Estimates for the enthalpy of formation of rare-earth oxyhalides with the P4/ nmm structure

Abstract

The enthalpies of formation of the rare-earth oxyhalides that have the P4/ nmm (PbFCl) structure are estimated through the energy of formation W t of the species from the gaseous ions. These estimates afford reasonable values for the energies of dissociation into oxide and halide phases. However, to obtain agreement with lattice energy calculations using the Busing potential energy function and lattice sum computer program, we were forced to abandon the assumption that the van der Waals and the repulsive energy parameters in this potential energy were unique for each type of ion. Rather, we required variabilities in these parameters for the oxide and for the halides across the rare-earth series and for the metal ions from halide to halide. The number of such parameters to be found increased from 54 to 405. To restore uniqueness, we chose to invoke the criterion of smooth behavior from species to species, in crystallographic parameters, in the repulsive and van der Waals parameters, in the total energy and its components, and in the individual bond energies and their components. A unique set of crystallographic parameters and Busing parameters was found for all oxyhalide phases which gave the values for W t as estimated, and which essentially satisfied the smoothness criterion. The criterion of smoothness leads to lattice descriptions somewhat different from published crystallographic data, particularly for the oxybromides.