STRUCTURES OF THE STUFFED TRIDYMITE DERIVATIVES, BAMSIO4 (M = CO, ZN, MG)

The crystal structures of the compounds BaMSiO4 (M = Co, Mg, Zn) have been refined by using single-crystal X-ray data (M = Co) and powder neutron data (M = Mg, Zn). The three compounds are isostructural Ba-stuffed derivatives of tridymite (SiO2) and crystallize with a (3 × A, C) superstructure of hexagonal (A, C) kalsilite (KAlSiO4). A clear correlation exists between the degree of collapse of the fully ordered tetrahedral framework and the size difference between the M and Si atoms. © 1993 Academic Press, Inc.