Low-temperature heat capacity and thermodynamic properties of crystalline [Re2(Ala)4(H2O)8](ClO4)6 (Re = Eu, Er; Ala = alanine)

[Re2(Ala)4(H2O)8](ClO4)6 (Re=Eu, Er; Ala=alanine) were synthesized, and the low-temperature heat capacities of the two complexes were measured with a high-precision adiabatic calorimeter over the temperature range from 80 to 370 K. For [Eu2(Ala)4(H2O)8](ClO4)6, two solid–solid phase transitions were found, one in the temperature range from 234.403 to 249.960 K, with peak temperature 243.050 K, the other in the range from 249.960 to 278.881 K, with peak temperature 270.155 K. For [Er2(Ala)4(H2O)8](ClO4)6, one solid–solid phase transition was observed in the range from 270.696 to 282.156 K, with peak temperature 278.970 K. The molar enthalpy increments, ΔHm, and entropy increments,ΔSm, of these phase transitions, were determined to be 455.6 J mol−1, 1.87 J K−1 mol−1 at 243.050 K; 2277 J mol−1, 8.43 J K−1 mol−1 at 270.155 K for [Eu2(Ala)4(H2O)8](ClO4)6; and 4442 J mol−1, 15.92 J K−1 mol−1 at 278.970 K for [Er2(Ala)4(H2O)8](ClO4)6. Thermal decompositions of the two complexes were investigated by use of the thermogravimetric (TG) analysis. A possible mechanism for the thermal decomposition is suggested.