Site symmetries of vanadium ions in La3Ga5.5Ta0.5O14 single crystals

Vanadium-doped La3Ga5.5Ta0.5O14 single crystal (0.05 mol% in the melt with respect to Ga) grown by the Czochralski method has been investigated using electron paramagnetic resonance (EPR) and optical absorption measurements. The optical energy gap of pure La3Ga5.5Ta0.5O14 single crystal is equal to 4.94 eV, while the value for vanadium-doped La3Ga5.5Ta0.5O14 single crystal is 3.44 eV. Optical absorption measurements have revealed 444 nm, 700 nm and 1100 nm bands arising mainly from V3+ ions at octahedral sites. Moreover, V4+ ions were found at octahedral sites (525 nm band) for the ''as-grown'' crystals and at tetrahedral positions (380 nm CT band and 600- and 800 nm dd transitions) for crystals irradiated with γ-quanta with a dose of 1.2 x 105 Gy. The X-band EPR measurements have been done at room temperature while rotating the single-crystal sample around a, b* and c crystal axes. For each rotation direction the observed spectra consist of an intense multiline signal centred at ∝340 mT and a second, weak one at ∝150 mT. The dominant signal can be ascribed to the V4+ ions (S=1/2,I=7/2) occupying the Ga1, Ta octahedral positions, with an axial D 3 point symmetry. The principal tensor values are determined as g parallel =1.925(5), g perpendicularto =1.958(5), A parallel =17.0(5) mT and A perpendicularto =5.8(5) mT. Each principal axis of the vanadium paramagnetic centres indeed reveals three different orientations in La3Ga5.5Ta0.5O14 crystal with respect to the space orientation of the Ga3 tetrahedra connected with the Ga1 octahedron. The low-field resonance signal could neither be ascribed to iron impurities nor to the V4+ ions (S=1/2,I=7/2) occupying other (tetrahedral) positions simultaneously creating the vacancy state. This signal was too weak to perform detailed analysis. There are also presented low-temperature EPR spectra that did not reveal any new lines. (copyright 2006 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)