The synthesis and thermal stability of CaKCe(PO4)2 phosphate. Phase equilibria in the CaKPO4–CePO4 system

The compound CaKCe(PO4)2 was obtained using respectively the ceramic method, the Pechini method and the hydrothermal method. The low-temperature (Pechini and hydrothermal) methods yielded homogenous powders with a crystallite size up to 24 nm.The compound has a hexagonal structure with parameters of an elementary cell: a = b = 7.0468(8) Å and c = 6.4367(0) Å. The CaKCe(PO4)2 phosphate occurs in one polymorphic form.A phase equilibrium diagram of the CaKPO4–CePO4 system has been established on the basis of XRD, DTA/DSC/TGA results. The initial phosphates were found to form one intermediate compound, CaKCe(PO4)2, with molar ratio CaKPO4:CePO4 = 1:1, melting incongruently at a temperature of 1530 °C according to the reaction: CaKCe(PO4)2(cr) → CePO4(cr) + L. At a temperature of 1440 °C an eutectic was found to be present in the CaKPO4–CePO4 system at x(CePO4) = 0.25.

► Thermal behavior of samples in CaKPO4–CePO4 system has been studied. ► The CaKCe(PO4)2 phosphate exists in one structural form and is thermally unstable. ► Nanocrystals of CaKCe(PO4)2 were obtained by Pechini and hydrothermal methods. ► Phase diagram of CaKPO4–CePO4 system has been proposed.