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The work considers the influence of various types of equilibrium defects on photovoltaic and diffusion fields and on the band gap of nominally pure and zinc-doped lithium niobate crystals. Concentrations of OH groups and point defects in the NbLi cationic sublattice (the deepest electron traps) and the Li/Nb ratio are calculated from the IR absorption spectra to estimate their contributions to proton conductivity. The IR absorption spectra reveal a correlation between the concentration of NbLi defect and the intensity of bands at ~3480 cm–1.

The laser conoscopy method confirms the high optical uniformity of LiNbO3:B crystals in the range of B2O3 concentrations from 0.008 to 1.24 mol. %. Minor signs of anomalous optical biaxiality appear in the conoscopic patterns of crystals at boron concentrations of 0.12 and 0.83 mol. % at a laser power of 90 mW. The maximum value of the angle of anomalous optical axes for the studied samples is 2V = 10 ́ for the sample (0.12 mol. %), аnd the value of anomalous birefringence corresponds to ∆n = 0.02·10-5.

It is shown that B3+ ions can embed in trace amounts into the faces of the oxygen tetrahedra in the LiNbO3 crystal structure (the faces bordering the lithium octahedron or the vacant oxygen octahedra) or into the oxygen plane separating oxygen-octahedral layers. By interacting with the oxygen atoms of octahedra O6, B3+ ions substantially distort the anionic framework of the crystal structure and change the octahedra′s polarizability determining nonlinear optical properties of the crystal. The structural units of the cationic sublattice in B3+ containing crystals are ordered along the polar axis while the Li/Nb ratio increases due to the reduced concentration of NbLi point defects serving as deep electronic traps. It is shown that the B2O3 flux (an active complexing agent) significantly changes the properties of the charge and those of the melt so that the melt acquires some specific structure, while the Curie temperature of the crystals increases.

It is shown that application of B2O3 as a flux allows one to obtain nominally pure composition-homogeneous LiNbO3 crystals characterized by high, close-to-stoichiometry ordering of structural units of the cation sublattice and high resistance to laser radiation. It have been established by calculations that boron can be incorporated into faces of oxygen tetrahedra of the LiNbO3 crystal structure. Trace amounts of boron in the LiNbO3:B crystal structure suppress to a great extent the formation of NbLi point defects. At the same time, boron distorts significantly the oxygen framework of the LiNbO3 crystal structure and thus changes the polarizability of oxygen octahedra, which determines the nonlinear optical properties of the crystal.

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In a series of initially polydomain crystals of LiNbO3:Zn,Mg obtained by the homogeneous doping of the crystals in the concentration ranges of around 1 ± 0.02 mol % MgO and 3.0–4.6 mol % ZnO, abrupt increases in temperature dependences of electrical conductivity σ(T) and dielectric constant e(T) with the manifestation of low-frequency dielectric dispersion are observed near T* ≈ 800 K. At T > T*, these crystals exhibit the activation enthalpy (Ha ≈ 1.76–2.07 eV) and transport enthalpy (Hm ≈ 1.55–2.01 eV) values that are unusually high for cationic conductors, while both values at T T*).

The main contribution to the luminescence of LiNbO3 : Zn (0,04 ÷ 2,01 mol. %) at 420 and 440 nm was established to be due to two electro-hole pairs of Nb4+–O-in niobium octahedron. Moreover, the luminescence weakly depends on Li / Nb ratio and Zn concentration. The increase of Zn concentration to 2,01 mol. % ZnO leads to monotonically increasing intensity in long-wave region of the photoluminescent spectra. This fact indicates that there are shallow energy levels near bottom of the conductivity band. Also the features of hydrogen bonds in LiNbO3structure effects on the location of the shallow energy levels as the complex defects caused by OH-group in the structure can contribute to the photoluminescence.

The analysis of structural particularities and optical properties of LiNbO3 : B (0,55, 0,69 and 0,83 mol. % B2O3) was performed by methods of Raman and photoinduced light scattering, infrared absorption and laser conoscopy. It has been established that the boron impurity brings to 1 the distribution coefficient of lithium and niobium in the growing process. Thus, LiNbO3: B crystals grown from a congruent melt approach the stoichiometric crystals in ordering the structural units of the cation sublattice and the Li / Nb ratio.

The analysis of structural particularities of nominally pure LiNbO3 : B grown with using of B2O3flux was performed by methods of Raman and photoinduced light scattering, infrared absorption andlaser conoscopy. It was established that the non-metallic element boron is included in the vacant tetrahedrals of LiNbO3crystal structure at trace amounts. In this case, boron substantially deforms the oxygen octahedra of the structure and changes their polarizability. It affects the nonlinear optical properties of the crystal. Boron, as an active complexing agent, structures the melt and aligns value of the distribution coefficients of lithium and niobium in the growing process. Boron in the tetrahedralsreduces both the number of NbLidefects and the content of multiply charged uncontrolled impurities in the crystal, enhancing the optical damage.

The photoluminescence intensity in lithium niobate crystals close to the stoichiometric composition being lower than in a congruent crystal was established. The increase of Li / Nb ratio leads to shifting the photoluminescence bands to the short-wavelength region of the spectrum and changing the fundamental absorption edge of the crystals under study. It was shown that, in addition to point defects in the cationic sublattice, complex defects due to the presence of OH groups and compensation defects (VLi / VNb) in the structure can also contribute to photoluminescence.

Nominally pure and doped with zinc in a wide concentration range lithium niobate crystals were studied using photoinduced light scattering. Using the parameters of photo-induced scattered light, we determined the values of the photovoltaic and diffusion field intensities in lithium niobate crystals of different compositions. It was found that the values of thephotoelectric field strengths depend on the state of the defect structure of the crystals.

We have performed a computer simulation of processes taking place at formation of energetic equilibrium oxygen-octahedral clusters in a ferroelectric phase of a lithium niobate crystal (LN, LiNbO3)...

It was shown that laser conoscopy can visually detect even slight changes in the optical characteristics of a crystal when it is doped. It has been found that the defective structure of LiNbO3:Zn (4.5 mol. %), LiNbO3:Mg (3.0–5.5 mol.%) crystals associated with an uneven entry of an impurity leads to a local change in the elastic characteristics of the crystal and the appearance of mechanical stresses that distort the conoscopic patterns. This can be an abnormal optical biaxiality, which manifests itself in the form of a rupture and enlightenment of the "Maltese cross" in the center of the conoscopic crystal pattern, or local birefringent inclusions that are recorded as additional interference patterns against the background of the main conoscopic pattern, both in the center of the field of view and in its peripheral area.

The photoluminescence intensity in lithium niobate crystals close to the stoichiometric composition is lower than in a congruent crystal was established. An increase in the Li / Nb ratio leads to a shift in the photoluminescence bands to the short-wavelength region of the spectrum and a change in the fundamental absorption edge of the crystals under study. It was shown that, in addition to point defects in the cationic sublattice, complex defects due to the presence of OH groups in the structure can also contribute to photoluminescence.

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The electrical conductivity σ of a series of multidomain LiNbO3:Zn,Mg crystals prepared via direct doping in the composition ranges ~1 ± 0.02 mol % MgO and ~2.8–4.6 mol % ZnO has been shown to undergo a sharp increase at T* ≈ 800 K, accompanied by anomalies in temperature dependences of their dielectric permittivity, e(T). The largest increase in their static bulk conductivity near T* has been observed in a narrow composition range (≈1 ± 0.02 mol % MgО and ≈3.9–4.3 mol % ZnO). Above T*, the LiNbO3:Zn,Mg crystals have an activation enthalpy for ionic conduction (Ha ≈ 1.76–2.15 eV) and enthalpy of transport (Hm ≈ 1.66–2.06 eV) unusually high for cation conductors, whereas below T* both quantities are characteristic of Li+ ion conduction in LiNbO3 crystals (На ≈ 1.15–1.35 eV and Нm ≈ 1.0–1.1 eV). The anomalous increase in Ha and Hm is caused by the formation of associated vacancies (divacancies) and correlated pair hopping of Li+ ions above T*.

The phenomenological mechanisms for increasing the unipolarity under thermal processing in the short-circuiting conditions for heavily doped LiNbO3:ZnO crystals are analyzed by comparing with the temperature behavior of nominally pure LiNbO3:ZnO crystals with the congruent composition. It is shown that an increase in the unipolarity and, hence, the disappearance of the domain structure in heavily doped LiNbO3:ZnO crystals is initiated by thermal decomposition of charged polar clusters stabilizing domain walls. The decomposition of polar clusters is accompanied with an abrupt jumpwise injection of extra charge carriers (Li+ cations). As a result, the conductivity of LiNbO3:ZnO crystals at a temperature above 800 K is an order of magnitude higher than that of nominally pure LiNbO3 crystals with the congruent composition. This leads to the degradation of the domain structure in LiNbO3:ZnO crystals in contrast to LiNbO3 crystals with the congruent composition.

The concentration of OH impurity groups, the Li/Nb ratio, the concentration of Nb $$_{{{\text{Li}}}}^{{4 + }}$$ and V $$_{{{\text{Li}}}}^{ - }$$ point defects in stoichiometric and congruent LiNbO3 crystals, as well as in crystals doped with magnesium and zinc at concentrations close to threshold concentrations, have been calculated from IR absorption spectra in the range of stretching vibrations of OH groups. The behavior of bands in the IR absorption spectra of heavily doped LiNbO3:Mg and LiNbO3:Zn crystals has been revealed to correlate with the behavior of the Raman spectrum line that corresponds to stretching bridge vibrations of oxygen atoms in the NbO6 octahedron along the polar axis.

Defects and optical properties of LiNbO3:Mg(5.05):Fe(0.009 mol%) are studied by IR absorption spectroscopy, Raman spectroscopy, laser conoscopy, and optical spectroscopy. Absorption bands corresponding to OH stretching vibrations are shifted to higher frequencies and are narrower in the spectrum of this crystal than in the spectrum of a nominally pure congruent LiNbO3 crystal. This indicates that the OH groups are more ordered and the O–H bonds are more rigid in the structure of LiNbO3:Mg(5.05 ):Fe(0.009 mol%) than in the congruent crystal.

—Improved methods and devices for nondestructive control of the degree of unipolarity of large LiNbO3 crystals are described. These methods are based on measurements of the static and dynamic piezoelectric characteristics of crystals and are applicable to other ferroelectric materials. These methods allow assessment of the degree of unipolarity and the relative volume of antiparallel domains in crystals.

A LiNbO3:Zn (4.69 mol % ZnO) crystal grown from a charge synthesized using homogeneously doped Nb2O5 as a precursor has been shown to contain less luminescence centers in comparison with a LiNbO3:Zn (4.60 mol % ZnO) crystal prepared via direct doping. In the case of LiNbO3:Zn crystals prepared via direct doping, the intensity of all luminescence bands varies monotonically as the ZnO concentration is raised to 4.60 mol %. At a ZnO concentration of ~4.60 mol %, the intensity of all the bands changes sharply, suggesting that the system of luminescence centers in the material undergoes structural changes.

Interference patterns of single crystals of LiNbO3:Gd, LiNbO3:Er, obtained by laser conoscopy, allow us to estimate the optical homogeneity of the samples under study. The observation of conoscopic patterns of the LiNbO3:Gd single crystal shows that with an increase in the percentage impurity concentration, the anomalous biaxiality in these samples manifests itself to a much greater degree. Anomalous biaxiality for the sample LiNbO3:Er [3.60 wt.%] is present at almost every point of the entrance face, which is manifested in conoscopic patterns in the form of enlightenment and rupture of the “Maltese cross”, which corresponds to the angle 2V~1° between the anomalous axes.

Bulk photoluminescence of congruent and near-stoichiometric lithium niobate crystals that were obtained by different technologies is studied. It is found that the luminescence intensity in crystals whose composition is close to stoichiometric is lower than that in congruent crystals. It is shown that not only luminescence centers of the main types, which are caused by the occurrence of NbLi, VLi, etc., point defects, but also complex defects that are caused by the occurrence of OH groups in the structure can contribute to luminescence.

Volume and surface photoluminescence of congruent and stoichiometric lithium niobate crystals obtained using various technologies was studied. The luminescence intensity of the stoichiometric crystal was less than that of the congruent one. Volume luminescence of the crystals was mainly caused by NbLi defects while luminescence quenching of the near-surface layer was observed in the long-wavelength spectral region (>500 nm) because of energy scattering in crystal-lattice vibrations and increased luminescence intensity of the $$ {\mathrm{Nb}}_{\mathrm{Nb}}^{4+}-{\mathrm{O}}^{-} $$ pair. Luminescence bands with maxima at 426 and 446 nm were caused by complex defects in the form of electron–hole pairs $$ {\mathrm{Nb}}_{\mathrm{Nb}}^{4+}-{\mathrm{O}}^{-} $$ in which the Nb atom was bound to oxygen atoms by covalent and ionic bonds. An increase of the Li/Nb ratio led to a shift of the luminescence bands to shorter wavelengths and a change of the fundamental absorption edge of the studied crystals.

GdNbхTa1 –хO4 and YNbхTa1 –хO4 (x = 0–1) ceramic samples have been prepared by a sol–gel process, and the luminescence properties of the solid solutions have been studied under UV excitation. The results demonstrate that even a relatively small amount of tantalum (x = 0.9) in GdNbхTa1 –хO4 leads to a sharp increase in the luminescence intensity related to Nb4+–O– centers, which exceeds the luminescence intensity in the GdNbO4 and GdTaO4 individual compounds by a factor of 3–6.5. The luminescence signal in the YNbхTa1 –хO4 solid solutions exceeds that in the YNbO4 and YTaO4 end-members at any Nb : Ta ratio, owing to energy transfer between Nb4+–O– and Ta4+–O– emission centers.

It has been shown that the growth of nominally pure lithium niobate crystals from a nonmetal (boron)-structured melt makes it possible to control the secondary phase, optical homogeneity, photoelectric fields, and bandgap of the material. From the characteristics of photoinduced light scattering, the photovoltaic and diffusion field strengths in nominally pure LiNbO3 : B crystals have been determined. It has been shown that the diffusion field governing the concentration of shallow electron traps in LiNbO3 : B crystals is between diffusion fields in crystals having a congruent and stoichiometric composition and depends on boron concentration in the charge. It has been found that the bandgap in LiNbO3 : B crystals is the same as in the stoichiometric crystal but the optical homogeneity of LiNbO3 : B crystals is closer to that of the congruent crystal. In addition, the concentration of OH groups in LiNbO3 : B crystals is lower and their arrangement in the structure is more regular than in the congruent crystal.

Nominally pure congruent LiNbO3 crystal and LiNbO3:ZnO ([ZnO] ~ 5.4–6.4 mol % in the melt) crystals of different genesis are studied using photoinduced light scattering, laser conoscopy, and X-ray diffraction (method of moments). The photorefractive properties and optical and structural uniformity are analyzed. It is found that LiNbO3:ZnO crystals obtained by direct solid phase doping have the best optical and structural uniformity, the lowest photorefractive sensitivity, and a high compositional uniformity along the polar axis. Crystals obtained by homogeneous doping are compositionally uniform, although their optical and structural uniformity is worse than those of LiNbO3:ZnO crystals obtained by direct solid phase doping. Anomalies of conoscopic patterns are caused by the presence of charged structural defects in LiNbO3:ZnO crystals and the distortion of the optical indicatrix induced by mechanical stresses and compositional inhomogeneity of the crystals. High temperature annealing in short-circuited state of LiNbO3:ZnO crystals with high dopant concentration leads to healing of charged defects and improvement of optical characteristics in general. The distribution coefficient for LiNbO3:ZnO crystals obtained by the homogeneous doping method is significantly higher than that for the crystals obtained by the direct solid phase doping method.

A comparative study of the optical and structural homogeneity of a LiNbO3 crystal with a congruent composition and LiNbO3 : ZnО ([ZnО] ≈ 5.4–6.4 mol % in melt) crystals grown from charges of different origin has been performed. It is shown that the LiNbO3 : ZnО crystals obtained by direct solid-phase doping have a higher general structural and optical homogeneity, provided that the crystal composition is sufficiently homogeneous along the growth axis. Homogeneously doped LiNbO3 : ZnО crystals, characterized by a high general structural homogeneity on the scale level of 10–500 µm, have poorer optical and structural homogeneity on the micron and submicron levels as compared with directly doped LiNbO3 : ZnО crystals.

An analysis of studies of single crystals of LiNbO3 stoich, LiNbO3 cong and LiNbO3 : B (0,83 mol.% B2O3 in the charge) was performed. It was established that boron present in the melt is unable to incorporate into the crystal structure of LiNbO3. Boron brings closer in magnitude to each other the effective distribution coefficients of lithium and niobium linking excess niobium in the process of complexation in a melt of a congruent composition. In this case, LiNbO3 : B crystals approach to the stoichiometric crystals in ordering the structural units.

The macro- and microstructure of doped LiNbO3 crystals is studied, their transmission spectra are investigated, and the effective coefficients characterizing the dopant impurity distribution are determined. We analyze the literature data on phase diagrams of the ternary system Li2O–Nb2O5–dopant oxide and the electron configurations of dopant elements, and the results suggest that this can be used as a basis for designing technological conditions for growing doped lithium niobate crystals and predicting their quality. With p‑elements (boron), structurally and compositionally uniform LiNbO3 crystals can be grown, with the doping impurity not being incorporated in the crystal. s- and d-metals (magnesium and zinc) have a similar effect on the melt and properties of resulting LiNbO3 crystals, which have periodic domain structures and similar types of point defects. Due to their electronic structure, f-metals (cerium) bring the melt to such a structure that this enables the formation of a periodic domain structure in LiNbO3 crystals during their growth.

Photoinduced light scattering, laser conoscopy, and IR spectroscopy in the absorption region of OH stretching vibrations are used to study photorefractive properties and compositional homogeneity of double doped (mol%) LiNbO3:Mg(5.05):Fe(0.009) and LiNbO3:Zn(4.34):Fe(0.02) crystals grown from a congruent melt. All frequencies in the IR spectrum of the LiNbO3:Mg(5.05):Fe(0.009) (mol%) crystal in the region of OH stretching vibrations are shifted to the high-frequency region compared with the spectrum of the nominally pure congruent crystal. This is caused by the absence of $$\rm{Nb}_{Li}^{4+}$$ point defects and the corresponding (VLi)–OH complex defects in the crystal structure and by the formation of new MgLi–MgNb–OH and MgLi–OH–FeNb complex defects. The LiNbO3:Zn(4.34):Fe(0.02) (mol%) crystal (zinc concentration is between threshold concentrations for ∼3.0 mol% and 7.0 mol% of ZnO) is found to be much more compositionally homogeneous than the LiNbO3:Mg(5.05):Fe(0.009) (mol%) crystal, despite much higher iron content in the former. This is explained by the fact that magnesium content in this crystal is close to the threshold concentration for ∼5.5 mol% of MgO. Neither of the crystals exhibits photoinduced light scattering.

—LiNbO3:Mg(~5.3 mol %) crystals grown from directly and homogeneously doped charges have been studied using a number of characterization techniques. Both crystals offer high chemical homogeneity, with an even Mg distribution over the boule. At the same time, the estimated effective distribution coefficient is larger in the homogeneously doped LiNbO3:Mg crystal. Moreover, the homogeneously doped crystal has a more ordered system of hydrogen bonds. Our results suggest that the distinctions between the fine structural features and chemical homogeneity of the crystals can be due to the effect of organic inclusions with rigid covalent bonds on the structure and physical characteristics of the growth charge prepared using a homogeneously doped Nb2O5:Mg precursor.

Second harmonic generation is studied in samples of LiTaO3 powder ceramics with different lithium concentrations (48.8 and 49.3%) under excitation by a femtosecond laser at a wavelength of 1.026 nm. The dependence of the second harmonic integral power on the incident radiation power is measured, and a method for estimating the nonlinearity coefficient and its homogeneity in green-body ceramics is proposed.

We have observed that LiNbO3:Mg (0.19–5.91 mol % MgO) crystals experience rearrangements in their defect structure with increasing concentration of Mg and exhibit optical anomalies related with them, which have a threshold character. We have shown that, at Mg concentrations close to the first concentration threshold at ~3.0 mol % MgO, the optical homogeneity of the crystals increases. When the Mg concentration approaches the second concentration threshold, we have revealed a significant shift (by ~50 cm–1 compared to that in other crystals) of the entire IR absorption spectrum in the range of stretching vibrations of OH groups to the high-frequency range, a sharp decrease in the number of protons, and a jumpwise shift of the absorption edge in the short-wavelength range. We have shown that the shift occurs because all NbLi point defects are replaced by Mg cations near the concentration threshold at 5.5 mol % MgO, and, subsequently, Mg cations occupy only positions of Li and Nb cations of an ideal stoichiometric structure, forming MgLi and MgNb point defects.

Structural defects of nominally pure congruent and stoichiometric LiNbO3 crystals and a series of congruent crystals doped with 0.91–5.91 mol% MgO were compared and comprised two concentration thresholds at ~3.0 and ~5.5 mol% MgO. The main band parameters of OH stretching vibrations in IR absorption spectra (frequency, half-width, intensity) displayed abrupt changes near the concentration threshold at 5.5 mol% MgO. The OH stretching-vibration band shifted to higher frequency by ~50 cm–1 because of the formation of (MgLi)+–(MgNb)3––OH–complexes upon reaching the second Mg concentration threshold.

It is shown that the method of high temperature top-seeded solution growth (HTTSSG) can be used to obtain near stoichiometric LiNbO3 crystals from a congruent melt with the addition of 6.0 wt.% of the K2O flux. The structure of complex defects containing OH− groups in these crystals is close to those in stoichiometric crystals grown from a melt containing ∼ 58.6 mol.% of Li2O. However, the former are significantly superior to the latter in terms of total optical and structural homogeneity. The obtained crystals are studied using laser conoscopy, photoinduced light scattering, measuring the fundamental absorption edge, IR absorption spectroscopy in the region of OH− stretchings, Raman spectroscopy in the region of two-particle states of acoustic phonons with zero total wave vector.

—The congruent charge LiNbO3:Zn (2.44 wt %) was synthesized using precursor Nb2O5:Zn (2.83 wt %) by the method of homogeneous doping. The LiNbO3:Zn crystal (2.12 wt %) was grown from this charge by the Czochralski method. The crystal demonstrates high chemical uniformity of Zn dopant distribution along the growth axis. The same distribution is characteristic of crystals obtained by usual direct doping. The homogeneously doped LiNbO3:Zn crystal (2.12 wt %) and the LiNbO3:Zn (2.02 wt %), LiNbO3:Zn (2.05 wt %), and LiNbO3:Zn (2.12 wt %) crystals close to this composition obtained by method of direct doping, as well as LiNbO(3cong), were compared to study defectiveness and optical and structural homogeneity. The method of IR absorption spectroscopy, photoinduced light scattering, and laser conoscopy were used for the study. All studied crystals show no photorefractive response, as can be seen from photoinduced light scattering. The conoscopic patterns of a crystal LiNbO3: Zn (2.12 wt %, homogeneous doping) are strained, which can be connected with a greater photoinduced ability to scatter light than in other crystals. The ability is caused by microstructures, clusters, and residual domain structures. However IR spectra demonstrate narrowing of bands, which can be explained by the fact that homogeneous doping of a Nb2O5 precursor with zinc contributes to the ordering of the lithium sublattice of a LiNbO3:Zn crystal and ordering of H+ protons compared to the ordering in a LiNbO3cong crystal. This effect is highly unusual at this concentration of the dopant.

Nominally undoped and doped LiNbO3 crystals have been studied using full-profile analysis of X‑ray diffraction data and Raman spectroscopy. The X-ray diffraction patterns and Raman spectra of the crystals contain weak superstructure lines forbidden for space group R3c. The observed superstructure lines are not reflections from a second phase. The intensity distribution and position of the additional reflections are essentially independent of nonstoichiometry and dopant concentration. The additional lines in the Raman spectra and X-ray diffraction patterns of the crystals can result from the formation of an ordered hexagonal defect superstructure sublattice with a doubled a cell parameter relative to the fundamental unit cell.

The variations of electrical conductivity and the primary pyroelectric coefficient with temperature for lithium niobate (LiNbO3) crystals grown from melt containing K2O flux are studied in the range of 292–450 K. These crystals are found to exhibit considerable anisotropy of electrical conductivity, and the proton conductivity is dominant in the studied temperature range.

IR absorption spectra of zinc-doped LiNbO3 crystals in the stretching region of OH groups, Raman spectra of the crystals, and their optical absorption spectra provide evidence of drastic changes in the mechanism responsible for the incorporation of Zn into the structure of the crystals and the nature of hydrogen bonds and the corresponding proton-containing charged defect complexes ( $$V_{{{\text{Li}}}}^{ - }$$ –OH–, NbLi–OH−, and $${\text{Zn}}_{{{\text{Nb}}}}^{{{\text{3}} - }}$$ –OH−) near a concentration threshold at ≈6.76 mol % ZnO in the melt.

We have studied the structural and optical characteristics and radiation hardness of LiNbO3:ZnO crystals containing ~0.15–5.9 mol % ZnO. The results demonstrate that anomalies in composition dependences of their structural and optical characteristics and radiation hardness are closely interrelated with threshold effects in the LiNbO3:ZnO crystals. We have confirmed the presence and determined the exact position of two concentration thresholds: at low dopant concentrations (~1.17 mol % ZnO in the crystals) and intermediate ones (~4.8 mol % ZnO in the crystals). Composition dependences of structural parameters for the crystals near the main concentration threshold (~6.8 mol % ZnO in the melt) have been determined with appreciably higher accuracy than before.