Preparation and investigation of regularly ordered LiNbO3 - the basis for tailoring fundamental properties and advanced applications

G.Malovichko, V.Grachev, O.Schirmer

The aim of the project was the optimisation of lithium niobate in order to arrive at a basis for better exploiting the numerous attractive properties of this well-known electro-optic material. The way of the solution of the problem was investigations by different complementary methods (magnetic resonances, optical absorption and luminescence, X-ray analysis and electrooptical measurements) and comparison of the properties of the conventional, congruently melting LiNbO3 (LN), which is strongly disordered and Li-deficient, and recently discovered ideally structured LN. The studies included: a) characterisation of the crystals by several methods, b) investigation of the chemical, geometrical, electronic and energetic structure of intrinsic (non-stoichiometric) and extrinsic (impurity) defects and their influence on LN properties.

Scientific Results

A new approach to the intrinsic and extrinsic defect subsystems, considering them as one integrated functional system, was developed for complex oxides. The strong interrelation of these subsystems becomes especially apparent when concentrations of both defect classes are comparable.  It is found that crystals with vanishingly small concentration of intrinsic defects offer extraordinary informative opportunities due to tremendous narrowing of resonance lines.

Besides the main axial Cr3+ center, eight satellite chromium centers were experimentally resolved by EPR and parameters of their spin-Hamiltonians were. From the correlation of EPR, optical absorption and luminescence spectra it was derived that the satellite centers include two defects - CrLi and niobium vacancy in the first or further neighbouring shells. Since in conventional congruent crystals the relative concentrations of additional satellite centers and the main center are comparable, both kinds of centers are equally responsible for many of the LN properties.

Doping by Cr  leads to a drastic decrease of the electrooptical coefficient r, which is still enhanced in the congruent crystal compared with the stoichiometric sample. In pure LN crystals, the electrooptical coefficients r22 and rc exhibit large and non monotonic dependences as a function of the deviation from stoichiometric composition.

All our results emphasize the necessity to check very precisely both the crystal composition and doping concentration. This is of the first importance for an appropriate use of LiNbO3 crystals in devices.

Main results were published in:

G.Malovichko, V.Grachev, E.Kokanyan, O.Schirmer. Axial and low-symmetry centers of trivalent impurities in lithium niobate. Chromium in congruent and stoichiometric crystals.- Physical ReviewB59, 9113-9125 (1999).

Influence of chromium doping on the electro-optic properties of lithium niobate. K.Chah, M.Aillerie, M.D.Fontana, G.I.Malovichko, K.Betzler, E.Kokanyan. -Opt. Communs., v. 136, pp. 231-234, 1997.

 Electro-optical properties of chromium-doped LiNbO3. K.Chah, M.Aillerie, M.D.Fontana, G.I.Malovichko, E.Kokanyan. -Ferroelectrics, v. 155, pp. 13-16 (963-966), 1996.

Effect of chromium concentration on site selective luminescence in nearly stoichiometric lithium niobate crystals. P.Bourson, G.Malovichko, A.Ridah, E.Kokanyan.- Ferroelectrics, v. 155, pp. 273-276 (907-910), 1996.