X-ray powder diffraction study of K₂MSiO₄, M=Mg, Zn, Co, Cd

Abstract

The title materials are stuffed cristobalites possessing moderate to extreme pseudosymmetry. On the bases of their X-ray powder diffraction patterns, the Mg, Zn, and Cd compounds had been previously reported as cubic and, more recently, the Zn phase as orthorhombic. Newly measured X-ray powder diffraction data demonstrate that all (including the hitherto unknown Co analog) have the Pca2₁ structure of Na₂BeSiO₄ at room temperature, but with a widely variable degree of cubic pseudosymmetry. Observed X-ray diffraction data are in good agreement with those calculated by the Rietveld method using a constrained model with Pca2₁ M²⁺/Si site occupancy and pseudocentrosymmetric Pcab atom locations. For the most nearly cubic phase, the Cd compound, there is too little deviation in the pattern from cubic symmetry to support atom coordinate refinement even with the constrained model. In these derivatives of the stuffed cristobalite structure family, M²⁺ and Si atoms form an ordered tetrahedral array which avoids M²⁺–O–M²⁺ connections. Potassium atoms fill all of the intervening large cavity sites.