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 Pb 5 (AsO 4 ) 3 Cl 0.8 (CO 3 ) 0.1 Pb 5 (AsO 4 ) 3 F, Pb 5 (AsO 4 ) 3 Br 0.8 (CO 3 ) 0.1 , Pb 5 (AsO 4 ) 3 OH 0.86 (CO 3 ) 0.07 , and Pb 5 (AsO 4 ) 3 I 0.45 OH 0.35 (CO 3 ) 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.