M. N. Palatnikov

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.

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.

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.

Mechanical properties of Nb2O5 and Ta2O5 prepared by different procedures

We have studied the mechanical properties of niobium pentoxide and tantalum pentoxide ceramics prepared by a conventional ceramic processing technique and by exposure to high-intensity light (HIL). The results demonstrate that, after HIL exposure in an optical furnace, the niobium pentoxide and tantalum pentoxide ceramics possess enhanced microhardness and improved mechanical properties (strength, fracture toughness, and brittle microstrength) owing to the formation of fractal micro- and nanostructures. With increasing exposure intensity, the strength of the Nb2O5 and Ta2O5 ceramics increases.

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.

Stoichiometry and doping effects on cation ordering in LiNbO3 crystals

The cation disorder in stoichiometric and congruent lithium niobate crystals and also in congruent crystals doped with Y3+ or Gd3+ has been studied by X-ray diffraction and Raman scattering spectroscopy using vacancy models. The results indicate that the structural disordering induced by doping with Y3+, revealed by both X-ray diffraction and Raman spectroscopy, can be understood in terms of the mechanism of yttrium incorporation into the cation sublattice of the crystal. Yttrium substitution for Nb5+ on its normal lattice site causes Nb5+ to occupy vacant octahedra, thereby increasing the cation and vacancy disorder along the polar axis and distorting the octahedra because the ionic radius of Y3+ exceeds that of Nb5+.

Complex investigations of structural and optical homogeneities of low-photorefractivity lithium niobate crystals by the conoscopy and photoinduced and Raman light scattering methods

Using photoinduced light scattering, conoscopy, and Raman spectroscopy methods, we have studied stoichiometric lithium niobate crystals and congruent crystals that were doped with Mg(0.078, 0.89 mas %), Zn(0.03, 0.52, 0.62), Cu(0.015), B(0.12), Gd(0.51), Y(0.46), Gd(0.23):Mg(0.75), Mg(0.86):Fe(0.0036), Ta(1.13):Mg(0.011), and Y(0.24):Mg(0.63) cations. It has been found that, depending on the kind of the pattern of photoinduced light scattering, investigated specimens can be divided into three groups. We have shown that the asymmetry of the indicatrix of photoinduced light scattering of LiNbO3 crystals is caused by birefringence of exciting laser radiation as it propagates perpendicularly to the polar axis of the crystal, whereas the asymmetry of the Raman spectrum arises due to the occurrence of spontaneous polarization, the vector of which is directed along the polar axis, and by birefringence. The pattern of the photoinduced light scattering depends on the difference of the refractive indices Δn = no − ne of the ordinary (no) and extraordinary (ne) rays and their energies E. If Eno {ie259-1} Ene, the proportion of the photoinduced light scattering has the shape of a three-layer round spot. For equal energies, the pattern has the shape of a symmetric figure-eight. At Eno < Ene, the figure-eight is asymmetric. In this case, its large “lobe” is directed in the positive direction of the polar axis of the crystal.

Photorefractive and Raman Light Scattering in Lithium Niobate Ferroelectric Crystal

Using the methods of photorefractive and Raman light scattering, we study subtle features of the structure of LiNbO3 single crystals with different Li/Nb ratios (pure and doped with nonphotorefractive cations) grown by different methods. We reveal that, upon the irradiation of a single crystal with visible laser light, locally fluctuating micro- and nanostructures are initially formed in it, with their physical parameters being different from the corresponding parameters of the single crystal in the absence of the photorefractive effect. Upon an increase in the irradiation intensity and in the course of time, more and more such micro-structures are formed, and they are transformed into static micro- and nanoformations, which are subsequently converted into a continuous laser track. The speckle structure of photorefractive scattering is studied in detail. We show that the photorefractive effect in single crystals of the stoichiometric composition is fairly strong for their use as materials for recording and storing information.