O. V. Makarova

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.

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.

Effect of the method used to prepare solid precursors Nb2O5:Mg on the characteristics of LiNbO3:Mg crystals produced on their basis

We investigated how the type of an extraction system affects the characteristics of precursors Nb2O5:Mg produced at the stage of extraction processing of niobium-containing HF-H2SO4 and HF-HCl solutions with an extractant comprising a mixture of dimethylamides of carboxylic acids of the C10–C13 fraction and 1-octanol in Escaid diluent. We also studied the optical properties of homogeneously doped crystals LiNbO3:Mg grown from a batch synthesized with the use of pentoxide Nb2O5:Mg obtained from various extraction systems. On Z-cut LiNbO3:Mg crystal wafers, such mechanical characteristics as Young’s modulus and microgravity were measured.

Structure and optical properties of LiNbO3:ZnO (3.43–5.84 mol %) crystals

LiNbO3:ZnO (3.43–5.84 mol %) crystals have been studied using Raman spectroscopy and fullprofile analysis of X-ray diffraction data. The results demonstrate that, at ZnO concentrations above 3.95 mol %, their structure is free of NbLi basic defects, which are characteristic of congruent lithium niobate crystals. Increasing the Zn concentration leads to changes in the arrangement of structural units in the cation sublattice along the polar axis and distorts the BO6 (B = Nb, Li, Zn, or a vacancy) oxygen octahedra. In the Raman spectra of the crystals, the width of the line at a frequency of 876 cm–1, which corresponds to stretching modes of the oxygens in the BO6 octahedra, has been shown to increase considerably, which may be due to changes in the character of bonding in the B–O–B bridges in response to changes in the Zn concentration in the crystals. The gradual increase in the electro-optical coefficients of the LiNbO3:ZnO crystals with increasing zinc concentration can be accounted for by changes in the ionic contribution to these bonds.

Anomalous dielectric and piezoelectric properties and electrical conductivity of heavily doped LiNbO3:Zn crystals

In LiNbO3:Zn crystals with near-threshold doping levels ( 4.5–4.7 mol % Zn), high-temperature measurements and high-temperature annealing under short-circuit conditions lead to considerable changes in spontaneous unipolarity. The considerable changes in spontaneous unipolarity are accompanied by well-defined low-frequency dielectric dispersion and sharp anomalies in temperature dependences of electrical conductivity and dielectric permittivity. As a result, the piezoelectric modulus d333 increases to a level approaching the highest values reported in the literature for single-domain nominally pure LiNbO3 crystals.

Spontaneous unipolarity and anomalies of the dielectric and piezoelectric properties and electrical conductivity of initially heavily doped polydomain LiNbO3: Zn crystals

A significant spontaneous unipolarity in initially polydomain LiNbO3: Zn crystals doped in the range of “threshold” impurity concentrations (∼4.5–4.7 mol %) has been revealed from high-temperature measurements and after high-temperature annealing under short circuit conditions. The effect of strong increase in the spontaneous unipolarity is accompanied by a pronounced low-frequency dielectric dispersion and abrupt anomalies in the temperature dependences of the electrical conductivity and dielectric permittivity. The piezoelectric coefficient d333 increases to nearly maximum values available in the reference literature for nominally pure single-domain LiNbO3 crystals.

Anisotropic electrical conductivity and dielectric properties of LiTaO3 crystals in the temperature range 290–900 K

We have studied anisotropic electrical conductivity and dielectric properties of a LiTaO3 crystal in the temperature range 290–900 K. The anisotropy in its dielectric characteristics is associated with specific features of dielectric relaxation in the polar and nonpolar directions of the crystal. In the temperature range 290–450 K, the electrical conductivity in a nonpolar direction slightly exceeds that in the polar direction and there is anisotropy in electron mobility μe. At temperatures from 600 to 900 K, conductivity anisotropy shows up in both the magnitude of conductivity and the energetic and kinetic characteristics of charge transport processes.

Radiation hardness of lithium niobate nonlinear optical crystals doped with Y, Gd, and Mg

We have studied the effect of gamma irradiation to various doses on optical characteristics (optical transmission spectra) of nominally undoped, rare earth-doped, and alkaline earth-doped lithium niobate crystals: LiNbO3, LiNbO3:Y (0.46 wt %), LiNbO3:Y,Mg (0.32, 0.24 wt %), LiNbO3:Mg (0.27 wt %), and LiNbO3:Gd (0.004, 0.04, 0.26, 0.43 wt %). We have examined the influence of the ionizing radiation dose, the nature of dopants, and their concentration on the optical transmission of the gamma-irradiated doped lithium niobate crystals. The results suggest that gamma-induced changes in the optical transmission of LiNbO3:Gd (≃0.004 and 0.04 wt %) crystals can be used for gamma dosimetry in the dose range ≃101 to 1.5 × 104 rad.

Structure and Properties of Boron-Doped LiNbO 3 Single Crystals

We have studied the macro-and microstructure and properties of LiNbO3〈B〉 crystals in comparison with LiNbO3〈Zn〉 and stoichiometric and congruent lithium niobate crystals. The optical characteristics of the LiNbO3〈B〉 crystals have been shown to approach those of the stoichiometric LiNbO3 and LiNbO3〈Zn〉 crystals.