The heat capacity, thermodynamic functions, and $4f$-electron states of rare-earth compounds: Rare-earth oxides of the $A$ type

The paper deals with possible ways of approximating the contributions of the crystal lattice and $4f$-electrons to the total heat capacity of rare-earth (RE) compounds. In estimating the heat capacity of the lattice, use is made of the Debye model with due regard for the temperature dependence of the Debye temperature. The calorimetrically identified values of the Schottky heat capacity are compared with the results of quantum-statistical calculations of the heat capacity of Stark sublevels, determined from the experimentally obtained optical spectra and interpreted within the framework of the crystal field theory. The contributions of the lattice and $4f$-electrons to the thermal and thermodynamic functions of $A$-type oxides of lanthanum, cerium, praseodymium, and neodymium are calculated. Good agreement is observed between the predicted functions and the accepted values of heat capacity, entropy, and enthalpy.