Thermodynamic properties of calcium alkali phosphates Ca(Na,K)PO4

Calcium alkali phosphates Ca(Na,K)PO4 are main constituents of bioceramics and thermochemically produced phosphorus fertilizers because of their bioavailability. Sparse thermodynamic data are available for the endmembers CaNaPO4 and CaKPO4. In this work, the missing data were determined for the low-temperature phase modifications of the endmembers CaNaPO4 and CaKPO4 and three intermediate Ca(Na,K)PO4 compositions. Standard enthalpy of formation ranges from - 2018.3 +/- 2.2 kJ mol(-1) to - 2030.5 +/- 2.1 kJ mol(-1) and standard entropy from 137.2 +/- 1.0 J mol(-1) K-1 to 148.6 +/- 1.0 J mol(-1) K-1 from sodium endmember beta-CaNaPO4 to potassium endmember beta '-CaKPO4. Thermodynamic functions are calculated up to 1400 K for endmembers and the sodium-rich intermediate phase beta-Ca(Na0.93K0.07)PO4. Functions above 640 K are extrapolated because of the phase transition from low- to high-temperature phase. Impurities in the synthesized intermediate phases gamma-Ca(Na0.4K0.6)PO4 and gamma-Ca(Na0.35K0.65)PO4 and one additional phase transition around 500 K impeded the determination of high-temperature thermodynamic functions. In general, data for phase transition temperatures agree with the previously reported phase diagrams.