N. V. Sidorov

Raman Spectra and Structural Perfection of Nominally Pure Lithium Niobate Crystals

We have studied the Raman spectra of nominally pure lithium niobate single crystals with congruent, near-stoichiometric, and stoichiometric compositions. The stoichiometric crystals have been grown by different procedures: from melts of congruent composition containing K2O (4.5 and 6 wt %) as an alkaline solvent (flux) and by a conventional procedure from a melt containing a considerable excess of Li2O (58.6 mol %). The results indicate that the growth procedure has a significant effect on the structural perfection and photorefractive properties of the crystals because it determines the nature and density of structural defects and, hence, the concentration of localized electrons. Stoichiometric crystals grown from a K2O flux, though less perfect than stoichiometric crystals grown by a conventional procedure, exhibit a reduced photorefractive effect in comparison with both stoichiometric crystals grown with no flux and crystals of congruent composition.

Effects of the ordering of structural units of the cationic sublattice of LiNbO3:Zn crystals and their manifestation in Raman spectra

We have studied the Raman spectra of congruent crystals of lithium niobate (LiNbO3) that were doped with Zn2+ ions in the range of concentration of 0–1.59 mol %. We have revealed a region of a more ordered structure such that the order of sequence of basic ions, impurity ions, and vacancies along the polar axis of the cationic sublattice is more regular, while the oxygen octahedra are close to ideal. In this case, crystals have a higher optical quality and are more stable with respect to optical damage. An increased ordering of the structure is realized because small amounts of Zn2+ cations displace NbLi defects and order the alternation of cations and vacancies along the polar axis and make the crystal less defect with respect to Li+ vacancies. Our results are important for industrial production of optically perfect lithium-niobate crystals by doping a congruent crystal with small concentrations of Zn2+ ions, since, in this case, technological regimes of crystal growth almost do not differ from regimes of growing of nominally pure congruent crystals.

Growth of heavily doped LiNbO3〈Zn〉 crystals

We have studied conditions for the growth of LiNbO3〈Zn〉 crystals in the composition range ≃4.0–8.91 mol % ZnO. It has been shown that, in the composition range ≃4–6.8 mol % ZnO in the melt, compositionally and optically homogeneous LiNbO3〈Zn〉 crystals can be grown. Above 6.8 mol % ZnO, imperfect crystals consisting of two distinct phases grow. We have accurately determined threshold impurity concentrations corresponding to significant changes in LiNbO3〈Zn〉 crystallization conditions.

Structure and Optical Homogeneity of LiNbO 3 〈Mg〉 Crystals Grown from Different Charges

This paper reports a comparative study of the homogeneity of LiNbO3〈Mg〉 (≥5 mol % Mg) crystals grown from a charge synthesized using Nb2O5〈Mg〉 prepared through homogeneous magnesium doping of a reextract in the Nb2O5 extraction step, and LiNbO3〈Mg〉 crystals prepared through direct doping of a growth charge. The LiNbO3〈Mg〉 crystals were characterized by optical microscopy and Raman spectroscopy. The results demonstrate that the LiNbO3〈Mg〉 crystals prepared using homogeneous doping are more structurally uniform.

Photorefractive Properties of Stoichiometric Lithium Niobate Single Crystals

The specific features of photorefractive light scattering in nominally pure stoichiometric (Li/Nb = 1) sin- gle crystals grown from a melt with 58.6 mol % Li2O (LiNbO3st) and in the stoichiometric single crystals grown from a melt of congruent composition in the presence of K2O flux (LiNbO3stK2O) have been investi- gated. At an excitation power of 30 mW, LiNbO3stK2O single crystals are found to exhibit a stronger photo- refractive effect than LiNbO3st single crystals.

Optical homogeneity, defects, and photorefractive properties of stoichiometric, congruent, and zinc-doped lithium niobate crystals

Using the laser-conoscopy method, the photorefractive light-scattering method, and the Raman light-scattering method, we have studied the structural and optical homogeneities and photorefractive properties of (i) stoichiometric lithium niobate crystals (LiNbO3(stoich)), which were grown from a melt with 58.6 mol % of Li2O; (ii) congruent crystals (LiNbO3(congr)); and (iii) congruent crystals that were doped with Zn2+ cations (LiNbO3:Zn; [Zn] = 0.03–1.59 mol %). We have shown that the speckle-structure of the photorefractive light scattering in all the crystals is three-layer. The shapes of the second and third layers repeat in general the shape of the first layer. We have shown that the differences that are observed between the Raman spectra, the photorefractive light scattering, and the conoscopic patterns of the examined crystals are caused by the fact that defects are distributed inhomogeneously over the volume of these crystals and that Zn2+ cations are incorporated inhomogeneously into the lattice. This leads to the appearance of local changes in the elastic characteristics of the crystal and to the appearance of mechanical stresses, which locally change the optical indicatrix and, correspondingly, the conoscopic pattern and the Raman spectrum.

Investigation of structural peculiarities of impure lithium niobate crystals by Raman spectroscopy

Abstract Polarized Raman spectra of lithium niobate single crystals doped with Mg 2+ , Gd 3+ , Y 3+ and Ta 5+ cations were studied. Threshold concentrations for Mg 2+ and Gd 3+ cations were shown to exist below which these dopants cause an ordering effect of the Li + and Nb 5+ cations and an octahedral alignment of the hollows along the crystal polar axis. This ordering can be seen in spectra as a narrowing of the 254 and 274 cm −1 modes corresponding to fundamental vibrations of the Li + and Nb 5+ ions located inside the oxygen octahedra and as a splitting into two components of the two-phonon line in the region of 100–120 cm −1 caused by the light scattering on two bound acoustic phonons of A 1 symmetry. Above the maximum concentrations Mg 2+ and Gd 3+ ions disorder the crystal structure, which leads to the loss of the fine structure for the two-phonon line whereas the 254 and 274 cm −1 modes considerably broaden and almost merge into one band. The Y 3+ and Ta 5+ ions worsen the ordering of the cations and the octahedral hollows along the polar axis and cause, in addition, deformation of the crystal oxygen carcass.

Effects of nonstoichiometry and doping on the curie temperature and defect structure of lithium niobate

LiNbO3 crystals doped with Gd, Zn, Cu, and Er to various levels were grown by the Czochralski technique. The structural parameters, Curie temperature, and density of the LiNbO3 crystals were determined as a function of composition. With increasing Gd content, thea parameter of the hexagonal cell increases, c remains constant within the experimental error, density increases, and Tc gradually decreases. The defect structure and properties of LiNbO3 doped with Gd, Zn, Cu, Er, Mg, and Ta are contrasted with those of nominally undoped LiNbO3 crystals differing in Li/Nb ratio. The results suggest that the variation of the Curie temperature with composition is determined primarily by the position that the dopant cations occupy in the structure of lithium niobate.

Growth of large LiNbO3〈Mg〉 crystals

We have analyzed conditions for the growth of large (≥80 mm in diameter) LiNbO3〈Mg〉 crystals in a wide range of dopant concentrations in the melt (2.9–5.8 mol % MgO). We have established conditions for the growth of large LiNbO3〈Mg〉 crystals uniform in doping level from melts containing magnesium concentrations above a certain threshold (Cmelt 5.0–5.8 mol % MgO) and optimized the high-temperature electrodiffusion annealing process, which allowed us to obtain single-domain, microstructurally homogeneous, large LiNbO3〈Mg〉 crystals containing 4.9–5.15 mol % magnesium.

Modeling of Cluster Formation in Nonlinear Optical Lithium Niobate Crystal

The processes occurring during the formation of energetically equilibrium oxygen-octahedral clusters in the ferroelectric phase of lithium niobate (LiNbO3) crystal, have been qualitatively modeled in dependence of the phase composition. The modeling results are compared with the data obtained within vacancy models. It is shown that the cluster structure constructed along the crystallographic Y axis is most ordered, while that constructed along the polar Z axis is least ordered. The largest spread in the ratio R = Li/Nb is observed in the direction of the Z axis.