Phase and melting relationships of β, α and α′-Ca3(PO4)2 polymorphs in the Ca3(PO4)2-Zn3(PO4)2 system

In order to provide an exact knowledge of the phase transitions and melting relationships of Ca3(PO4)2 (TCP) in the presence of zinc, a revisited version of the rich-Ca3(PO4)2 region of the phase diagram of the system Ca3(PO4)2-Zn3(PO4)2 has been established in the present work. Experimental determination of this diagram was carried out by solid-state reactions of samples prepared from pure NH4H2PO4, CaCO3 and ZnO raw materials. X-ray Diffraction, Differential Thermal Analyses and Field Emission Scanning Electron Microscopy studies allowed to revise the α, β, α + β-TCP phase stability fields, delimitating for the first time the biphasic α + α′-TCP field and the melting relationships in the high temperature region of the system. The results allowed to determine two peritectic invariant points, at ≈1400 °C for 95 mol% Ca3(PO4)2 and at ≈1490 °C for ≈99.5 mol% Ca3(PO4)2.