STRUCTURAL RELATIONSHIPS AMONG SOME BEPO-, BEASO-, AND ALSIO-RHO FRAMEWORKS

The structures of Tl+-exchanged and Rb+-exchanged forms of AlSiO-, BeA sO-, and BePO-rho have been investigated using neutron or synchrotron X-ray powder diffraction data collected at room temperature. All mater ials, except Tl-AlSiO-rho, crystallize in the space group I23, with a = 13.6469(6) Angstrom A for Tl-BePO-rho, a = 13.4791(5) Angstrom A for Rb-BePO-rho, and a = 13.998(1)Angstrom A for (Rb,Tl)-BeAsO-rho; for T l-AlSiO-rho (I $($) over bar$$ 43m), a = 14.4636(7) Angstrom A. The ex traframework cations are distributed over the double 8-ring (D8R), sin gle 8-ring (S8R), and two single 6-ring (S6R) sites. Dehydrated single crystals of the (Ca,Li)-beryllophosphate mineral Pahasapaite were als o studied to temperatures of 500 degrees C. Unlike AlSiO-rho containin g singly charged extraframework cations, the (Ca,Li)-BePO material sho ws little variation with temperature. Once account is taken of differe nces in the ionic radii of framework cations, all materials with the R HO framework fall on the same universal curve that describes the disto rtion of elliptical double 8-ring pore opening as a function of unit c ell parameter.