Nanomaterials for remediating synthetic dyes are often limited by non-selective electrostatic interactions. Here we show that intentional atomic disorder can be programmed to overcome this limitation by fundamentally altering the thermodynamic landscape of a surface. Using methylene blue (MB) a cationic dye selected as a mechanistic probe to test the electrostatic paradox and enable direct Lewis base coordination with defect-generated Nb5+ Lewis acid sites we demonstrate that this defect engineering resolves a key thermodynamic paradox: surfaces with a strong binding enthalpy (Delta H degrees = -14.90 kJ/mol) are rendered inactive by a prohibitive entropic penalty (Delta S degrees = -51.77 J/(mol & centerdot;K)), making adsorption non-spontaneous (Delta G degrees > 0). Strategic milling engineers a favourable entropic landscape (Delta S degrees = -37.95 J/(mol & centerdot;K)), overcoming this barrier and switching the process to be spontaneous (Delta G degrees < 0). This results in a 25-fold capacity increase (to 100 mg/g) and resolves the electrostatic paradox of cationic dye uptake on a highly negative (-40.3 mV) surface. Concurrently, the defect landscape provides substantial bandgap tuning (to 4.13 eV). We establish atomic disorder as a design principle for programming the surface thermodynamics and functionality of advanced materials.

It is found that the speckle structure of the photoinduced light scattering indicatrix of the LiNbO3:Y3+(0.46 wt%) crystal and its behavior with the time of crystal irradiation with a laser undergo an atypical behavior caused by the features of the dissipation processes of laser-induced defects in the crystal. In the frequency range of 100–4000 cm−1, the Raman spectra of the LiNbO3:Y3+(0.46 wt%) single crystal were recorded upon excitation by visible (532 nm) and near-IR (785 nm) laser radiation. Five second-order Raman scattering lines were detected in the frequency range of 1000–2100 cm−1, with the frequencies of two of them (of about 1790 cm−1 and 1940 cm−1) somewhat exceeding the doubled value of the frequencies of fundamental vibrations of the 4A1(z)LO (longitudinal optical) and 9E(x,y) symmetry types, which allows us to attribute these lines to the overtones of the fundamental vibrations of 4A1(z)LO and 9E(x,y). It is found that only one Raman scattering line is observed in the region of stretching vibrations of OH-groups (3200–3800 cm−1). The frequency of this line is found to depend on the scattering geometry, varied within 3431–3438 cm−1, and to be shifted to the low-frequency region by about 30–50 cm−1 relative to the frequencies in the IR absorption spectrum. This finding may be due to the alternative prohibition rule due to the presence of the center of symmetry of the oxygen octahedra O6 of the crystal structure.

Features of the defect structure of a nominally pure LiNbO3stoich crystal and double-doped LiNbO3:Zn:Mg (3.45:1.41 mol

We proposed and investigated a refinement of technology for obtaining Mg-doped LiNbO3 (LN) crystals by co-doping it with B. LN:Mg (5.0 mol%) is now the most widely used material based on bulk lithium niobate. It is suitable for light modulation and transformation. We found that non-metal boron decreases threshold concentrations of the target dopant in many ways. In addition, we earlier determined that the method of boron introduction into the LN charge strongly affects the LN:B crystal structure. So we investigated the point structural defects of two series of LN:Mg:B crystals obtained by different doping methods, in which the stage of dopant introduction was different. We investigated the features of boron cation localization in LN:Mg:B single crystals. We conducted the study using XRD (X-ray diffraction) analysis. We have confirmed that the homogeneous doping method introduces an additional defect (MgV) into the structure of LN:Mg:B single crystals. Vacancies in niobium positions (VNb) are formed as a compensator for the excess positive charge of point structural defects. According to model calculations, boron is localized in most cases in the tetrahedron face common with the vacant niobium octahedron from the first layer (VNbIO6). The energy of the Coulomb interaction is minimal in the LN:Mg:B crystal (2.57 mol% MgO and 0.42 × 10−4 wt% B in the crystal); it was obtained using the solid-phase doping technology. The solid-phase doping technology is better suited for obtaining boron-containing crystals with properties characteristic of double-doped crystals (LN:Mg:B).

The non-equilibrium conditions inherent in femtosecond laser ablation in liquids (LAL) offer a versatile platform for synthesizing metastable nanomaterials, yet predicting the structural evolution of complex oxides under rapid quenching remains a challenge. Here, we elucidate the divergent structural and functional outcomes of LAL applied to two related wide-bandgap niobium-based oxides: LiNbO3 and Nb2O5. We find that the intrinsic crystallization kinetics of the materials dictate their response to laser-induced fragmentation and condensation. Nb2O5, a strong glass-former with complex polymorphism, is trapped in an amorphous state. In contrast, LiNbO3 exhibits robust thermodynamic stability, favoring rapid nucleation and growth to form polycrystalline, albeit defect-rich, nanoparticles. These structural differences profoundly impact their electronic landscapes. Amorphization in Nb2O5 introduces a broad continuum of localized states that facilitate rapid charge recombination. Conversely, defect engineering in crystalline LiNbO3 yields discrete mid-gap states that enhance visible-light absorption and prolong carrier lifetimes. Consequently, LiNbO3 nanoparticles demonstrate sustained hydroxyl radical generation under visible irradiation, achieving a photocatalytic dye degradation rate threefold higher than their amorphous Nb2O5 counterparts and enabling 90% dye removal after 150 minutes at low catalyst loading. This investigation underscores the critical role of intrinsic crystallization kinetics in LAL synthesis and establishes defect-mediated crystallinity as a superior strategy over amorphization for activating wide-bandgap materials for solar-driven photocatalysis.

LiNbO3:Mg crystals ([Mg] = 6.0 and 5.54 mol

LiNbO3 crystal with a lithium composition gradient of Li/Nb = 0.8 wt%/cm (Li0.97..1.01Nb1.03..0.99O3) were obtained. A monotonic change in the edge of the UV absorption edge is observed when scanning the surface of the gradient crystal along the growth direction. Raman spectra from different areas of studied crystal were analyzed in a wide frequency range, which includes the region of fundamental vibrations of the crystal lattice (100-900 cm(-1)) and the region of overtone processes (900-3000 cm(-1)). A compositionally homogeneous, congruent LiNbO3 crystal was used as a comparison sample. It was found that in the spectra obtained from different parts of the gradient crystal, there is a significant scatter in the frequency values of the lines corresponding to the fundamental vibrations of the crystal lattice, but at the same time, the number of lines corresponding to the fundamental vibrations of the lattice for the gradient and compositionally homogeneous LiNbO3 crystals is the same. Moreover, in the spectrum of a gradient crystal in the region of overtone processes of fundamental vibrations, significantly more lines (35 lines) are observed than in the spectrum of compositionally homogeneous crystals (15 lines). The data obtained show that the state of the defect structure of compositionally homogeneous crystals and gradient LiNbO3 crystal is significantly different. The discovered differences between the defective structure of a gradient crystal and the defective structure of a compositionally homogeneous crystal may be the reason for compensation (damping) of distortions during nonlinear optical conversion of laser radiation by a gradient crystal due to the uneven temperature distribution along the length of the crystal. In compositionally homogeneous crystals, such temperature distortions significantly limit the efficiency of nonlinear optical conversion.

The paper presents comparative studies of the results obtained by the methods of IR absorption spectroscopy, photoluminescence and Raman scattering of LiNbO3:Zn (0,04–2,01 mol. % ZnO) crystals. It has been established that Zn doping reduces complex OH-group defects. This is confirmed by the fact that the volume concentration of OH-groups decreases. It has been shown that the decrease in the luminescence intensity in LiNbO3:Zn (0,04–2,01 mol. % ZnO) crystals is associated with a decrease in the concentration of NbLi point defects when Zn is introduced into lithium positions. Anomalous behavior of luminescence intensity was found in LiNbO3:Zn (1,39 mol. % ZnO) crystal. This correlates well with weakly expressed concentration threshold according to Raman spectroscopy data. Thus, it has been shown that the use of the photoluminescence method in conjunction with Raman spectroscopy in the study of concentration series of crystals can be extremely informative from the standpoint of analyzing changes in the secondary structure of doped crystals.

Lithium tantalate (LT) ceramics of various stoichiometry in the concentration range 48.5-51.4 mol% Li2O has been synthesized. The dependence of microstructure on the ceramics composition has been studied. Ceramic samples of LT of composition 48.5, 48.7, 49.0, 49.1, 49.3, 49.5, 50.5, 50.8, 51.0, 51.2 and 51.4 mol% Li2O have been investigated by Raman spectroscopy. Weak bands have been observed for the first time in the spectral region of 1000-2500 cm(-1) of all the studied samples. Regular Raman bands have been detected in the fundamental vibrations of crystal lattice in the 150-900 cm(-1) region. The bands are characteristic of single crystal states. Theoretical modeling suggests that the appearance of a second-order spectrum may be due to the small size of crystallites in ceramic samples. These investigations have helped to develop a method for controlling the structural uniformity of LT ceramic samples using Raman spectra.

Four double doped LiNbO3:Zn: B crystals has been grown from a melt containing 1.5 mol

Single crystals LiNbO3:Cu(0.015 wt%) and LiNbO3:Cu(0.042 wt%) have been studied by laser conoscopy, photoinduced light scattering, optical spectroscopy and Raman spectroscopy. The crystals are promising materials for the functional elements development for devices generating terahertz radiation. Exciting radiation is converted into the terahertz range with a high coefficient. Both crystals have been determined to have photorefraction effect. LiNbO3:Cu(0.015 wt%) has been shown to be perfect single-axis crystal and LiNbO3:Cu(0.042 wt%) to have a weak optical biaxiality. We have determined transfer of energy from E(X,Y) to A1(Z) symmetry type phonons that happens when photorefractive crystals LiNbO3:Cu(0.015 wt%) and LiNbO3:Cu(0.042 wt%) are irradiated by visible laser radiation. In this case Raman bands corresponding to 4A1(Z)- symmetry type phonons appear; these bands are prohibited in a given scattering geometries by the selection rules. This can increase the conversion efficiency to the terahertz range. When the Raman spectrum is excited by near-IR laser radiation, there is no photorefractive effect. The intensity of the line corresponding to phonons of the 4A1(Z) type of symmetry in the spectrum of the LiNbO3:Cu (0.042 wt%) crystal is noticeably lower than in LiNbO3:Cu (0.015 wt %). Thus, the parameters of the LiNbO3:Cu(0.015 wt%) crystal are more suitable for converting exciting radiation into the terahertz range.

The defect structure of LBO crystals of different orientations has been studied. Various types of macro-, meso- and micro-defects have been discovered in the crystal structure. Optical microscopy studies revealed the greatest defects in z-oriented LBO crystalline samples. Dendrite fractal structures have spontaneously formed on the surface of LBO samples. Features of the structures indicate internal stresses in the crystalline plates upper layers, and crystal defects. The anisotropy of ionic conductivity of LiB3O5 (LBO) crystals containing H+ cations as a technological impurity has been studied. A pronounced effect of high-temperature annealing of LBO crystals on the dielectric properties and conductivity features has been found. The obtained results explain some contradictions in the literature data on the phase states, conductivity character and pyroelectric properties of LBO crystals. The contributions of intrinsic (Li+) and impurity (H+) conductivity have been separated in LBO samples of x, z orientation with small values of intrinsic conductivity, and their temperature dependences have been obtained. The transition temperatures from impurity to intrinsic conductivity with the corresponding increase in the activation enthalpy for x, y, z cuts of LBO crystals have been established. It has been shown that in LBO crystal technology it is desirable to reduce the concentration of doping H+ cations.

The results of studying the vibrational spectra of the gradient LiNbO3:Er3+(congruent in the main components, the Er3+ gradient is 0.55 at%/cm) crystal grown by the Czochralski method are presented. The obtained data were compared with the spectra of LiNbO3cong (R = [Li]/[Nb] = 0.946) and the LiNbO3:Er3+(3.1 wt.%) crystals which were also grown by the Czochralski method. It was found that in a graded LiNbO3:Er3+ crystal (congruent in terms of its main components, with an Er3+ gradient of 0.55 at.%/cm), the intensity of second-order Raman scattering bonds significantly exceeds the intensity of fundamental vibrations. This is due to the high microscopic inhomogeneity of the graded crystal. The absence of the Raman spectrum in the scattering geometry was established, an explanation for which has not been found at the moment. According to the IR absorption spectra in the region of stretching vibrations of the OH--groups, it was found that the oxygen-octahedral clusters МеО6 (Ме–Li+, Nb5+, vacant octahedron V, impurity ion) of the structure of the gradient LiNbO3:Er3+ crystal have a shape close to regular. In this case, the value of R ≈ 1, and point defects of NbLi are almost absent in the structure. The volume concentration of OH--groups in the gradient LiNbO3:Er3+ crystal is almost an order of magnitude less than in the samples for comparison.

LiTaO3 crystals doped with Cr3+ and Nd3+ ions are promising for developing active nonlinear laser media. In this work, the defect structure of LiTaO3 crystals, including those doped with Cr3+ and Nd3+, is examined. X-ray patterns of all six investigated LiTaO3:Cr:Nd crystals are identical and correspond to a highly perfect structure. Using optical microscopy, the presence of defects of various shapes, microinhomogeneities, and lacunae was revealed. The optical absorption and Raman scattering spectra of a series of nonlinear, optical, double-doped LiTaO3:Cr3+:Nd3+ (0.06 ≤ [Cr3+] ≤ 0.2; 0.2 ≤ [Nd3+] ≤ 0.45 wt%) crystals showed that at concentrations of doping Cr3+ ions less than 0.09 wt% and Nd3+ ions less than 0.25 wt%, the crystal structure is characterized by a low level of defects, and the optical transmission spectra characterized by narrow lines corresponding to electron transitions in Nd3+ ions. In this case, for the radiative transition in the cation sublattice, the existence of three nonequivalent neodymium centers is observed, and for the radiative transition, two nonequivalent centers are observed. IR absorption spectroscopy in the OH−-stretching vibration range revealed two main spectral regions: 3463–3465 cm−1, associated with stoichiometry changes, and 3486–3490 cm−1, linked to complex defects such as (V-Li)-OH and (Ta4+Li)-OH. A distinct low-intensity line at ~3504 cm−1 was observed only in doped crystals, attributed to (Nd2+Li)-OH defects that significantly distort the oxygen-octahedral clusters due to the larger ionic radius of Nd3+ compared to Ta5+. In contrast, Cr-related defects cause only minor distortions. The Klauer method indicated that the highest concentration of OH−-groups occurs in the LiTaO3:Cr3+ (0.09 wt%):Nd3+ (0.25 wt%) crystal, where multiple complex defects are present.

Based on the analysis of the IR transmission spectra in the region of stretching vibrations of hydrogen atoms of OH−-groups, it was established that the oxygen-octahedral МеО6 clusters (Ме-Li+, Nb5+, vacant octahedron V, impurity ion) of the structure of the compositionally homogeneous crystal LiNbO3:Er3+(3.1 wt%) and the gradient crystal LiNbO3:Er3+(congruent composition by the main components, Er gradient of 0.55 at%/cm) have a shape close to the regular one. In this case, the value of R = [Li]/[Nb] ≈ 1, and in the structure of both crystals, there are practically no point defects in NbLi responsible for the photorefraction effect. By using the IR transmission spectra and Klauer’s method, it was found that the volume concentration of OH−-groups in the gradient crystal LiNbO3:Er3+ is almost an order of magnitude lower than in the compositionally homogeneous LiNbO3:Er3+(3.1 wt%) crystal. This fact explains the lower hydrogen conductivity of the gradient crystal LiNbO3:Er3+ and the lower photorefraction effect compared to the compositionally homogeneous LiNbO3:Er3+(3.1 wt%) crystal. The results obtained are important for the development of materials for active nonlinear laser media and for the conversion of laser radiation.

A comparative analysis of the photoluminescent properties, concentration of OH--groups and optical quality of double-doped crystals obtained from charges of different genesis has been performed. In a LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal obtained by solid-phase doping, the content of OH--groups is higher than in a LiNbO3:Er(0,75 mol.%):Zn(3,82 mol.%) crystal obtained by homogeneous doping. These changes occur as a result of the simultaneous formation of two types of complex defects in the LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal structure: Zn-Nb(3-)-OH and VLi- OH . It has been established that photoluminescence in the visible region is caused by radiative transitions of Er3+ without the manifestation of the host's own luminescence in the studied crystals. For the LiNbO3:Er:Zn crystal obtained by solid-phase doping, the luminescence intensity is 77% higher than in the crystal obtained by homogeneous doping. This may be due to the participation of OH--groups in the energy transfer between the host and the Er3+ ions.

Features of the defect structure of nominally pure LiNbO3:В crystals were investigated by X-ray diffraction analysis and photoluminescence. Crystals were grown by the Czochralski from a mixture of congruent composition charge containing 0.08 and 0.12 wt. % boron. At this, the concentration of boron in crystals is at the level of trace amounts of metallic impurities and is ~ 10–4 wt%. MeO6 oxygen-octahedral clusters are responsible for the ferroelectric and nonlinear optical properties of the crystal. It has been found that in LiNbO3:B crystals, the lengths of O–O, Me–O, and Me–Me (Me–Li, Nb) bonds in clusters, the arrangement of Me cations, vacancies, and NbLi point defects along the polar axis differ significantly from those for nominally pure congruent crystal. NbLi defects and transition metals are deep electron traps responsible for the photorefraction effect. The photoluminescence spectra showed that the concentration of defects and metals in the investigated LiNbO3:B crystals is lower than in the congruent crystal. these differences can be due to both a change in the properties of the boron-containing melt and the localization of trace amounts of boron in the O4 tetrahedral gaps of the LiNbO3 crystal structure. In the first case, reactive boron binds cations of niobium and transition metals in the melt into stable complexes.

Comparative studies of double-doped LiNbO3:Er:Zn crystals of different genesis have been carried out using infrared absorption spectroscopy (in the region of OH-- group stretching vibrations) and Raman spectroscopy. A LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal obtained by solid-phase doping and a LiNbO3:Er(0,75 mol.%):Zn(3,82 mol.%) crystal obtained by homogeneous doping were used in the study. No significant changes have been recorded in the infrared absorption spectra and Raman spectra of crystals obtained using different technologies. Minor changes in the main parameters of the absorption bands with frequencies of 3483 and 3492 cm-1 have been detected in the infrared absorption spectra. This may be due to the higher concentration of zinc dopant in the LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal. Measuring the half-width parameter of the band with a frequency of 271 cm-1 in the Raman spectra of the studied crystals helped to establish that the LiNbO3:Er(0,53 mol.%):Zn(4,02 mol.%) crystal has a higher ordering of the structural units of the cation sublattice compared to the LiNbO3:Er(0,75 mol.%):Zn(3,82 mol.%) crystal.