Thermodynamic properties of calcium alkali phosphates CaPO.sub.4

• Published in: Journal of materials science Vol. 55; no. 20; p. 8477
• Main Authors: Herzel, Hannes, Grevel, Klaus-Dieter, Emmerling, Franziska, Dachs, Edgar, Benisek, Artur, Adam, Christian, Majzlan, Juraj
• Format: Journal Article
• Language: English
• Published: Springer 01-07-2020
• Subjects: Hydroxylapatite; Phosphates; Phosphatic fertilizers; Thermodynamics
• ISSN: 0022-2461
• DOI: 10.1007/s10853-020-04615-5

Calcium alkali phosphates Ca(Na,K)PO.sub.4 are main constituents of bioceramics and thermochemically produced phosphorus fertilizers because of their bioavailability. Sparse thermodynamic data are available for the endmembers CaNaPO.sub.4 and CaKPO.sub.4. In this work, the missing data were determined for the low-temperature phase modifications of the endmembers CaNaPO.sub.4 and CaKPO.sub.4 and three intermediate Ca(Na,K)PO.sub.4 compositions. Standard enthalpy of formation ranges from - 2018.3 ± 2.2 kJ mol.sup.-1 to - 2030.5 ± 2.1 kJ mol.sup.-1 and standard entropy from 137.2 ± 1.0 J mol.sup.-1 K.sup.-1 to 148.6 ± 1.0 J mol.sup.-1 K.sup.-1 from sodium endmember [beta]-CaNaPO.sub.4 to potassium endmember [beta]'-CaKPO.sub.4. Thermodynamic functions are calculated up to 1400 K for endmembers and the sodium-rich intermediate phase [beta]-Ca(Na.sub.0.93K.sub.0.07)PO.sub.4. 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(Na.sub.0.4K.sub.0.6)PO.sub.4 and [gamma]-Ca(Na.sub.0.35K.sub.0.65)PO.sub.4 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.