Melilite-type borates Bi2ZnB2O7 and CaBiGaB2O7

The bismuth borates Bi2ZnB2O7 and CaBiGaB2O7 have been synthesized by solid-state reactions at temperatures in the 650−825 °C range at 1 atm pressure. These compounds are the only synthetic diborate members of the melilite family, A2XZ2O7, in which layers of A cations alternate with XZ2O7 tetrahedral layers. Except for CdBiGaB2O7, the synthesis of other substituted bismuth borate melilites has been unsuccessful. The crystal structures of Bi2ZnB2O7 and CaBiGaB2O7 have been determined by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction data. CaBiGaB2O7 adopts the regular tetragonal melilite structure (P421m space group, Z = 2) containing B2O7 tetrahedral dimers. The refinement of split eight-coordinated sites for the Ca2+ and Bi3+ interlayer cations suggests the presence of additional disorder. Bi2ZnB2O7 adopts a unique orthorhombic melilite superstructure (Pba2 space group, Z = 4) containing both tetrahedral B2O7 and triangular B2O5 dimers. The Bi3+ cations occu...