Többens, D. M. and Depmeier, W. (2001) Superstructure of strontium chromate aluminate sodalite at low temperatures. Zeitschrift Fur Kristallographie, 216 (11). pp. 586-590. DOI 10.1524/zkri.216.11.586.22483.

Abstract

At high temperatures, yellow strontium chromate aluminate sodalite Sr-8[Al12O24](CrO4)(2) has cubic symmetry with orientational disorder of the CrO42--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. 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 Al-27-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.

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