Structure and crystal chemistry of fluorite-related Bi38Mo7O78 from single crystal X-ray diffraction and ab initio calculations

The floating-zone furnace method was used to synthesize single crystals of the fluorite-related δ-Bi2O3-type phase Bi38Mo7O78 for the first time. Single crystal synchrotron X-ray diffraction data, in conjunction with ab initio (density functional theory) calculations, were used to solve, optimize, and refine the 5×3×3 commensurate superstructure of fluorite-type δ-Bi2O3 in Pbcn (a=28.7058(11) Å, b=16.8493(7) Å and c=16.9376(6) Å, Z=4, RF=11.26%, wRI=21.67%). The structure contains stepped channels of Mo6+ in tetrahedral environments along the b axis and chains of Mo6+ in octahedral environments along the ac plane. The role of the stepped channels in oxide ion conduction is discussed. The simultaneous presence of both tetrahedral and octahedral coordination environments for Mo6+, something not previously observed in Mo6+-doped δ-Bi2O3-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis.