STRUCTURAL BEHAVIOR, CRYSTAL-CHEMISTRY, AND PHASE-TRANSITIONS IN SUBSTITUTED LEUCITE - HIGH-RESOLUTION NEUTRON POWDER DIFFRACTION STUDIES

High-resolution neutron powder diffraction was used to study phase tra nsitions in the leucite phases of KAlSi2O6, RbAlSi2O6, CsAlSi2O6, and KFeSi2O6. The temperature-dependent structural behavior involves two m echanisms: relaxation of the tetrahedral framework about channel catio ns, and slowly changing T-O bond lengths. The high-temperature cubic p hase is characterized by a fully-extended tetrahedral framework; therm al expansion occurs by an increase in mean T-O bond lengths. On decrea sing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetragonal prisms of corner-linked (Al,Si)O-4 tetrahedra about [001] leads to collapse o f the [111] structural channels and concomitant volume reduction.