Heat capacities and entropies of La2O2CO3fromT = (12 to 300) K and of Nd2O2CO3fromT = (12 to 930) K, and their interpretation

Abstract

The heat capacities of La2O2CO3and Nd2O2CO3were determined by adiabatic shield calorimetry fromT= (12 to 300) K andT= (12 to 930) K, respectively. For the dineodymium dioxide carbonate a Schottky-type heat capacity with maximum nearT= 60 K was observed. The transition is due to splitting of the4I9/2ground term in the crystal field of the Nd3+. The vibrational contributions to the heat capacities of La2O2CO3and Nd2O2CO3are discussed in terms of a modified Kieffer model. Although the heat capacity can be modelled reasonably well, deviations, especially at low temperatures, give rise to significant errors in entropy. Thermodynamic properties at selected temperatures have been evaluated, and the values of the molar heat capacitiesCp,m, enthalpy increments ΔT0Hm, and entropies ΔT0Smfor La2O2CO3atT= 298.15 K and for Nd2O2CO3atT= (298.15 and 900) K are (R= 8.3145J· K−1·mol−1): [table]