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11 October 2005 EPR and optical properties of La3Ga5.5Ta0.5O14:Co2+ single crystals
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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 ~150 mT magnetic field and a second wider line at ~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 C3 symmetry (octaheder), whereas dominating in spectrum, more complicated signal at higher fields arises from cobalt located at Ga positions (tetraheder) with C2 point symmetry. The whole spectrum repeats every 60° as is expected for trigonal crystal symmetry. Optical measurements have confirmed the conclusion especially for γ-irradiated crystals which reveal additional Co3+ content after the treatment. Co3+ ions are also seen in "as-grown" (not irradiated) crystals as 5T2-5E 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.
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H. Fuks, S. M. Kaczmarek, and M. Berkowski "EPR and optical properties of La3Ga5.5Ta0.5O14:Co2+ single crystals", Proc. SPIE 5958, Lasers and Applications, 595826 (11 October 2005);

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