Reversible pressure-induced first-order phase transitions in anion-excess fluorite KY3F10 studied with single-crystal X-ray diffraction

• Pressure-induced phase transitions in KY3F10 with 2×2×2 fluorite superstructure.
• Anion-excess fluorites tend to transform to the CaF2 structure at high pressures.
• Excessive fluorine is located in the F8 cubes that are empty in the ideal fluorite.
• First-order phase transitions are explainable using group–subgroup relationships.

KY3F10 (Fm3¯mZ = 8) has been studied with single-crystal X-ray diffraction to 9.5 GPa in a diamond anvil cell under hydrostatic conditions. At atmospheric pressure, it is an anion-excess 2 × 2 × 2 superstructure of fluorite with two building blocks Y6F36 and Y6F32. In the Y6F36 block, six square YF8 antiprisms share their corners to form an empty F12 cubooctahedron. In the Y6F32 block, six square YF8 antiprisms share their edges to form an empty F8 cube. Upon compression, there occur phase transitions between 5 and 7 GPa due to an instability of the empty F12 cubooctahedra in the 2 × 2 × 2 superstructure. At about 7 GPa, the cubooctahedra collapse completely and all the cations in the new structure (Pm3¯m, Z = 1) are surrounded by cubes of the F atoms. The excessive fluorine is located in the F8 cubes. At hydrostatic conditions, all the transformations are reversible. These observations imply that at high pressures the anion-excess fluorite superstructures in general tend to transform to the CaF2 structure.

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