Growth and properties of Ru-doped lithium niobate crystal

Ruthenium (Ru)-doped lithium niobate (LiNbO₃) single crystals were grown by the Czochralski method from a congruent melt composition. The color of the Ru: LiNbO₃ was red and darkened with increasing Ru concentration. The Ru concentration in the grown crystal gradually decreased along the pulling direction because the effective segregation coefficient of Ru in lithium niobate is greater than one, and also because RuO₂ evaporated during the crystal growth period. However, a solubility limit does exist in Ru: LiNbO₃ crystals, so the maximum amount of Ru doped into LiNbO₃ single crystals is about 0.2 mol%. In addition, the lattice constants of Ru: LiNbO₃ on the A- and C-axis decreased with the concentration of Ru in the crystal. An absorption spectra examination of Ru: LiNbO₃ showed that there were two absorption peaks around 370 and 530 nm that is within the UV/VIS region. The absorption coefficients should increase, and the absorption edges shift toward longer wavelength as the Ru concentration increases. The OH^- absorption spectra confirm that Ru ions may have substituted for both Li vacancies (V^-_Li) and antisite Nb ions (Nb⁴⁺_Li) as the Ru doping concentration in the present case is well below the threshold limit.