Optical spectrum of Cr3+ in octahedral fluoride lattices: Refinements on an open-shell SCF MO calculation

Abstract

The six lower electronic transitions observed in octahedral fluoride compounds containing Cr3+ are calculated in the framework of an open-shell SCF MO methodology. An initial, isolated-cluster description is presented and compared with a more elaborate representation which includes the external lattice potential, a cluster correlation energy correction, and spin-orbit and configuration interactions. When these refinements are included, the six transition energies observed in K2NaCrF6 are computed with a mean deviation of 0.6 kK, a sixth of the initial value. The theoretical equilibrium distance is only 0.014 Å removed from experiment. CI and cluster correlation energies play a competitive role in locating the t3 doublets; the calculation places t3−2T1g above t3−2Eg and below t2e−4T2g. Due to the variation of spin-orbit interaction with distance, these levels change in character so rapidly that a more realistic procedure than the usual Franck–Condon approximation has been employed in the calculation of the transition intensity pattern via the magnetic dipole mechanism. Surprisingly, the equilibrium distance of 1Γ7−4T2g is computed only 0.01 Å greater than that of the ground state, but this small separation still allows at least one vibrational overtone to have observable intensity. Some of our results may be related to cubic oxide compounds and in this context quantitative discussions are presented about the doublet-quartet mixing, the structure of the 4A2g→2T2g broad band, and the Stokes shift observed in the emission spectra of those crystals.