Molecular dynamics study of the thermodynamic properties of calcium apatites. 2. Monoclinic phases

Structural and thermodynamic properties of crystalline monoclinic calcium apatites, Ca10(PO4)6(X)2 (X=OH, Cl), were investigated for the first time using a molecular dynamics (MD) technique under a wide range of temperature and pressure conditions. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, yielding maximum deviations of ca. 2%. The standard molar lattice enthalpy (DeltalatHo298) of the apatites was also calculated and compared with previously published experimental and MD results for the hexagonal polymorphs. High-temperature simulation runs were used to estimate the isobaric thermal expansivity coefficient and study the behavior of the crystal structure under heating. The heat capacity at constant pressure, Cp, in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, Hm=(Hm,298-298Cp,m)+Cp,mT, yielding Cp,m=635+/-7 J.mol-1.K-1 and Cp,m=608+/-14 J.mol-1.K-1 for hydroxy- and chlorapatite, respectively. High-pressure MD experiments, in the 0.5-75 kbar range, were performed to estimate the isothermal compressibility. The Parsafar-Mason equation of state was successfully used to fit the high-pressure p-Vm data, with an accuracy better than 0.03%.