Heat capacities and entropies of La2O2CO3 from T = (12 to 300) K and of Nd2O2CO3 from T = (12 to 930) K, and their interpretation

The heat capacities of La2O2CO3 and Nd2O2CO3 were determined by adiabatic s hield calorimetry from T = (12 to 300) K and T = (12 to 930) K, respectivel y. For the dineodymium dioxide carbonate a Schottky-type heat capacity with maximum near T = 60 K was observed. The transition is due to splitting of the I-4(9/2) ground term in the crystal field of the Nd3+. The vibrational contributions to the heat capacities of La2O2CO3 and Nd2O2CO3 are discussed in terms of a modified Kieffer model. Although the heat capacity can be mo delled reasonably well, deviations, especially at low temperatures, give ri se to significant errors in entropy. Thermodynamic properties at selected t emperatures have been evaluated, and the values of the molar heat capacitie s C-p,C-m, enthalpy increments Delta(0)(T)H(m), and entropies Delta(0)(T)S( m) for La2O2CO3 at T = 298.15 K and for Nd2O2CO3 at T = (298.15 and 900) K are (R = 8.3145 J.K-1.mol(-1)): [GRAPHICS] (C) 1999 Academic Press.