Molecular dynamics simulations of molten calcium hydroxyapatite

Molecular dynamics simulations of molten hydroxyapatite were performed, for the first time, in the range 2000 K < T < 3000 K and pressures up to 20 GPa. The all-atom Born–Huggins–Mayer potential energy function employed had been previously used to study the thermodynamic properties of the solid compound. High-temperature simulation runs were used to generate the p–Vm–T surface of the melt, from which properties like the isobaric thermal expansion coefficient, αp and the isothermal compressibility, κT, could be evaluated. The heat capacity at room pressure, Cp, in the range 2000–3000 K, was estimated from the plot of the molar enthalpy of the melt as a function of temperature, Hm = A0 + AT + BT2 + C/T (A0 = −3.7490 × 104 kJ mol−1, A = 3.5842 kJ mol−1 K−1, B = −5.6989 × 10−4 kJ mol−1 K−2, C = −3.0061 × 105 kJ mol−1 K). Cp varies from 1373 J mol−1 K−1 (T = 2000 K) to 180 J mol−1 K−1 (T = 3000 K). The intermolecular atom–atom distribution functions, at several temperatures and pressures, were also investigated. A universal EoS proposed by Parsafar et al. was shown to give a good account of the MD data, the precision being better than 0.5%. Likewise, the Parsafar–Mason regularity which assumes a linear dependence of (Z − 1)V2 on ρ2, has been established for molten hydroxyapatite.