Stability of Transition-Metal Impurities in Ionic Fluorides from Approximate Hartree-Fock Cluster Calculations

Abstract

The location of the 3d levels of 10 transition-metal impurities in the band gaps of four ionic fluorides has been investigated by independent Hartree-Fock (HF) cluster calculations on open-shell M Fn-6 systems [M = Crz+ (z = 1-3), Mnz+ (z = 1-3), Fez+ (z = 1-2), and Vz+ (z = 2-3)] and closed-shell AFn-6 systems (A = Li+, Na+, Mg2+ and Zn2+). The variation of the 3d orbital energies of the impurities with the M-F distance has been deduced from the corresponding MFn-6 calculations at different cluster sizes. The band gap edges for LiF, NaF, KMgF3, and KZnF3 have been estimated from the A Fn-6 calculations: the highest occupied level has been estimated from HF orbital energies of the 1A1g ground state, and the lowest unoccupied level, from HF orbital energies of the first excited 3T1u state. These orbital energies have been combined to prepare approximate energy-distance diagrams that describe the location of the 3d impurity levels relative to the band gap of the host crystal. The diagrams give very valuable information on the stability of the 3d impurity in the host and the probable lattice relaxation induced upon impurity substitution. Such information is difficult to obtain either experimentally or by means of large-scale quantum-mechanical calculations.