A molecular dynamics study of the thermodynamic properties of calcium apatites. 1 – Hexagonal phases (original) (raw)

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca 10 (PO 4 ) 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, ∆ lat H 298°, 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, C p,m , 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,298 -298C p,m ) + C p,m T, yielding C p,m ) 694 ( 68 J‚mol -1 ‚K -1 , C p,m ) 646 ( 26 J‚mol -1 ‚K -1 , C p,m ) 530 ( 34 J‚mol -1 ‚K -1 , and C p,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, κ 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%.