EPR and optical properties of La 3 Ga 5.5 Ta 0.5 O 14 :Co2+single crystals

Abstract

Cobalt doped La 3 Ga 5.5 Ta 0.5 O 14 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 Co 2+ 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° as is expected for trigonal crystal symmetry. Optical measurements have confirmed the conclusion especially for γ-irradiated crystals which reveal additional Co 3+ content after the treatment. Co 3+ ions are also seen in as-grown (not irradiated) crystals as 5 T 2 - 5 E transition giving absorption bands in the range 700-1100 nm. So in the as-grown crystals there arise Co 2+ ions at octahedral or tetrahedral Ga sites and Co 3+ ions at octahedral Ga sites.