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 299 K and 289 K, which result in superstructures of tetragonal and orthorhombic symmetry, respectively, is well known. The structure of the orthorhombic low-temperature phase has been solved recently. In this work we discuss the crystal structure of the intermediate phase and its determination from neutron and x-ray powder diffraction data. The structure of this phase shows only very small distortions of the framework of AlO₄-tetrahedra. The CrO₄-tetrahedra within the sodalite cages are partially ordered. The observed distribution of their orientations can be described as a combination of those of the low- and the high-temperature phase. ²⁷Al-MAS-NMR spectra reveal the dynamic nature of the structure.