EPR study of charge compensation of chromium centers in the strontium titanate crystal

Chromium centers and their charge compensation in two single strontium titanate crystals, i.e., SrTiO$_3$: Cr (0.05 at%) and Sr$_{0.9995}$TiO$_3$:Cr$_{0.0005}$ grown with strontium deficiency, have been studied by the electron paramagnetic resonance method. The crystals have been investigated both immediately after growth and after oxidation and reduction procedures. Oxidation and reduction are performed by crystal annealing in a corresponding gas atmosphere at high temperature. Chromium centers associated with oxygen vacancy (Cr$^{3+}$ – V$_{\mathrm{O}}$) are detected in the reduced crystals. It is shown that strontium vacancies are formed in the crystal grown with strontium deficiency, which leads to a lowering of the tetragonal symmetry of (Cr$^{3+}$ – V$_{\mathrm{O}}$) and Cr$^{5+}$ centers to the orthorhombic symmetry. Possible compensation mechanisms for charges of various chromium centers are considered.