Electronic structure, interactions between Frenkel defects and ionic conductivity characteristics in the MeF2 (Me = Ca, Sr, Ba, Pb) fluorite-type compounds - Ab initio LMTO-TB supercell approach

摘要

The electronic band structure for a series of perfect fluorite-type crystals MF2, M = Ca, Sr, Ba, Pb, and for the crystals containing Frenkel defects has been studied by means of the LMTO-ASA-TB approach. As basis the supercell M8F16 has been used. The energies of the defect formation and migration have been evaluated in reasonable agreement with experiment. The interactions between anionic defects (AED) have been studied depending on the number of defects, on the density of the AFD location and on the geometric parameters. It has been shown that at the shortest distance an attraction between two AFD exists, so a tendency to close packing of AFD appears. With the increase of the AFD concentration the energy of AFD creation decreases. The parameters describing the creation energy of a noninteracting AFD and the interactions between the AFD have been calculated. The peculiarities of chemical bonding in PbF2 have been discussed. It has been shown that some covalency exists in this compound that favours the location of the AFD in the trigonal interstitial site. [References: 40]