Nefedovite, Na5Ca4(PO4)4F is a rare sodium and calcium fluorophosphate. The crystal structure of nefedovite could be described as antiperovskite - type. It is based upon anion-centered [FCa4Na2]9+ octahedra, which link through Na1 cations in chains located along the direction [001]. (PO4)3- tetrahedra and Na2 cations are located in cavities between the chains. Nefedovite is tetragonal, I-4, a = 11.6582(2), c = 5.4111(1) Å, V = 735.45(5) Å3, Z = 2. Thermal behavior of mineral was studied by high-temperature X-ray powder diffraction method (30 - 600 º C) and by single crystal X-Ray diffraction analysis at -173 ºC, room temperature and at 150 ºC. The analysis of the data showed that the thermal expansion has anisotropic character and the anisotropy increases with temperature. The structure expands more intensively in the direction perpendicular to anion-centered chains. Thus the anion-centered chains are the strongest structural units in the structure of nefedovite and practically unchanged with temperature increasing whereas the PO4 tetrahedra rotate around the diagonal [-110] of the base of the tetragonal cell. This fact could be an explanation of this kind of thermal expansion.
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