Structural behavior, crystal chemistry, and phase transitions in substituted leucite; high-resolution neutron powder diffraction studies

Abstract

High-resolution neutron powder diffraction was used to study phase transitions in the leucite phases of KAlSi206, RbAlSi20, CsAlSi206, and KFeSi206. The temperature-dependent structural behavior involves two mechanisms: relaxation of the tetrahedral framework about channel cations, and slowly changing T-O bond lengths. The high-temperature cubic phase is characterized by a fully-extended tetrahedral framework; thermal expansion occurs by an increase in mean T-0 bond lengths. On decreasing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetragonal prisms of comer-linked (AI,Si)04 tetrahedra about [001] leads to collapse of the (111) structural channels and concomitant volume reduction.