Crystal-structure refinement, thermal expansion, and chemical distortion of Bi2Ga4O9

Single crystals of Bi2Ga4O9 are grown from a solution in a bismuth oxide melt. The structure (orthorhombic, space group Pbam, a = 7.918(2)Delta, b = 8.299(2)Delta, c = 5.894(2)Delta, Z = 2) is refined to R = 0.052 in the anisotropic approximation based on single-crystal X-ray diffraction data. The structure is a framework. The bismuth(III) atoms are sixfold coordinated; gallium(III) exists in both tetrahedral and octahedral coordinations. The thermal expansion of Bi2Ga4O9 is studied by high-temperature X-ray powder diffraction method and is found to be sharply anisotropic. A structural interpretation of the anisotropy is proposed. Chemical distortion in the Bi2M4O9 compounds with M = Fe(III), Al, or Ga is analyzed and compared with the thermal expansion of Bi2Ga4O9.