A molecular dynamics study of the thermodynamic properties of calcium apatites. 1. Hexagonal phases

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca-10(PO4)6(X)2 (X = OH, F, Cl, Br), were investigated using an all-atom Born-Huggins-Mayer potential by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of ca. 4% for the haloapatites and 8% for hydroxyapatite. The standard molar lattice enthalpy, Delta H-lat(298)degrees, of the apatites was calculated and compared with previously published experimental results, the agreement being better than 2%. The molar heat capacity at constant pressure, Cm, in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, H-m = (H-m,H-298 - 298C(p,m)) + Cp,mT, yielding C-p,C-m 694 +/- 68 J(.)mol(-1.)K(-1), C-p,C-m = 646 +/- 26 J(.)mol(-1.)K(-1), C-p,C-m = 530 +/- 34 J(.)mol(-1.)K(-1), and C-p,C-m = 811 +/- 42 J(.)mol(-1.)K(-1) for hydroxy-, fluor-, chlor-, and bromapatite, respectively. High-pressure simulation runs, in the range 0.5-75 kbar, were performed in order to estimate the isothermal compressibility coefficient, kappa(T), of those compounds. The deformation of the compressed solids is always elastically anisotropic. with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state with an accuracy better than 1%.