Entropies of phase transitions in the M3LnCl6 compounds (M ≡ K, Rb, Cs; Ln qq La, Ce, Pr, Nd) and K2LaCl5

The molar enthalpies of the solid-solid and solid-liquid phase transitions were determined by differential scanning calorimetry for the compounds K2LaCl5, Rb3LaCl6, Cs3LaCl6, K3CeCl6, Rb3CeCl6, Cs3CeCl6, K3PrCl6, Rb3PrCl6, Cs3PrCl6, K3NdCl6, Rb3NdCl6 and Cs3NdCl6. The K3LnCl6 (Ln ≡ Ce, Pr, Nd) and Rb3LaCl6 compounds, which do not exist at room temperature, are formed at high temperature with a considerable loss of enthalpy (48-55 kJ mol-1) compensated by a high gain in entropy (63-65 J mol-1 K-1). The Cs3LnCl6 (Ln - La, Ce, Pr, Nd) compounds are stable at ambient temperature; they undergo a transition in the solid state with a corresponding enthalpy and entropy of 7.4-7.8 kJ mol-1 and 10.9-11.2 J mol-1 K-1. The calorimetric measurements show that Rb3CeCl6, Rb3PrCl6 and Rb3NdCl6 are also probably stable at room temperature. The enthalpies and entropies of fusion are reported for all the compounds mentioned above.