- Email: [email protected]
Mechanism of superprotonic conductivity in CsHSO4
ELSEVIER
Physica B 241-243 (1998) 323 325
Mechanism of superprotonic conductivity in
CsHSO 4
A.V. Belushkin a'*, R.L. McGreevy b, P. Zetterstrom b, L.A. Shuvalov c a Frank Laborato~ o f Neutron Physics, JINR, 141980 Dubna, Moscow Region, Russian Federation b NFL Studsvik, Uppsala University, S-61182 Nykoping, Sweden c Institute o f C~stallography RAS, 117333 Moscow, Russian Federation
Abstract
Cesium hydrogen sulphate is one of the most extensively studied superprotonic conductors with hydrogen bonds. The first-order phase transition, from the tow-conductivity to high conductivity phase takes place at 414 K. The crystallographic structure of both phases has been established using high-resolution powder diffraction. Quasi-elastic neutron scattering gave information about the spatial and temporal characteristics of proton transport. The present paper reports the results of reverse Monte Carlo modelling of diffraction data which makes it possible to obtain the diffusion pathways for protons. © 1998 Elsevier Science B.V. All rights reserved.
Keywords: Superprotonic conductor; Proton diffusion; Reverse Monte Carlo modelling
Cesium hydrogen sulphate (CsHSO4) was the first crystal with hydrogen bonds found to undergo a phase transition characterised by a sharp increase in the protonic conductivity up to 10 - 2 ~ - 1 cm-1 [1]. This superprotonic transition is of improper ferroelastic type with a large spontaneous shear strain. The transition temperature is 414 K for the protonated compound and 412 K for the deuterated one. The structural mechanism of the superprotonic phase transition was studied by high-resolution neutron powder diffraction using a deuterated sample [-2]. In the low-conductivity phase the crystal is monoclinic, space group P21/c and the SO4 tetrahedra are linked by hydrogen bonds so that *Corresponding author. Tel.: ( + 7 09621) 65657; fax: ( + 7 09621 ) 65429; e-mail: [email protected].
zig-zag chains propagate along the [0 0 1] crystallographic direction. The D atoms and SO4 tetrahedra are fully ordered in this phase. When the temperature increases above the phase transition the crystal becomes tetragonal, space group I41/amd. In this phase each SO4 tetrahedron can have four different orientations with equal probability and, therefore, is disordered over them. As a result, the number of crystallographically equivalent positions for each deuteron increases from one in the low-conductivity phase to six in the highconductivity phase. The hydrogen bond network becomes disordered and the unoccupied proton positions provide the possibility for proton hopping through the lattice. The diffusion of protons in CsHSO4 was studied by quasi-elastic neutron scattering [3]. This made it possible to conclude that in the high-conductivity
0921-4526/98/$19.00 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 5 7 4 - 7
324
A. ~ Belushkin et al. /Physica B 241 243 (1998) 323 325
phase the hydrogen bond network is subjected to permanent reconstruction as the protons change their crystallographic sites. It was also established that there are two types of diffusion one is the fast reorientation of HSO4 defects and the other is the slower long-range translational proton diffusion. The latter is responsible for the high protonic conductivity. It is worth noting that the results described above are in very good agreement with the available experimental data obtained by different methods: calorimetry, optical and neutron spectroscopy and NMR. The diffraction (Bragg scattering) experiments, however, can give only the average structure of the sample and the quasi-elastic data interpretation is based on a certain model. Therefore, it is quite interesting and important to have an independent, model free method to check if the proposed picture of proton transport in the superprotonic phase is correct. Such a possibility is provided by Reverse Monte Carlo (RMC) modelling of diffraction (total scattering) data [4]. This method allows one to obtain very detailed information on structural disorder even on the basis of diffuse scattering from powders. RMC analyses the total structure factor, that is both Bragg and diffuse scattering, and is related to the instantaneous crystal structure. Therefore, one gets a description of structure deviations from the time average. In this way RMC can provide a picture of pathways for protons in the high-conductivity phase, as well as verify if SO4 groups really rotate in this phase. The experiments have been performed using the SLAD diffractometer at N F L Studsvik in Sweden. The deuterated sample was used. The data were fully corrected for background scattering, container scattering and absorption, and normalised to a vanadium standard. The resulting total structure factor was the subject of further analysis. The RMC method applied to the data at 295 K gave a very good agreement with the structural data obtained by Rietveld refinement of high-resolution neutron powder diffraction [2]. Some new features of the hydrogen bonds in the low-conductivity phase obtained by RMC will be discussed elsewhere. The analysis of the data obtained at 423 K clearly revealed the reorientational disorder of SO4 tetrahedra and confirmed its dynamical
Fig. I. The constant-density isosurface for the deuterium ions showing the diffusion routes in the high-conductivity tetragonal phase of C s D S O 4.
character. It was also possible to trace the diffusion pathways for deuterium ions in the crystal lattice. While the most probable location of the deuterons obtained by RMC matches well the crystallographic sites defined by the Rietveld method, there exists a high probability of finding these ions at other locations. Fig. 1 shows a constant density isosurface for the deuterium ions in the high-conductivity tetragonal phase of CsDSO4. One can
A.V. Belushkin et al. / Physica B 241 243 (1998) 323-325
clearly see the percolation paths which ensure proton diffusion through the lattice. The circles in the figure show Cs (largest size), S and O (smallest size) atoms, respectively. For clarity, only one possible SO4 orientation is shown. A more detailed analysis reveals that the density isosurfaces of oxygens and deuterons are very similar and, therefore, there exists a correlation between SO4 rotations and deuteron diffusion. We conclude that the Reverse Monte Carlo method applied to the analysis of the structure of superprotonic conductor CsDSO4 gave results fully consistent with the earlier high-resolution neutron powder diffraction and quasi-elastic neutron-scattering results. In addition, it made it possible without any model approximations to clearly see the reorientation of SO4 tetrahedra and D long-range translational diffusion in the high-con-
325
ductivity phase. Some additional new features of both low and high conductivity phases of CsDSO4 revealed by RMC will be discussed elsewhere. One of us (A.V.B.) expresses sincere gratitude to the NFL Directorate for the access to the Laboratory facilities and the NFL staff for the support during the experiments.
References [1] A.I. Baranov, L.A. Shuvalov, N.M. Shchagina, JETP Lett. 36 (1982) 459. [2] A.V. Belushkin, W.I.F. David, R.M. Ibberson, L.A. Shuvalov, Acta Crystallogr. B 47 (1991) 161. [3] A.V. Belushkin, C.J. Carlile, L.A. Shuvalov, J. Phys.: Condens. Matter 4 (1992) 389. [4] R.L. McGreevy, Nucl. Instr. and Meth. A 354 (1995) 1.
Physica B 241-243 (1998) 323 325
Mechanism of superprotonic conductivity in
CsHSO 4
A.V. Belushkin a'*, R.L. McGreevy b, P. Zetterstrom b, L.A. Shuvalov c a Frank Laborato~ o f Neutron Physics, JINR, 141980 Dubna, Moscow Region, Russian Federation b NFL Studsvik, Uppsala University, S-61182 Nykoping, Sweden c Institute o f C~stallography RAS, 117333 Moscow, Russian Federation
Abstract
Cesium hydrogen sulphate is one of the most extensively studied superprotonic conductors with hydrogen bonds. The first-order phase transition, from the tow-conductivity to high conductivity phase takes place at 414 K. The crystallographic structure of both phases has been established using high-resolution powder diffraction. Quasi-elastic neutron scattering gave information about the spatial and temporal characteristics of proton transport. The present paper reports the results of reverse Monte Carlo modelling of diffraction data which makes it possible to obtain the diffusion pathways for protons. © 1998 Elsevier Science B.V. All rights reserved.
Keywords: Superprotonic conductor; Proton diffusion; Reverse Monte Carlo modelling
Cesium hydrogen sulphate (CsHSO4) was the first crystal with hydrogen bonds found to undergo a phase transition characterised by a sharp increase in the protonic conductivity up to 10 - 2 ~ - 1 cm-1 [1]. This superprotonic transition is of improper ferroelastic type with a large spontaneous shear strain. The transition temperature is 414 K for the protonated compound and 412 K for the deuterated one. The structural mechanism of the superprotonic phase transition was studied by high-resolution neutron powder diffraction using a deuterated sample [-2]. In the low-conductivity phase the crystal is monoclinic, space group P21/c and the SO4 tetrahedra are linked by hydrogen bonds so that *Corresponding author. Tel.: ( + 7 09621) 65657; fax: ( + 7 09621 ) 65429; e-mail: [email protected].
zig-zag chains propagate along the [0 0 1] crystallographic direction. The D atoms and SO4 tetrahedra are fully ordered in this phase. When the temperature increases above the phase transition the crystal becomes tetragonal, space group I41/amd. In this phase each SO4 tetrahedron can have four different orientations with equal probability and, therefore, is disordered over them. As a result, the number of crystallographically equivalent positions for each deuteron increases from one in the low-conductivity phase to six in the highconductivity phase. The hydrogen bond network becomes disordered and the unoccupied proton positions provide the possibility for proton hopping through the lattice. The diffusion of protons in CsHSO4 was studied by quasi-elastic neutron scattering [3]. This made it possible to conclude that in the high-conductivity
0921-4526/98/$19.00 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 5 7 4 - 7
324
A. ~ Belushkin et al. /Physica B 241 243 (1998) 323 325
phase the hydrogen bond network is subjected to permanent reconstruction as the protons change their crystallographic sites. It was also established that there are two types of diffusion one is the fast reorientation of HSO4 defects and the other is the slower long-range translational proton diffusion. The latter is responsible for the high protonic conductivity. It is worth noting that the results described above are in very good agreement with the available experimental data obtained by different methods: calorimetry, optical and neutron spectroscopy and NMR. The diffraction (Bragg scattering) experiments, however, can give only the average structure of the sample and the quasi-elastic data interpretation is based on a certain model. Therefore, it is quite interesting and important to have an independent, model free method to check if the proposed picture of proton transport in the superprotonic phase is correct. Such a possibility is provided by Reverse Monte Carlo (RMC) modelling of diffraction (total scattering) data [4]. This method allows one to obtain very detailed information on structural disorder even on the basis of diffuse scattering from powders. RMC analyses the total structure factor, that is both Bragg and diffuse scattering, and is related to the instantaneous crystal structure. Therefore, one gets a description of structure deviations from the time average. In this way RMC can provide a picture of pathways for protons in the high-conductivity phase, as well as verify if SO4 groups really rotate in this phase. The experiments have been performed using the SLAD diffractometer at N F L Studsvik in Sweden. The deuterated sample was used. The data were fully corrected for background scattering, container scattering and absorption, and normalised to a vanadium standard. The resulting total structure factor was the subject of further analysis. The RMC method applied to the data at 295 K gave a very good agreement with the structural data obtained by Rietveld refinement of high-resolution neutron powder diffraction [2]. Some new features of the hydrogen bonds in the low-conductivity phase obtained by RMC will be discussed elsewhere. The analysis of the data obtained at 423 K clearly revealed the reorientational disorder of SO4 tetrahedra and confirmed its dynamical
Fig. I. The constant-density isosurface for the deuterium ions showing the diffusion routes in the high-conductivity tetragonal phase of C s D S O 4.
character. It was also possible to trace the diffusion pathways for deuterium ions in the crystal lattice. While the most probable location of the deuterons obtained by RMC matches well the crystallographic sites defined by the Rietveld method, there exists a high probability of finding these ions at other locations. Fig. 1 shows a constant density isosurface for the deuterium ions in the high-conductivity tetragonal phase of CsDSO4. One can
A.V. Belushkin et al. / Physica B 241 243 (1998) 323-325
clearly see the percolation paths which ensure proton diffusion through the lattice. The circles in the figure show Cs (largest size), S and O (smallest size) atoms, respectively. For clarity, only one possible SO4 orientation is shown. A more detailed analysis reveals that the density isosurfaces of oxygens and deuterons are very similar and, therefore, there exists a correlation between SO4 rotations and deuteron diffusion. We conclude that the Reverse Monte Carlo method applied to the analysis of the structure of superprotonic conductor CsDSO4 gave results fully consistent with the earlier high-resolution neutron powder diffraction and quasi-elastic neutron-scattering results. In addition, it made it possible without any model approximations to clearly see the reorientation of SO4 tetrahedra and D long-range translational diffusion in the high-con-
325
ductivity phase. Some additional new features of both low and high conductivity phases of CsDSO4 revealed by RMC will be discussed elsewhere. One of us (A.V.B.) expresses sincere gratitude to the NFL Directorate for the access to the Laboratory facilities and the NFL staff for the support during the experiments.
References [1] A.I. Baranov, L.A. Shuvalov, N.M. Shchagina, JETP Lett. 36 (1982) 459. [2] A.V. Belushkin, W.I.F. David, R.M. Ibberson, L.A. Shuvalov, Acta Crystallogr. B 47 (1991) 161. [3] A.V. Belushkin, C.J. Carlile, L.A. Shuvalov, J. Phys.: Condens. Matter 4 (1992) 389. [4] R.L. McGreevy, Nucl. Instr. and Meth. A 354 (1995) 1.