Apatite-structured compounds: Synthesis and high-temperature investigation

The phase transitions and thermal expansion of apatite-structured compounds with the general formula MII5(AVO4)3L (MII = Ca, Sr, Cd, Ba, Pb; AV = P, V, Cr, Mn; L = OH, F, Cl, Br, I) have been studied by high-temperature X-ray diffraction and differential thermal analysis. Polymorphic transition nature was explained on the basis of the structure data. Connection between thermal stability and peculiarities of thermal expansion, which were explained in assumption of composition and structure of apatites was ascertained.

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► 25 Apatite-structured compounds of different composition were synthesized.
► Phase transformations of them in wide temperature interval were investigated.
► For the first time polymorphic transitions were observed on 6 apatites.
► Thermal expansion coefficients of all studied phases were calculated.
► The direct communication between thermal stability and expansion anisotropy was observed.