COMPARISON OF THE STRUCTURAL MECHANIZM OF SUPERIONIC PHASE TRANSITION IN Me4LiH3(SeO4)4 (NH4)4H2(SeO4)3 AND Me3H(SeO4)2
Adam Pietraszko , Kazimierz Lukaszewicz and Bozena Hliczer*.
Institute for Low
Temperature and Structure Research Polish Acad. of Sc. Wroc3aw,
Poland,
*Institute of Molecular
Physics, Polish Acad., of Sci. Poznan, Poland
Keywords:
Structural phase transitions; superprotonic phases; fast proton
transport.
Superionic phase transitions found in a number of ionic solids are connected with a disordering of one of the ion sublattice in these materials. The behaviour of the protons as charge carriers differs considerably from that of ions due to their small mass and size, and also the to tendency to form hydrogen bonds. The electric conductivity in the superionic phase depends on the concentration of protons involved in the fast transport and the number of sites available for protons.
The compounds Me4LiH3(SeO4)4, (NH4)4H2(SeO4)3 and Me3H(SeO4)2 where Me=Rb, K, NH4 belong to protonic conductors with the electric conductivity varying from s =10-4 to s=10-1 (S/m.). The comparison of structural mechanism of phase transitions in these three compounds type shows a correlation between proton migration and the dynamics of the proton nearest neighbourhood. In general, the proton migration in the above materials can be described by Grothuss process.
The crystals of Me4LiH3(SeO4)4 undergo at low temperature a ferroelastic phase transition and at high temperature (above Ts> 400K) they become superionic conductors . In the paraelastic phase at room temperature Me4LiH3(SeO4)4 crystals have a P41 space group with four chemical units in the lattice unit cell. The crystal structure of Me4LiH3(SeO4)4 consists of XO4 tetrahedra and MeI atoms distributed in layers perpendicular to c-axis. The XO4 tetrahedra are connected by the hydrogen atoms H1 and H2 within layers and H3 atoms between the layers. The crystal structure can be characterised by four "sandwiches" related by a screw rotation of 41-axis. Li+1-ions are located between the sandwiches. A monoclinic deformation of the low temperature ferroelastic phase can be regarded as an order parameter of the ferroelastic phase transition. Below Ts the SeO4 tetrahedra form tetrammers, which disappear gradually above Ts.
The Rb4LiH3(SeO4)4 [RLHSe] and (NH4)4LiH3(SeO4)4 [ALHSe] crystals undergo ferroelastic and superionic phase transitions at 101K, 448K and 268K, 415K, respectively. The temperature dependence of lattice parameters of RLHSe confirmed the superionic phase transition at 488K observed by DTA, as well as conductivity and dielectric measurements [2]. The crystal structure of RLHSe determined at 295, 440K, 452K and 465K shows no changes of the space group during the superionic phase transition, though, the distances for SeO4 tetrahedra become more distorted with decreasing temperature. The X-ray study indicates that the fast proton transport in the superionic phase is related to delocalisation of H2 atoms.
The crystal structure of ALHSe has been determined below the superionic phase transition at 295K and 400K. After first order transition at Ts =415K, single crystals specimens become polycrystalline. The crystal structure above Ts has been determined with space group P41 by using Ritveld method.
At room temperature (NH4) 4H2(SeO4)3 belong to the triclinic system with P.1 space group. Above Ts 378K they become triclinic with the ionic conductivity jump of approximately three orders of magnitude. The high isotropic proton conductivity is due to the melting of hydrogen bond sublattice. Changes of the crystal structure above Ts indicate the breaking of hydrogen bonds linking SeO4 tetrahedra in trimmers.
Characteristic feature of superionic phase transitions for the (NH4)3H(SeO4)2 crystals is a distortion of dimmers of the SeO4 tetrahedra linked by hydrogen bonds which above Ts form two dimensional lattice of hydrogen bonds. This transformation is correlated with change of symmetry from the triclinc P.1 at room temperature to R3m. at 360K.
An increase in the vibrational and librational modes of the SeO4 tetrahedra at high temperature and the increase the number of sites available for protons are the necessary conditions for fast protonic transport in the investigated materials. Basing on riding model analysis of anisotropic temperature factors we have calculated mean distances of hydrogen bonds for dimmers, trimmers and tetrammers in the normal phases and the superionic phases.
This work was supported by grant 7 TO8A 027 09 Polish State Committee for Scientific Research.