EPR study of the Jahn–Teller effect of Cu$^{2+}$ ions in ZnGa$_2$O$_4$

The Jahn-Teller effect in the ZnGa$_2$O$_4$ spinel single crystal has been investigated using electron paramagnetic resonance of Cu$^{2+}$ ions in the temperature range 110–560 K. It has been shown that copper ions occupy octahedral sites 16$d$ in the ZnGa$_2$O$_4$ crystal with cubic symmetry $O_h^7$ (Fd-3m). At $T <$ 560 K, the octahedra undergo tetragonal distortions (predominantly tension) and rotation around the fourfold axes by the angle $\theta\approx$ 2.6$^\circ$. The parameters of the spin Hamiltonian, which characterize the prolate ($g_\parallel$ = 2.355, $g_\perp$ = 2.077, $A_\parallel$ = 116 Oe, $A_\perp$ = 12 Oe) and oblate ($g_\parallel$ = 2.018, $g_\perp$ = 2.246, $A_\parallel$ = 75 Oe, $A_\perp$ = 44 Oe) octahedra, have been determined. At temperatures above 560 K, the static Jahn-Teller effect transforms into the dynamic effect and the spectrum of the magnetic resonance becomes isotropic with $g$ = 2.116 (the experimental frequency corresponds to the X band).