Multiphonon optical bands of Cr³⁺ ion in sulphide spinels: A pseudo Jahn-Teller model

In this article we report the adiabatic and dynamic solutions of the pseudo Jahn-Teller (JT) problem in the orbital triplet ⁴T₂(t₂²e) of Cr³⁺ ion resulting from the combined effect of tetragonal (E) vibrations and trigonal crystal field in sulphide spinels lattice. We discuss the general properties of the adiabatic surfaces that give a clear qualitative insight on the main features of the optical bands. The shapes of the absorption bands are calculated on basis of the quantum-mechanical solution of the dynamic pseudo JT problem taking into account the effective electric-dipole moment involving odd-parity crystal field potential. Pseudo JT (⁴A₁+⁴E)⊗e interaction is shown to produce a complicated structure of the multiphonon optical bands and leads to a specific polarization effect. Calculated shape function for the U-band absorption proves to be in a good agreement with the excitation spectrum of α - ZnAl₂S₄ tiospinel doped with chromium ions, the experimentally detected peculiarities of the structure of this band are shown to be an evidence of strong pseudo JT interaction in the excited state.