Superstructure of strontium chromate aluminate sodalite at low temperatures

Abstract

At high temperatures yellow strontium chromate aluminate sodalite Sr₈[Al₁₂O₂₄](CrO₄)₂ has cubic symmetry with orientational disorder of the CrO₄^2--ions within the sodalite cages. The existence of two phase transitions at 299K and 289K, which result in superstructures of tetragonal and orthorhombic symmetry, respectively, is well known. In this work we describe the crystal structure of the orthorhombic low temperature phase which was determined from neutron and x-ray powder diffraction. Additional information from ²⁷Al-MAS-NMR spectra and the Monte-Carlo-method was used to get a starting model for the structure refinement. In the orthorhombic phase the CrO₄-tetrahedra are fully ordered. Two thirds of the rings of four AlO₄-tetrahedra forming the sodalite framework are nearly undistorted. The remaining, distorted, rings form chains parallel to the a-axis. The distortion of the rings is of a type which occurs in other aluminate sodalite structures as well and is known as the shearing mechanism. Despite of the different symmetry the structure of this phase is similar to the structure of strontium tungstate aluminate sodalite. The characteristic differences can be explained by the smaller size of the CrO₄-tetrahedra.