EPR and optical properties of La3Ga5.5Ta0.5O14:Co2+ single crystals

Cobalt doped La3Ga5.5Ta0.5O14 single crystal (0.5 mol.% in the melt with respect to Ga) grown by the Czochralski method has been investigated using electron paramagnetic resonance and optical absorption measurements. X-band EPR spectra observed at 8 K under rotation around c crystal axis consists of two asymmetric lines: a narrow line at similar to 150 mT magnetic field and a second wider line at similar to 350 mT. These lines could be ascribed to the high spin S=3/2 Co2+ ions located at two different crystallographic positions. The low field signal arises from cobalt occupying Ga positions with C-3 symmetry (octaheder), whereas dominating in spectrum, more complicated signal at higher fields arises from cobalt located at Ga positions (tetraheder) with C-2 point symmetry. The whole spectrum repeats every 60 degrees as is expected for trigonal crystal symmetry. Optical measurements have confirmed the conclusion especially for gamma-irradiated crystals which reveal additional Co3+ content after the treatment. Co3+ ions are also seen in "as-grown" (not irradiated) crystals as T-5(2)-E-5 transition giving absorption bands in the range 700-1100 nm. So in the "as-grown" crystals there arise Co2+ ions at octahedral or tetrahedral Ga sites and Co3+ ions at octahedral Ga sites.