubna, Russian Federation b DRAL, ISIS Facili~, Chilton, Didcot, Oxon OXl l OQX, UK Institute ~?fSolid State Physics, RAS, 142432 Chernogolovka, Russian Federation d Institute of Co,stallography, RAS, 117333 Moscow, Russian Federation
Abstract
Caesium hydrogen sulphate (CsHSO4) shows a rich variety of phases in the pressure temperature phase diagram. The crystal structures of these phases have been studied by the neutron-diffraction method. The lattice dynamics of several crystallographic phases have also been studied by the inelastic neutron scattering. It is shown that anharmonicity plays an important role in the dynamic properties of phases occurring at ambient pressure. High pressure reduces the effects of anharmonicity and the crystal phase which appears at a pressure of 1.7 G P a exhibits harmonic behaviour.
1. Introduction
Caesium hydrogen sulphate (CsHSO4) shows a rich variety of phase transitions in its P T phase diagram as shown in Fig. 1 [1,2]. The structure and mechanism of the phase transitions at ambient pressure have been studied previously (see Refs. [-3,4] and references thereint. As a hydrogenated crystal of CsHSO4 is grown, it crystallizes into monoclinic phase IlI (according to the notations accepted in Ref. [2]) with space group P21/c. When heated above 349 K, phase III transforms into phase II. This phase is also monoclinic with space group P21/c. It differs from phase IlI by its lattice parameters, smaller unit cell volume and by the organization of hydrogen bonds. When phase II is obtained, it can be cooled down to liquid-helium temperatures. At 414 K
* Corresponding author.
phase II transforms into the superprotonic phase I. This phase is tetragonal with space group I4x/amd. The deuterated samples of CsHSO4 are similar to the hydrogenated samples in all respects. The only substantial difference is the absence of phase III in the deuterated crystals. A theoretical consideration [5] of the phase transitions predicts the P - T diagram shown in Fig. 2. The theory correctly explains the direct transition from the tetragonal high-temperature I4x/amd phase to the monoclinic low-temperature P21/c phase. This fact demonstrates that the predictions of theory can help interpret highpressure diffraction experiments. The present paper concerns structural phase transformations due to the application of high pressure and the dynamical properties of three different crystal phases which can exist at low temperature. The experiments were performed on the POLARIS neutron powder diffractometer [6] and the TFXA high-resolution inelastic
0921-4526/95/$09.50 i' 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 3 5 7 6
A.V. Belushkin et al./Physica B 213&214 (1995) 1034 1036
'*L
,
440 400 36O 320 280
v,y.
Temperature t ~ ~
VI~V~
14Jamd ~ ~
P2jc
0.4 0.8 1.2 1.6 6Po
,
P4n2 1~2d
,
I
monoclinic
,
,
P41212 Pnma Pnna Pnn2 62221 P212121 P21/c P21
D4
DI~6 h
D6h CI~)
/ i
I
D4
c
\
monoclinie
P21/c
D5
i
P211m
monoclini¢
1~211C
(phase VII) / phase VI most
probably'Pnma
P21/m- - P21 ~ Pressure or
i
orthorhombic
? /
P21/m
Fig. 1. Pressure temperature phase diagram for CsHSO4 as defined in Ref. [2].
tetragonal
1035
I
P'2tc
Fig. 2. Theoretically predicted possible symmetries of CsHSO,~ phases on a P T diagram. neutron scattering spectrometer [7] at the ISIS neutron facitility, DRAL, UK.
Fig. 3. The scheme of phase transitions proposed on the basis of the analysis of both experimental and theoretical results. the experiment, but it is impossible to distinguish between these two groups because they have the same selection rules. We cannot, therefore, be certain about the exact symmetry of phases IV and V, but assuming in addition that the phase transition from phase I to phase 1V goes through the intermediate phase VI (see Fig. 1) the most probable space group for phase IV is P21/m. Then the symmetry of phase VI should be Pnma (see Fig. 2). For phase V both symmetries P2~/m, P2~ are possible. The P T phase diagram proposed on the basis both the experimental and theoretical results is shown in Fig. 3. The lattice parameters for phases IV and V obtained on the basis of the analysis described above are: Phase IV: a = 10.752(10)A, b=4.593(6)A, c=9.861(18) A, [~ = 113.42(11)~', V = 446.9A 3, most o probable space group P21/m. Phase V: a = 10.574(4)A, b = 4.225(3) A, c = 9.920(8)A, [~ = 94.70(6)°, V = 441.7A 3, most probable space groups P2~/m or P21.
3. Inelastic scattering 2. Neutron diffraction The neutron-diffraction experiments were performed at ambient temperature on a deuterated sample of CsDSO4. A test at ambient pressure confirmed that the sample was in phase II. To obtain the high-pressure phases the sample was mixed with a pressure-transmitting fluid (Fluorinert FC-75) to ensure hydrostaticity and inserted into a pressure cell of the McWhan design. Pressure of 1.25 GPa was applied to obtain phase IV and 2 G P a to obtain phase V [2]. Clear changes in the diffraction patterns were observed which reflected the phase transformations. An autoindexing program [8] and a Pawley-type refinement program [9] were used to define the space groups of high-pressure phases. Two monoclinic space groups P21 and P21/m gave a very good description of
A detailed description of the experimental procedure and data analysis is published elsewhere [10]. Here we will briefly outline only those results obtained in the study of anharmonic effects. High-resolution inelastic neutron scattering spectra were obtained for phases II, llI and V of CsHSO4. Phase V was produced at room temperature from phase III by the application of a hydrostatic pressure of 1.7 GPa. All experiments were performed at a temperature of 30 K. The results are shown by solid lines in Fig. 4. The dashed lines show the multiphonon contribution in harmonic approximation as estimated from the experimentally obtained scattering law. It was found [10] that phase V shows harmonic behaviour due to the effect of high pressure. Analysis of the experimental data for phases II and III, together with the
1036
A.V. Belushkin et al./Physica B 213&214 (1995) 1034-1036
30
,
~
25
E
"10 ~t. =[z~~ . ~ _
o
o
~
'
,
4. Conclusions
i
E
~
o
" ,~ p h a s e V
=
+
+
CsHS04
We performed neutron diffraction on CsDSO~ and inelastic neutron scattering experiments on CsHSO4 to study the structures of high-pressure phases and the lattice dynamics of three different low-temperature crystalline phases of CsHSO4. The symmetry and lattice parameters of the high-pressure phases were obtained. The inelastic neutron scattering study showed that in the two crystal phases which are realised at low temperature and ambient pressure, the proton dynamics are strongly anharmonic. The high pressure phase on the other hand shows harmonic behaviour.
°
~" ~
41~
o
25 |
~
phase
20 -
~
'
III
A 25 "
20
~f2
~
,,
The authors wish to thank the Daresbury and Rutherford Appleton Laboratory for access to the ISIS facilities. We also express our gratitude to Dr. V.S. Shakhmatov for many useful discussions.
u phase II
j~
~
Eo References
0
50
100 150 Energy transfer (meV)
200
250
Fig. 4. The inelastic neutron scattering spectra from phases [!, Ill and V of CsHSO¢. The solid lines with error bars show the experimental data and the broken lines correspond to the estimated multiphonon contribution.
multiphonon calculation results, revealed that the proton vibrations in these phases are anharmonic. In phase 11 the anharmonicity can be explained using the simple model of an anharmonic oscillator for the proton vibrations. The estimated anharmonicity parameter was found to be 0.0472. In phase IIl the anharmonic effects cannot be explained in the same way as they are connected with the creation of a bound two-phonon state [11].
[1] A.I. Baranov, L.A. Shuvalov and N.M. Shchagina, JETP Lett. 36 (1982) 459. [2] E.G. Ponyatovskii, V.I. Rashchupkin, V.V. Sinitsyn, A.I. Baranov, L.A. Shuvalov and N.M, Shchagina, JETP Lett. 41 (1985) 139, [3] A.V. Belushkin, C.J. Carlile, W.I.F. David, R.M. Ibberson, L.A. Shuvalov and W. Zajac, Physica B 174 (1991) 268. [4] A.V. Belushkin, C.J. Carlile and L.A. Shuvalov, J. Phys.: Condens. Matter 4 (1992) 389. [5] V.S. Shakhmatov. Kristallografiya 38 (1993) 176. [6] S. Hull, R.I. Smith, W.I.F. David, A.C. Hannon, J. Mayers and R. Cywinski, Physiea B 180&181 (1992) 1000. [7] J. Penfold and J. Tomkinson, RAL report, RAL-86-019, Chilton (1986). [8] V.B. Zlokazov, J. Appl. Crystallogr. 25 (1992) 69. [9] W.I.F. David, R.M. Ibberson and J.C. Matthewman, RAL report, RAL-92-032, Chilton (1992). [10] A.V. Belushkin, M.A. Adams, A.1. Kolesnikov and L.A. Shuvalov. J. Phys.: Condens. Matter 6 (1994) 5823. [11] V.M. Agranovich, in: Spectroscopy and Excitation Dynamics of Condensed Molecular Systems, eds, V.M. Agranovich and R.M. Hochstrasser (North-Holland, Amsterdam, 1983) p. 83.