ELECTRONIC FACTORS INFLUENCING THE LOCAL AND COOPERATIVE JAHN-TELLER INTERACTIONS IN SPINELS WITH COPPER(II) AND NICKEL(II)

Optical spectra and structural data of mixed crystals CuxZn1-xCr2O4, NixZn1-xCr2O4, and Cu1-xNixCr2O4 are reported. The optical results are used as a probe characterizing the d -contributions to the Cu-O bonds. The cooperative Jahn-Teller forces are analyzed, extending the statistical model by Wojtowicz to the tetrahedral sublattice. Thermodynamical and optical data are used to estimate the corresponding cooperative strain energies. A microscopic bonding model is proposed which is based on the bonding properties of the oxygen ligands with respect to Cu2+ due to the presence of the “contrapolarizing” Cr3+ cations. This concept makes it possible to understand the enhancement of the local Jahn-Teller distortions with increasing cooperativeness. © 1993 Academic Press, Inc.