Gitterenergieberechnungen zur Ermittlung der wahrscheinlichsten Strukturtypen für Verbindungen MX ₆

Abstract

Lattice-energy calculations were performed in order to find out which of the possible structure types for compounds MX 6 are the most probable to occur. The structure types considered were those previously derived by assuming a close-packed arrangement of X atoms in which one-sixth of the octahedral holes are occupied by M atoms. The consequences of distortions of the close packing were studied by allowing the occupied octahedra to be widened or shrunk relative to the unoccupied octahedra. An (exp-6-1) interatomic potential function was used; as for heavier atoms only very few of the corresponding parameters are known, values for X= Cl and M= Se were taken. Atomic site charges were varied from 0 to 6 for M and from 0 to -1 for X and the lattice constants yielding maximum lattice energies were determined. For octahedra that had been shrunk, the favoured structure type is,the t~-WCl 6 type when the M site charge is between +1.5 and +4 and the UF 6 type when the site charge is higher. For widened octahedra, the UF 6 type becomes favored with M site charges higher than +2. A structure type in the space group Pnmn which has not yet been found seems to be as probable as the mentioned structure types for M site charges of about +2. The optimum structure for nonpolar MX 6 molecules (M site charges < 1) should be an unknown structure type in which the molecules are stacked to form columns (space group P31m). Structures with cubic close packing of the X atoms are not expected to exist.