Estudo das transições de fases estruturais em cristais mistos LiK1-xRbxSO4 .

Abstract

The phase transition sequence of the LiK1-xRbxSO4 mixed crystals (x 0,50) has been investigated by various experimental techniques. The purpose of these studies is to contribute to the understanding of the mechanisms of the different structural phase transitions presented by thepure and mixed LiKSO4 crystals. The main effects of the random substitution of rubidium ions in the potassium sites are seen by the changes in the intervals of existence of the crystalline phasesand by the increasing of the diffuse character of the first order transitions, mainly those of reconstructive type. The electro-optic properties of these compounds are affected by the compositional disorder created in their structures. We observed that the electro-opticcoefficients, obtained by a modulation method based upon the Sénarmont set-up, shows an important decrease when compared with those obtained for the pure LiKSO4 crystal. By theother hand, the ionic conductivity of the system did not show important changes, before and after the hexagonal orthorhombic transition, in the high temperature region. The thermal analysis of our compounds allowed us to determine the entropy changes at this transition, and revealed that its diffuse character is related to the non-uniform distribution of the rubidium ions into the samples. Neutron powder diffractometry has been used in order to confirm the symmetries of the structural phases presented by these crystals. The results allowed us to propose the phasediagram of our mixed system, in the temperature interval from 50 to 800K, for the rubidium concentration x = 0 to 0.50. We observed the trigonal phase stabilization in a larger temperature interval and the progressive reduction of the region of hexagonal symmetry, with increasingrubidium concentration. We realized a more detailed study of the crystal x = 0.50, by means of Raman spectroscopy and neutron diffraction techniques, since this crystal presents the more important changes due to the highest rubidium concentration. We present a global analysis of thestructural modifications of the system, based upon the variations of the lattice parameters and steric repulsion caused by the random introduction of rubidium ions, changing the phase diagramin comparison with the pure compound. The temperature evolution of the c/a ratio shows characteristic behavior for each phase, being almost independent of the rubidium concentration.