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 2–298.15 K. 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 respectively. Some of the thermodynamic values deviate from previously determined data, most likely because of carbonate substitution in our samples.