Energetics and IR spectroscopy of Pb5(AsO4)3X (X = F, OH, Cl, Br, I) synthetic apatite phases

In this work, we measured heat capacity and enthalpy of formation of synthetic mimetite phases with composition Pb5(AsO4)3Cl0.8(CO3)0.1 Pb5(AsO4)3F, Pb5(AsO4)3Br0.8(CO3)0.1, Pb5(AsO4)3OH0.86(CO3)0.07, and Pb5(AsO4)3I0.45OH0.35(CO3)0.1. Heat capacity has been measured using relaxation calorimetry and differential scanning calorimetry in the temperature range of 2.1–300 K and 280–770 K, respectively. The standard third-law entropy of phases in question, derived from the low-temperature heat capacity measurements, is S°298.15K = 638.9 ± 4.5, 627.8 ± 4.4, 639.7 ± 4.5, 630.5 ± 4.5 and 637.7 ± 4.5 J mol−1 K−1, respectively. The measured enthalpies of formation derived from acid solution calorimetry are − 3068.7 ± 9.4, − 3001.4 ± 3.5, − 2963.4 ± 9.2, − 2919.1 ± 9.1, and − 2936.3 ± 8.7 kJ mol−1, respectively. Some of the thermodynamic values deviate from previously determined data, most likely because of carbonate substitution in our samples.