Melilite-Type Borates Bi₂ZnB₂O₇ and CaBiGaB₂O₇

The bismuth borates Bi 2 ZnB 2 O 7 and CaBiGaB 2 O 7 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, A 2 XZ 2 O 7, in which layers of A cations alternate with XZ 2 O 7 tetrahedral layers. Except for CdBiGaB 2 O 7, the synthesis of other substituted bismuth borate melilites has been unsuccessful. The crystal structures of Bi 2 ZnB 2 O 7 and CaBiGaB 2 O 7 have been determined by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction data. CaBiGaB 2 O 7 adopts the regular tetragonal melilite structure ( P 4̄2 1 m space group, Z = 2) containing B 2 O 7 tetrahedral dimers. The refinement of split eight-coordinated sites for the Ca 2+ and Bi 3+ interlayer cations suggests the presence of additional disorder. Bi 2 ZnB 2 O 7 adopts a unique orthorhombic melilite superstructure ( Pba 2 space group, Z = 4) containing both tetrahedral B 2 O 7 and triangular B 2 O 5 dimers. The Bi 3+ cations occupy two distinct interlayer sites with strongly asymmetric 6-fold coordination environments. The preliminary measurement of second harmonic generation efficiencies ( d eff ) of powder samples has yielded values of 4.0 (Bi 2 ZnB 2 O 7 ) and 1.6 (CaBiGaB 2 O 7 ) relative to a KH 2 PO 4 standard.