Low-temperature heat capacity and thermodynamic properties of [Re-2(Ile)(4)(H2O)(8)](ClO4)(6) (Re = Nd, Er, Ile = isoleucine)

The heat capacities of two kinds of rare-earth element solid complexes with isoleucine [Re-2(Ile)(4)(H2O)(8)](ClO4)(6) (where Re = Nd, Er, and Ile = isoleucine) have been measured by an automatic adiabatic calorimeter in the temperature range from 80 to 370 K. Two solid-solid phase transitions were found from the C-p curve of Nd formed complex in the range of 165-175 K with a peak temperature of 167.88 K and in the range of 195-210 K with a peak temperature of 202.13 K. The corresponding molar enthalpies of these phase transitions were determined to be 404.61 J mol(-1) and 2.955 kJ mol(-1), respectively. One solid-solid phase transition was found for the Er formed complex in the range of 190-205 K with a peak temperature of 193.42 K. The corresponding molar enthalpy of this transition was 14.11 kJ mol(-1). Smooth heat capacities and thermodynamic functions relative to the standard state (298.15 K), H-T - H-298.15, S-T - S-298.15 and -[G(T) - G(298.15)], of the two compounds, were calculated on basis of experimental heat capacity data. Possible mechanisms of thermal decompositions for the pair of compounds were suggested according to the thermogravimetric (TG) analysis. (C) 2002 Elsevier Science B.V. All rights reserved