Electron paramagnetic resonance and luminescence of chromium in calcium germanate crystals

The luminescence of Ca₂GeO₄: Cr⁴⁺ single crystals at wavelengths in the range of 1.3 μm upon excitation with a 1-μ m semiconductor laser is investigated in the temperature range up to 573 K. At T<110 K, the Ca₂GeO₄: Cr⁴⁺ crystals are characterized by the electron paramagnetic resonance, which is attributed to the Cr⁴⁺ ions substituted for Ge⁴⁺ ions. The components of the g tensor and its principal axes are determined. It is revealed that the Cr⁴⁺ impurity centers in calcium germanate affect the crystal symmetry to a lesser degree compared to Cr⁴⁺ ions in forsterite. The observed deviation of the temperature dependence of the electron paramagnetic resonance from the Curie law is explained by the transition to the excited state with a low activation energy, as is the case in impurity 3d ions in diamond-like semiconductors. The inference is made that the giant effective degeneracy multiplicity of the excited state is associated with the initiation of soft phonon modes in the crystal upon excitation of the defect.