The Jahn-Teller effect of octahedrally co-ordinated 3d⁴ions

Abstract

The chromous ion in magnesium oxide is studied and an interpretation is given of the acoustic paramagnetic resonance results of Marshall and Rampton. Because of the very strong orbit-lattice interaction it is important to take full account of the Jahn-Teller effect. The cluster approximation justified previously is used, and then are added the spin-orbit coupling, anharmonic forces, external magnetic field and random lattice strains. The resultant secular equations are usually solved numerically in terms of parameters to describe the strains and anharmonic couplings. It is postulated that the observed line shapes reflect random strain distributions and that peaks in the lines are correlated with turning points in the strain-resonance field curves. Parameters are chosen to fit the observations for a definite field orientation and it is shown that a good interpretation can then be given for the results for a variety of orientations. The energy-level diagram of the chromous ion is shown to be quite complicated, and it is pointed out that many more transitions than have yet been observed can be expected.