Estudos por difração de raios X da fase incomensurável de alta temperatura do LiKSO4

Abstract

The structural phases observed in LiKSO4 crystals due to thermotropic transitions have been intensively studied in the last few decades. Many different phases are reported but some of the results are still controversial. The present work reports a detailed X-ray diffraction study of the intermediate phase (II) of LiKSO4, observed between 708 and 943 K. The purpose of this study was to solve the reported controversies. Single crystals, obtained using classical slow evaporation methods, were used in X-ray diffraction experiments. Studies of the high temperature phases were performed using precession method by applying uniaxial pressure in order to check the existence of ferroelastic domains in Phase II. The obtained results allowed us to describe the crystal structure in this phase as composed by three orthorhombic domains. The possibility of a structural modulation and the presence of broad line profiles in the diffraction results obtained with conventional scans lead us to perform a careful study in the temperature range from 730 to 830 K. Three-dimensional maps of the diffraction peaks were obtained by scanning the diffraction peaks in a four-circle diffractometer in the neighbourhood of a set of h k l reflections, which can be characterised either as the main ones those present in Phase II and Phase III and for which h and k are both even or as the secondary ones those appearing in Phase II, for which h and/or k are odd. It was observed that the main reflections are split in three peaks, each of them associated with one ferroelastic domain. The secondary reflections are composed of two peaks whose separation depends on the temperature in the whole investigated temperature range. The structure of Phase II is interpreted in terms of three incommensurate domains, each one corresponding to a single q incommensurate structure, described by the wave vector q = (/2)a*ort , with average orthorhombic symmetry. The set of the systematic absences leads us to propose the Cmmm(00)111 as the super space group for Phase II symmetry.