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A Molecular Dynamics Study of the Thermodynamic Properties of Calcium Apatites. 1. Hexagonal Phases

2005; American Chemical Society; Volume: 109; Issue: 51 Linguagem: Inglês

10.1021/jp054304p

1520-6106

Fernando J. A. L. Cruz, José N. Canongia Lopes, Jorge C. G. Calado, Manuel E. Minas da Piedade,

Crystallography and molecular interactions

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(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, ΔlatH298°, 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, Cp,m, 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 = 694 ± 68 J·mol-1·K-1, Cp,m = 646 ± 26 J·mol-1·K-1, Cp,m = 530 ± 34 J·mol-1·K-1, and Cp,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, κT, 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%.