CRYSTAL-STRUCTURE OF ORIENTATIONALLY DISORDERED NA2(CA,SR)SIO4

Na2CaSiO4, Na2SrSiO4, and Na2(Ca0.5Sr0.5)SiO4, are shown by X-ray powder diffraction to have the cubic, Fm3mBAR crystal structure of orientationally disordered Na3PO4. The distribution of SiO4 orientations are well described by cubic harmonics and show a high probability of Si-O bonds pointing within about 20-degrees of <100> and low probabilities in other directions. The disordered SiO4 groups form a cubic close packed array with cations in both ''tetrahedral'' and ''octahedral'' interstices. There is apparent preference of the divalent Ca and Sr cations for the tetrahedral sites over the octahedral sites. About 92% of the cations in the ideal formulae are accounted for on these two sites. Electron mcroprobe analyses suggest that at least part of any remaining deficit is attributable to loss of Na atoms during synthesis but additional, weakly occupied sites may also be present in the structure.