X-RAY STUDY OF THE INCOMMENSURATE PHASE OF LiKSO₄ UNDER UNIAXIAL PRESSURE. D. R. Ventura, N. L. Speziali and M. A. Pimenta. Departamento de Física, ICEx- UFMG, Belo Horizonte 30161-970 Brazil

Lithium potassium sulfate (LiKSO₄) crystals display a very interesting sequence of structural phase transitions at high temperature as schematized in the diagram below.

The symmetries of phases I and III are well established in the literature. In this work we present a X-ray study of the intermediate phase II. Our results show that this phase is, in fact, incommensurate, contrarily to a previous work which supported an orthorhombic normal Pmcn structure. The modulation wave vector was observed to be approximately constant with temperature. Experiments with applied uniaxial pressure showed that this phase corresponds to a superposition of three kinds of ferroelastic incommensurate domains oriented at 120deg. to one another; the modulation is described by a single wave vector (1-q model) in each domain, with q = (a*. The application of uniaxial pressure inhibits one or two kinds of domains depending on the direction of applied pressure. The inhibition occurs for the domains that have a component of the wave vector q in the direction of the applied pressure. Structural studies of the intermediate phase II are in progress.