High-frequency epr spectroscopy of Cr$^{3+}$ ions in $\beta$-Ga$_2$O$_3$ single crystals

Beta-phase gallium oxide ($\beta$-Ga$_2$O$_3$) bulk single crystals doped with Cr$^{3+}$ ions were investigated by the electron paramagnetic resonance (EPR) at a frequency of 94 GHz (W-band) and a temperature of 6 K. In the EPR spectra of samples with a high concentration of trivalent chromium, in addition to three well-known lines of the Cr$^{3+}$ ions ($S$ = 3/2) in an octahedral oxygen environment, three less intense lines were observed. These signals were attributed to chromium ions in a tetrahedral position based on the analysis of anisotropic EPR spectra and numerical modeling of experimental angular dependences. The fine structure parameters of the spin Hamiltonian, the directions of the magnetic axes and energy level diagram for the ground state of this new center were determined in this paper.