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The crystal structure of bismutocolumbite, Bi(Nb 0.79 Ta 0.21 )O 4  with orthorhombic (sp. gr.  Pnna ) stibio-tantalite structure type at ambient conditions has been de-termined at 2.59 and 9.56 GPa, and 23 o C. Crystal data and results of structure refinement using powder X-ray diffrac-tion and Rietveld analysis for Bi-rich stibiocolumbite, (Sb 0.52 Bi 0.48 )(Nb 0.71 Ta 0.29 )O 4 , 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.  Pn 2 1  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 BiVO 4 , 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)O 6  octahedra within octahedral sheets parallel to (010). (Nb,Ta)O 6  octahedra remain rigid with smaller change in size at high pressure.

Phase transitions in solid solution series bismutocolumbite – stibiocolumbite (Bi-Sb)(Nb 0.79 Ta 0.21 )O 4