Jahn-Teller coupling in spinel-type crystals doped with transition metal ions

In this article, we report the theoretical evaluation of the vibronic constants for the spinel-type crystals ZnAl2S4, ZnAl2Se4, CdAl2Sc4, and CdIn2S4 doped with the transition metal ions Cr3+, Cr4+, Ti2+, and Ti3+. In these crystals the ions Cr3+, Ti3+ and Cr4+, Ti2+ occupy positions with octahedral and tetrahedral surrounding correspondingly. In the case of a tetrahedral complex the interaction with full symmetric A(1), tetragonal E and two trigonal vibrations T-2((1)) and T-2((2)) is considered, while for the octahedral cluster along with the A(1) and E vibrations only one trigonal vibrational mode is taken into account. The examined semiconductor systems are mainly covalent and application of the crystal field point charge model loses proper exactity. For this reason, we employ the exchange charge model for the crystal field that accounts for the covalence effects and provides relatively simple expressions for the crystal field and vibronic parameters keeping at the same time a reasonable level of accuracy. The vibronic coupling constants are numerically calculated and for each case an appropriate multi-mode Jahn-Teller vibronic problem is found out. (c) 2007 Elsevier B.V. All rights reserved.