Multi-minimum adiabatic potential in the single crystal normal spinel ZnAl₂O₄, doped by Cu²⁺ ions

Spectroscopic investigations of a ZnAl₂O₄ spinel doped with bivalent copper ions of 0.05% concentration have been carried out in the temperature range 4.2-290K using a 3cm^-1 range electron paramagnetic resonance (EPR) spectrometer having an operational frequency f = (9.241 ±0.001)GHz. The spectrum can be represented as a superposition of two components: a low-temperature (LT) and a high-temperature (HT) one. Redistribution of integrated intensity between HT and LT components of the spectra occurs with temperature change that is typical of systems with multi-minimum adiabatic potential. Spectra observed are explained within the modified theory of crystalline field (MTCF). The electron levels of a Cu ²⁺ ion placed in an octahedral coordination center with trigonal distortion [CuO₆]^10- have been calculated. The influence of possible types of oxygen octahedron distortions and possible displacement of copper ions from the symmetry center on the electron spectrum, as well as the shape of the adiabatic potential, has been analyzed. It is shown that in the low-temperature phase the multiple minima of the adiabatic potential occur due to tetragonal distortions while the depth of a minimum is determined by the degree of trigonal octahedron distortions. Tetragonal distortion values and multi-minimum potential barrier heights have been determined.