A calorimetric and thermodynamic investigation of strontium fluorapatite [formula omitted

Abstract

A calorimetric and thermodynamic investigation of one of the representatives of full-substituted fluorapatites, strontium-substituted fluorapatite {Sr10PO46F2}, was undertaken. The phase was synthesized according to the technique developed by the authors of the article based on the classical sol–gel process. The synthetic product was characterized by X-ray powder diffraction (XRD), X-ray fluorescence analysis (XRF), Scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and Differential thermal/gravimetric analysis (DTA-TG). The enthalpy of formation of the phase was determined using HNO3-solution calorimetry giving ΔfHmo(T=298.15K,Sr10PO46F2,cr)=(-13721±8)kJ∙mol-1. The low-temperature heat capacity, Cp,mo, was measured using adiabatic calorimetry from T = (6.8 to 307.0) K. The high-temperature heat capacity, Cp,mo, was measured using DSC calorimetry from T = (306.7 to 651.7) K. Using these Cp,mo(T) data, the third law entropy at T = 298.15 K, Smo, is calculated as (938.7 ± 3.8) J∙K−1∙mol−1. This new experimental result, together with literature data, are used to calculate the Gibbs energy of formation, ΔfGmo, giving: ΔfGmo(T=298.15K,Sr10PO46F2,cr)=(-13000±9)kJ∙mol-1. Smoothed Cp,moT values between T → 0 K and T = 650 K are presented, along with values for Smo and the functions [HmoT-Hmo0] and [GmoT-Gmo0]. Thermodynamic stability of cation-substituted fluorapatites, M10PO46F2 with M = Ca, Ba, Sr, Mg, was analyzed based on literature and experimental data.