A rietveld refinement of the structure of K2ZnGeO4 and its relationship to a revised model of the KGaO2 structure type

The K2ZnGeO4 structure has been refined, from X-ray powder diffractometer data, in space group Pca21 and with the unit cell a = 11.0769(10) Å, b = 5.5216(5) Å, c = 15.8465(13) Å, V = 969.2 Å3, to residual factors RF = 4.9% and Rwp = 16.2%. Constraints on the positional parameters (pair-wise linking) were used in order to obtain a convergent refinement. The structure is a stuffed cristobalite-type structure with the K atoms within voids in a framework of corner-sharing ZnO4 and GeO4 tetrahedra. Guinier-Hägg photograph data indicate that K2ZnSiO4 and Rb2ZnGeO4 are isostructural compounds. The K2ZnGeO4 structure is found to be isotypic with the KGaO2 structure. A reevaluation of the published single crystal X-ray data for KGaO2 suggests that one reported oxygen position is erroneous.