A calorimetric and thermodynamic investigation of strontium fluorapatite Sr10PO46F2

Abstract A calorimetric and thermodynamic investigation of one of the representatives of full-substituted fluorapatites, strontium-substituted fluorapatite { S r 10 P O 4 6 F 2 } , 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 Δ f H m o ( T = 298.15 K , S r 10 P O 4 6 F 2 , c r ) = ( - 13721 ± 8 ) k J ∙ m o l - 1 . The low-temperature heat capacity, C p , m o , was measured using adiabatic calorimetry from T = (6.8 to 307.0) K. The high-temperature heat capacity, C p , m o , was measured using DSC calorimetry from T = (306.7 to 651.7) K. Using these C p , m o ( T ) data, the third law entropy at T = 298.15 K, S m o , 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, Δ f G m o , giving: Δ f G m o ( T = 298.15 K , S r 10 P O 4 6 F 2 , c r ) = ( - 13000 ± 9 ) k J ∙ m o l - 1 . Smoothed C p , m o T values between T → 0 K and T = 650 K are presented, along with values for S m o and the functions [ H m o T - H m o 0 ] and [ G m o T - G m o 0 ] . Thermodynamic stability of cation-substituted fluorapatites, M 10 P O 4 6 F 2 with M = Ca, Ba, Sr, Mg, was analyzed based on literature and experimental data.