Zeeman Study of the Jahn-Teller Effect in theT2g3State ofAl2O3:V3+

Abstract

The fine structure of the zero-phonon $^{3}T_{2g}$ state of ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$: ${\mathrm{V}}^{3+}$ arising from the trigonal field and spin-orbit coupling is anomalously small. Ligand-field theory predicts a 400-${\mathrm{cm}}^{\ensuremath{-}1}$ splitting, while the observed levels, first fully identified by Scott and Sturge, span only 14 ${\mathrm{cm}}^{\ensuremath{-}1}$. This has been attributed to a large tetragonal Jahn-Teller effect, and a detailed analysis of the zero-phonon levels was made by Scott and Sturge on the basis of Ham's model calculations for octahedral molecules. We have made new measurements on the 0-0 $^{3}T_{2g}$ band, including the axial spectrum, which shows one $\ensuremath{\sigma}$-polarized transition to be magnetic dipole in origin, and the Zeeman effect of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ spectra at 10\ifmmode^\circ\else\textdegree\fi{}K in fields up to 40 kG parallel and perpendicular to the trigonal axias. The validity of the Ham-Scott-Sturge model has also been further investigated. We have shown that the details of the Scott-Sturge theory require modification, and that it is possible to explain the observed zero-field level pattern within the framework of Ham's general model if, but only if, the assumption of equal quenching of first-order trigonal field and spin-orbit splittings is dropped. In addition, we have obtained values of the first- and second-order trigonal field and spin-orbit coupling energies which reproduce quantitatively the zero-field energies, Zeeman behavior, selection rules, and intensities of all transitions. It is found that the first-order spin-orbit splitting is some four times less quenched than the first-order trigonal-field splitting.