The efficiency of the process of increasing the volume concentration of hydrogen in LiNbO 3 crystals of congruent composition by heating in a humid atmosphere has been studied. It is shown that pretreatment of LiNbO 3 crystals in adipic acid makes it possible to significantly increase the volume concentration of hydrogen in the sample during subsequent thermochemical treatment in wet air. Keywords: lithium niobate, hydrogen, electrical conductivity, IR spectroscopy.

IR absorption spectroscopy and electrical conductivity have been studied in a series of lithium niobate crystals. We have established that mobility of Li+ ion sharply increases when a crystal approaches stoichiometric composition. This is especially true along the polar direction of a crystal. We have also established that doping of lithium niobate crystals with under-threshold concentrations of non-photorefractive Mg and Zn substantially decreases mobility of Н+ ions.

Using the results of studying the electrical conductivity in the temperature range ~295–460 K and the IR absorption spectra of a series of LiNbO3:Mg crystals, it is shown that an increase in the Mg content leads to sharp decrease in the H+ ion mobility during their displacement along the polar direction. This effect is observed most clearly in LiNbO3:Mg crystals with high (superthreshold) impurity content.

This paper is dedicated to a study of conductivity and current-voltage characteristics (I-V curve) of lithium niobate (LN, LiNbO3) single crystals. The studied crystals have been turned to a single domain state. We have researched nominally pure congruent (CLN) and strongly doped LiNbO3:Mg and LiNbO3:Zn crystals at T = 300-450 K. Magnitude, activation energy and type of conductivity have been established in CLN, LiNbO3:Mg and LiNbO3:Zn in the temperature range T = 300-450 K. Coercive field in LiNbO3:Mg and LiNbO3:Zn crystals is much lower than in CLN. However, we have determined that nonlinearity of I-V curve exists in CLN crystals and is absent from LiNbO3:Mg and LiNbO3:Zn crystals at temperatures up to 450 K and external electric field strength up to 12 kV/mm. The paper suggests possible explanation for the observed effects.

The pyroelectric response has been studied for electro-optic modulators utilizing X-cut LiNbO3 integrated-optical chips. Since this response induces the modulator drift that appears only at fast change of a chip temperature, it causes the temperature and temporal instabilities of integrated-optical devices utilizing these chips. This drift was significantly reduced with the aid of extra electrodes providing the significant shielding of the pyroelectric field.

The temperature dependence of the electrical conductivity of four samples of LiNbO3 crystals, which are significantly different in their electrical properties, is analyzed. It is shown that the corresponding experimental data can be correctly described within the framework of the simultaneous presence of mobile carriers with activation energies of 0.29 eV, 1.03 eV, and 1.33 eV, which correspond to polaron, proton, and lithium conductivity, respectively.

The influence of a static electric field on the dielectric properties of nominally pure (impurity-free) LiNbO 3 crystals with congruent composition has been studied in the low frequency range. It has been shown that an external static electric field codirected or counterdirected with the polar axis of the crystal, as well as a pyroelectric field up to 14 kV /cm in strength, does not influence the low-frequency dielectric properties of the crystal within experimental accuracy.

Electrical conductivity and dielectric constant have been researched in a series of crystals LiNbO3:Zn(~4.0–9.0 mol% ZnО in a melt) by application of constant voltage with a couple of electrodes (~<500 K) and impedance spectroscopy (~>500 К). In order to compare results we have also researched electrical conductivity σdc of congruent LiNbO3congr and magnesium doped LiNbO3:Mg crystals. We have established that electron contribution to electrical conductivity of doped crystals LiNbO3:Zn and LiNbO3:Mg is greatly smaller than that of a congruent crystal LiNbO3congr. A decrease in conductivity is apparently connected with a restructure of LiNbO3 crystals at high concentrations of magnesium and zinc. Doping of crystals with Zn leads to drastic changes not only in the electron conductivity area (T < 500 K), but also in a lithium ion conductivity area (Т > 500 K). At this, parameters of temperature dependence of lithium ion conductivity depend on both zinc concentration and crystallographic orientation of samples.

The electrical conductivity of a series of LiNbO3 crystals grown from the charge of the congruent composition with relatively low iron concentration (up to 0.15 wt % Fe2O3) has been studied experimentally. It is found that, at temperatures close to 300 K, the main contribution to the electrical conductivity of these crystals is from two types of the centers with close activation energies. The first type of centers are Fe2+ ions, which determines the impurity electron conductivity with the activation energy (0.34 ± 0.01) eV. The second type of centers are $${\text{N}}_{{{\text{Li}}}}^{{{\text{4 + }}}}$$ polarons of small radius with the activation energy (0.29 ± 0.02) eV. The polaron conductivity is shown to be dominate for nominally impurity-free and low-doped “as-grown” LiNbO3 crystals at T = 300 K.

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.

The nuclear magnetic resonance (NMR) of 7Li and 6Li nuclei in a LiTaO3 single crystal sample of congruent composition has been experimentally investigated. It is found that there are weak side lines in the NMR spectra of 7Li nuclei, which have not been observed before; these lines are due to cation sublattice defects. A computer simulation of NMR spectra has revealed the most likely configuration of $$({\text{Ta}}_{{{\text{Li}}}}^{{{\text{5}} + }} + {\text{4}}{{V}_{{{\text{Li}}}}})$$ defect complexes that are present in nonstoichiometric LiTaO3 crystals.

Experimental study of the pyroelectric effect has been made for multi-function integrated-optical circuits (MIOC) utilizing x-cut LiNbO 3 chips. It has been experimentally established that pyroelectric voltage is proportional to the temperature scanning rate, and a voltage magnitude depends significantly on capacitances of chip parts between the MIOC electrodes. The pyroelectric effect is considered by us to be an important source of the thermal instability of MIOC. The model of the pyroelectric contribution to the MIOC thermal instability is used to suggest the new methods reducing this instability.

Using the method of impedance spectroscopy and optical density measurements, the diffusion of oxygen in single crystals of lithium niobate of the congruent composition after the reductive thermochemical processing is studied. The parameters describing the diffusion of oxygen in the temperature range 493–693 K are established.

The effect oxidizing annealing has on the electrical properties of nominally pure congruently grown LiNbO3 crystals is investigated via impedance spectroscopy. A set of samples is preliminarily reduced in saturated D2O or H2O vapor. It is shown that reducing annealing in saturated water vapor at T = 773 K results in partial deprotonation of LiNbO3 crystals. It is assumed that such annealing does not change the charge state of impurity ions with variable valence.

We have carried out a comparative study of the electrical properties of lithium tantalate (LiTaO3) crystals in a wide temperature range (300–900 K) before and after reductive treatment in H2O vapor and subsequent oxidative annealing. The results demonstrate that, in the temperature range of Li+ ion conduction (550–900 K), the activation enthalpy for ionic conduction in the reduced lithium tantalate crystal is H a = 1.37 eV, which slightly exceeds that in the initial state of the crystal (1.34 eV). In the temperature range 390–450 K, the σ(T) data for the unannealed crystal are well represented by the Arrhenius law in the presence of two carrier types, with activation energies E 1 = 1.03 eV and E 2 = 0.29 eV, characteristic of proton and electron hopping conduction, respectively. After reductive annealing, the activation energy for conduction is ~0.65 eV, characteristic of the activation energy for bipolaron conduction. After subsequent oxidative annealing of the reduced crystals in dry air, the activation energy is ~1.2 eV. It seems likely that the presence of oxygen vacancies in the reduced LiTaO3 crystal stimulates hydrogen release from the crystal during oxidative annealing.

Possible combinations of the electronic polarizability tensor components for oxygen and tantalum ions have been established based on the analysis of the refraction coefficients of LiТаO 3 crystal in the far-IR region. The local electric field on structurally nonequivalent ions in the LiТаO 3 unit cell was calculated within the modified point-multipole model. A combination of the electronic polarizability tensor components for oxygen and tantalum ions and their effective charges is established, which provides a good correspondence of the calculation results with the experimental data on LiТаO 3 crystals obtained by 7 Li nuclear magnetic resonance and 181 Та nuclear quadrupole resonance and with the known value of the crystal spontaneous polarization.

The instability of the electrical properties of lithium niobate single crystals of congruent composition subjected to reducing thermochemical treatment has been investigated by impedance spectroscopy. It has been shown that the subsequent heating of the reduced lithium niobate samples in dry air up to 380 K or higher is accompanied by the progressive increase in their electric resistance, which is due to the oxidization of the crystal surface layers.

We have studied anisotropic electrical conductivity and dielectric properties of a LiTaO 3 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 μ e . 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.

The temperature dependence of the dark electrical conductivity of the LiNbO3(LN) crystals annealed in saturated H2O and D2O vapor in the range 293...400 K is investigated. It is found that the activation energy of the electrical conductivity is equal to (0.71 ± 0.02) eV and is close this value of LN samples, reduced in hydrogen. Annealing in ampoules with H2O vapor also lead to LN optical spectra changes such annealing in H2. The nature of this phenomenon is discussed.

The investigation of the temperature dependence of the dark electrical conductivity of as grown LiTaO3 single crystals and the crystals reduced in vacuum was carried out. The reducing of the LT in vacuum at 830 K causes a removal diffusion of hydrogen and a partial loss of oxygen. It leads to recharging of transition metal impurities to the lowest valence state and blocking of hopping electronic conductivity. After filling of electronic traps, the process of small polaron formation takes place.