Enthalpy of formation of Ln2O2CO3 II (Ln = La, Nd, Eu) and thermodynamics of decomposition equilibria

High temperature oxide melt solution calorimetry using molten 2PbO·B2O3 at 1078 K and molten 3Na2O·4MoO3 at 976 K has been applied to determine standard enthalpies of formation, ΔfHm° (298 K), of Ln2O2CO3 II (Ln = La, Nd, Eu) from their oxide components Ln2O3 and CO2 at 298 K. Two solvents were applied to cross-check the thermodynamic cycles. Using molten 2PbO·B2O3, ΔfHm°(298 K) for La2O2CO3 II, Nd2O2CO3 II and Eu2O2CO3 II were found as −198.0 ± 1.7, −176.2 ± 3.1 and −151.4 ± 6.5 kJ mol−1, respectively. Using molten 3Na2O·4MoO3 the corresponding values for Nd2O2CO3 II and Eu2O2CO3 II were determined as −176.6 ± 3.5 and −154.9 ± 3.2 kJ mol−1, respectively. These enthalpies of formation are discussed in view of prior thermodynamic data, discrepancies are analyzed, and recommended values for enthalpy, entropy and free energy of formation for La2O2CO3 and Nd2O2CO3 are given.