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 transitions in the leucite phases of KAlSi₂O₆, RbAlSi₂O₆, CsAlSi₂O₆, and KFeSi₂O₆. 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-O bond lengths. On decreasing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetragonal prisms of corner-linked (Al,Si)O₄ tetrahedra about [001] leads to collapse of the 〈111〉 structural channels and concomitant volume reduction.