Abstract
Abstract The crystal structure of bismutocolumbite, Bi(Nb0.79Ta0.21)O4 with orthorhombic (sp. gr. Pnna) stibio-tantalite structure type at ambient conditions has been de-termined at 2.59 and 9.56 GPa, and 23oC. Crystal data and results of structure refinement using powder X-ray diffrac-tion and Rietveld analysis for Bi-rich stibiocolumbite, (Sb0.52Bi0.48)(Nb0.71Ta0.29)O4, are reported. At the pressure ~2.9 GPa bismutocolumbite exhibits phase transition which is accompanied by the change of its symmetry: sp. gr. Pnna is replaced by the sp. gr. Pn21 a. This kind of transformation is anticipated at ambient pressure in bismutocolumbite- stibiocolumbite solid solution series in the compounds with Sb/(Bi+Sb) ratio higher than 0.52 which characterises the studied crystals of stibiotantalite. Similarly with scheelite-like BiVO4, in bismutocolumbite the reduction of the unit cell volume under HP is mainly a result of Bi-O polyhedral compression, which is accompanied by a rearrangement of (Nb,Ta)O6 octahedra within octahedral sheets parallel to (010). (Nb,Ta)O6 octahedra remain rigid with smaller change in size at high pressure.
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