Crystal structures of new gallo-borates MGa2B2O7, M = Sr, Ba

Two new gallo-borates, MGa2B2O7 (M = Sr, Ba) have been synthesized and structurally characterized. The structure of SrGa2B2O7 (Cmcm, a = 11.7260(7) Angstrom, b = 8.3745(4) Angstrom, c = 5.7062(3) Angstrom, Z = 4) has been determined by single-crystal X-ray diffraction (wR(F-2) = 0.089 for 732 unique reflections). BaGa2B2O7 (Cmcm, a = 11.7434(6) Angstrom, b = 8.7076(4) Angstrom, c = 5.8038(3) Angstrom, Z = 4) has been shown to be isostructural by ab initio structure determination and Rietveld refinement using powder X-ray data (R-wp = 0.119, chi (2) = 2.16). The MGa2B2O7 structure, formulated as MGa2(B2O5)O-2, consists of a framework of corner-sharing tetrahedral (GaO4) chains and pyroborate (B2O5) groups with the M atoms filling eight-coordinated framework cavities, It represents a new structure-type for anhydrous berates and, in particular, is structurally distinct from the MAI(2)B(2)O(7) (M = Ca, Sr, Ba) alumino-borate analogs. (C) 2000 Academic Press.