Excess heat capacity and entropy of mixing along the hydroxyapatite-chlorapatite and hydroxyapatite-fluorapatite binaries.

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The heat capacity, , of synthetic hydroxyapatite [Ca(PO)OH-OH-Ap], as well as of ten compositions along the OH-Ap-chlorapatite (Cl-Ap) join and 12 compositions along the OH-Ap-fluorapatite (F-Ap) join have been measured using relaxation calorimetry (heat capacity option of the Physical Properties Measurement System-PPMS) and differential scanning calorimetry (DSC) in the temperature range of 5-764 K. Apatites along the Cl-OH and F-OH joins were synthesized at 1100 °C and 300 MPa in an internally heated gas pressure vessel via an exchange process between synthetic fluorapatite or chlorapatite crystals (200-500 μm size) and a series of Ca(OH)-HO solutions with specific compositions and amounts relative to the starting apatite. The standard third-law entropy of OH-Ap, derived from the low-temperature heat capacity measurements, is ° = 386.3 ± 2.5 J mol K, which is ~ 1% lower than that resulting from low-temperature adiabatic calorimetry data on OH-Ap from the 1950's. The heat capacity of OH-Ap above 298.15 K shows a hump-shaped anomaly centred around 442 K. Based on published structural and calorimetric work, this feature is interpreted to result from a monoclinic to hexagonal phase transition. Super ambient up to this transition can be represented by the polynomial: . The DSC data above this transition were combined with heat capacities computed using density functional theory and can be given by the polynomial: . Positive excess heat capacities of mixing, ∆ , in the order of 1-2 J mol K, occur in both solid solutions at around 70 K. They are significant at these conditions exceeding the 2σ-uncertainty of the data. This positive ∆ is compensated by a negative ∆ of the same order at around 250 K in both binaries. At higher temperatures (up to 1200 K), ∆ is zero within error for all solid solution members. As a consequence, the calorimetric entropies, S, show no deviation from ideal mixing behaviour within a 2σ-uncertainty for both joins. Excess entropies of mixing, ∆S, are thus zero for the OH-Ap-F-Ap, as well as for the OH-Ap-Cl-Ap join. The - behaviour of the OH-Ap endmember is discussed in relation to that of the F- and Cl-endmembers.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00269-021-01167-1.