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) ?, b=8.3745(4) ?, c= 5.7062(3) ?, Z=4) has been determined by single-crystal X-ray diffraction (wR(F2)=0.089 for 732 unique reflections). BaGa2B2O7 (Cmcm, a=11.7434(6) ?, b=8.7076(4) ?, c= 5.8038(3) ?, Z=4) has been shown to be isostructural by ab initio structure determination and Rietveld refinement using powder X-ray data (Rwp=0.119, chi2=2.16). The MGa2B2O7 structure, formulated as MGa2(B2O5)O2, consists of a framework of corner-sharing tetrahedral (GaO4) chains and pyroborate (B2O5) groups with the M atoms filling eightcoordinated framework cavities. It represents a new structure-type for anhydrous borates and, in particular, is structurally distinct from the MAl2B2O7 (M=Ca, Sr, Ba) alumino-borate analogs.