The properties of (3d)4ions in corundum. III. The Jahn-Teller theory of the5E ground state and the parameters for Cr2+

Abstract

For pt.II see abstr. A8428 of 1973. A detailed theoretical discussion is given of the Jahn-Teller theory of a ground state orbital doublet in a trigonal environment. In particular, the isoelectronic ions Cr²⁺ and Mn³⁺ in Al₂O₃ are studied and it is found that the site symmetry may be approximated to C_3v. A cluster model is used to evaluate the coefficients in the hamiltonian describing the spin system which is derived using symmetry arguments. Various approximations in the theory are discussed in detail. It is shown that the system is characterized by four parameters. The more important are the inversion splitting delta and lambda ²/ Delta . Data previously obtained by APR experiments when the magnetic field is along the c axis are used to obtain estimates for the parameters for Cr²⁺. The best fit to the data is obtained by taking the anharmonic coefficient B to be positive, delta approximately 3.0 cm^-1. and lambda ²/ Delta approximately 0.6 cm^-1 An energy level diagram appropriate to Cr²⁺ in constrained sites in Al₂O₃ is also given.