CENTROSYMMETRIC PEROVSKITE CRYSTAL STRUCTURESWITH SPACE GROUP Pbnm: CRYSTALLOGRAPHIC PARAMETERIZATIONOF KCaF_3 BETWEEN 100 AND 400 K IN TERMS OF THE AMPLITUDESOF SYMMETRY-ADAPTED BASIS VECTORS OF THE CUBIC ARISTOTYPE PHASE

摘要

The crystal structures of centrosymmetric, zone-boundary-tilted perovskite-structured phases with space group Pbmn can be decomposed in terms of an appropriate set of symmetry-adapted basis vectors, for irreducible representations with wave vectors on the surface of the Brillouin zone of the cubic aristotype phase. Such a decomposition potentially allows a physical basis to be derived for any observed structural distortion (bond lengths, bond angles, polyhedron volumes) as a function of an applied thermodynamic variable (temperature, pressure, magnetic or electric field), or indeed chemical composition in studies of solid olutions. The methodology is illustrated using results from a parametric study of the temperature dependence of the crystal tructure of KCaF3 between 100 and 400 K.