Pyrochlores. X. Madelung energies of pyrochlores and defect fluorites

Abstract

Electrostatic (Madelung) lattice energies have been calculated for a wide range of compounds A2B2X6Y with the pyrochlore-type structure. The cases of disorder and partial disorder on the non-metal sites have been examined, as well as the inverse case, where Y is a positive ion. The results of the calculations are compared with those for defect fluorite structure and an attempt is made to relate them to the conditions under which the pyrochlore phase is found. The effect of the site potential in restricting the existence boundaries of the pyrochlore phase is examined, as the potential must have the correct sign relative to the charge occupying the site. Alternative pyrochlore arrangements in which the 8(b) or 32(e) equipoints are occupied have also been explored.