Исследование структурного совершенства монокристаллов ниобата лития разного состава и генезиса методом ИК спектроскопии в области валентных колебаний водородных связей

We have analyzed complex defects due to the presence of hydrogen bonds in the crystal structure in nominally pure lithium niobate crystals with different Li/Nb ratio, in crystals alloyed with magnesium and zinc in a wide concentration range (LiNbO3 : Mg (0.19-5.91 mol.% MgO) and LiNbO3 : Zn (0.04-6.5 mol. % ZnO)) and in the double-alloyed crystals (LiNbO3 : Y(0.24) : Mg(0.63 wt. %) and LiNbO3 : Gd (0.25) : Mg(0.75 wt. %)), obtained by technology of direct melt alloying, and also in the double-alloyed crystal (LiNbO3 : Mg(5. 05 mol.% MgO) : Fe(0.009 mol.% Fe2O3)) grown from a charge synthesized using the technology of homogeneous alloying with magnesium and iron Nb2O5. We revealed the influence of doping impurities on the concentration of OH-groups, the type and localization of complex defects in the crystal structure. The change in the number of hydrogen atom positions in the structure of the LiNbO3 crystal allow us to judge with sufficient accuracy whether the crystal composition is stoichiometric or congruent. For doped crystals of different compositions data were obtained testifying to changes in the character of complexation of OH-groups with point defects of the cationic sublattice with formation of defects: MеLi-OH-, MеLi-MеNb-OН. A change in the mechanism of entry of the dopant cation into the structure dramatically affects the change in the properties of the crystal.The difference in the frequencies (and, correspondingly, in the values of the quasi-elastic constants of the O-H bonds) in the spectrum of a congruent crystal and doped crystals can also be contributed by differences in the electronegativity and ionic radii of the principal and doping cations.

Influence of cation substitution on electrical conductivity of microcrystalline ceramics based on (Cu1-xAgx)7GeSe5I solid solutions

Ceramic samples based on the microcrystalline powders (Cu1–хAgx)7GeSe5I (x = 0, 0.25, 0.5, 0.75, 1) were prepared by pressing them at the pressure close to 400 MPa and subsequent annealing at 873 K for 36 hours. Using the microstructural analysis, the average size of microcrystallites of these samples was determined. Investigation of electrical conductivity of ceramics based on (Cu1 хAgx)7GeSe5I solid solutions was carried out using the method of impedance spectroscopy in the frequency range from 10 Hz to 3×105 Hz and within the temperature range 293...383 K. Analysis of the Nyquist plots allowed determining the contributions of ionic and electronic components to the total electrical conductivity. The temperature dependence of ionic and electronic conductivity in Arrhenius coordinates is linear, which indicates their thermoactivation character. The compositional behaviour of ionic and electronic conductivity, as well as their activation energies have been studied. Their nonlinear character can be explained by the complex process of recrystallization and Cu+ ↔ Ag+ cation substitution within the cationic sublattice.

Understanding cation-disordered rocksalt oxyfluoride cathodes

A “concerted-densification” based failure mechanism, involving atomic-level changes in both transition-metal cationic sublattice and oxygen/fluorine anionic sublattice, is proposed for the degradation of F-DRX cathode materials.

Photoluminescence and features of the defective structure of nominally pure lithium niobate single crystals.

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.

Photoluminescence and Features of the Defective Structure of Nominally Pure Lithium Niobate Crystals

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.

Oxygen Lattice Distortions and U Oxidation State in UO2+x Fluorite Structures

The structural changes induced by the changes of the uranium oxidation state in the ideal fluorite lattice of pure UO2 are discussed. Experimental results evidence strong distortions of the oxygen sub-lattice due to dynamic (at high temperature) or static (at low temperature) fluctuations of the local charges in the cationic sublattice. These changes in the oxidation state are often described using the Vegard’s law because a linear dependence of lattice parameter is observed over a wide range of compositions. Nevertheless, an ideal solid solution model cannot explain this behavior where the elastic effects are directly related to the ionic radii of the cations. Strong evidence is provided that enthalpy effects are relevant in these systems and that they are directly associated with the local structural changes observed during neutron scattering experiments.