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

ABSTRACT: The heat capacity, C _p, of synthetic hydroxyapatite [Ca₅(PO₄)₃OH-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)₂-H₂O 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 S° = 386.3 ± 2.5 J mol^-1 K^-1, 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 C _p up to this transition can be represented by the polynomial: CpOH - Ap298K-442KJ mol-1K-1=1013.7-13735.5T-0.5+2.616718107T-2-3.551381109T-3. . The DSC data above this transition were combined with heat capacities computed using density functional theory and can be given by the C _p polynomial: CpOH - Ap>442KJ mol-1K-1=877.2-11393.7T-0.5+5.452030107T-2-1.3941251010T-3 . Positive excess heat capacities of mixing, ∆C _p ^ex, in the order of 1-2 J mol^-1 K^-1, occur in both solid solutions at around 70 K. They are significant at these conditions exceeding the 2σ-uncertainty of the data. This positive ∆C _p ^ex is compensated by a negative ∆C _p ^ex of the same order at around 250 K in both binaries. At higher temperatures (up to 1200 K), ∆C _p ^ex is zero within error for all solid solution members. As a consequence, the calorimetric entropies, S_cal, show no deviation from ideal mixing behaviour within a 2σ-uncertainty for both joins. Excess entropies of mixing, ∆S^ex, are thus zero for the OH-Ap-F-Ap, as well as for the OH-Ap-Cl-Ap join. The C _p-T 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.

SUBMITTER: Dachs E 

PROVIDER: S-EPMC8553719 | biostudies-literature |

REPOSITORIES: biostudies-literature