HYDROGEN BONDS IN LITHIUM NIOBATE CRYSTALS OF DIFFERENT COMPOSITION

Методом ИК-спектроскопии в области валентных колебаний OH - групп выполнен анализ комплексных дефектов, обусловленных наличием в структуре кристалла водородных связей, в номинально чистых кристаллах ниобата лития конгруэнтного и стехиометрического состава с разным отношением Li / Nb, а также в кристаллах LiNbO: Zn(0,04 - 6,5 мол.% ZnO), легированных цинком в широком диапазоне концентраций, полученным по технологии прямого легирования расплава. Выявлено влияние легирующих примесей на концентрацию OH - групп, вид и локализацию комплексных дефектов в структуре кристалла. Показано, что изменение количества позиций атомов водорода в структуре кристалла LiNbO позволяет с достаточной точностью судить о соответствии его состава стехиометрическому или конгруэнтному составу. Для легированных кристаллов LiNbO : Zn(0,04 - 6,5 мол.% ZnO) получены данные, свидетельствующие об изменении при прохождении концентрационных порогов характера комплексообразования OH - групп с точечными дефектами катионной подрешетки. При этом, вследствие изменения механизма вхождения легирующего катиона в структуру кардинально изменяются свойства кристалла. Вклад в различие частот (и, соответственно, в значение квазиупругих постоянных связей OH -) в спектре конгруэнтного кристалла и легированных кристаллов может вносить также различие электроотрицательностей и ионных радиусов основных и легирующих катионов. An analysis of complex defects was carried out by IR-spectroscopy method in the area of OH - groups stretching vibrations. The defects are caused by hydrogen bonds present in the structure of nominally pure congruent lithium niobate crystals, crystals of stoichiometric composition with a different Li / Nb ratio, as well as in LiNbO: Zn(0,04 - 6,5 мол.% ZnO) crystals doped in a wide range of concentrations due to direct doping of the melt method. Dopants were determined to influence OH - groups concentration, type and localization of complex defects in the crystals structure. A change in the amount of hydrogen sites in the LiNbO crystals structure was shown to evaluate the composition either stoichiometric or congruent. The character of OH - groups complexing with cation sublattice point defects was shown to change when doped crystals LiNbO: Zn(0,04 - 6,5 мол.% ZnO) trespass concentration thresholds. Dopant incorporation mechanism changes at this drastically, thus crystal properties also change quite sharply. Frequencies (as well as quasi-elastic constants of OH - bonds) change in congruent and doped crystals due to a difference in electronegativities and ionic radii of the main and dopant cations.

A NEW METHOD OF THE STOICHIOMETRY INCREASE OF THE LITHIUM NIOBATE NONLINEAR-OPTICAL CRYSTAL

Experimental and theoretical data on the influence of В2О3 flux on the crystal-melt system, the structural features, and the optical properties of a crystal of lithium niobate are summarized. The Gibbs energies of the borate impurities formation (Al4B2O9, CaB2O4, CaB4O7, Ca2B2O5, Ca3B2O6, PbB2O4) in a congruent composition charge of lithium niobate are calculated. It was found that the element boron, as an active complexing agent, in the composition of the В2О3 flux aligns the distribution coefficients of lithium (KLi) and niobium (KNb). Also, the element boron is able to prevent the transition of trace amounts of impurity metals into the structure of a lithium niobate crystal. Boron increases the ordering of structural units of the cation sublattice and distorts the anionic framework of the crystal. This is due to the fact that boron is embedded in the tetrahedral voids faces of the crystal structure in trace amounts (4∙10-4 mol.%). This leads to changes in bond lengths O-O of the oxygen octahedra O6, thereby changing polarizability oxygen-octahedral cluster NbO6, determining nonlinear optical and ferroelectric properties of the crystal.

Gd-admixed (Lu,Gd)AlO3 single crystals: breakthrough in heavy perovskite scintillators

We report a breakthrough concept for a bulk single crystal as a heavy aluminum perovskite scintillator, where due to bandgap engineering by a balanced Gd admixture in a Lu cation sublattice, the scintillation performance dramatically increases. In an optimized composition of (Lu, Gd)AlO3:Ce (LuGdAP:Ce), the light yield approaches 21,000 phot/MeV, which is close to that of classical but much less dense YAP:Ce and 50% higher than the best LuYAP:Ce reported in the literature. Moreover, contrary to LuYAP:Ce, the LuGdAP host maintains a high effective atomic number close to that of LuAP:Ce (Zeff = 64.9), which is comparable to commercial LSO:Ce. An enormous decrease in afterglow on the millisecond time scale and acceleration in the rise time of the scintillation response further increase the application potential of the LuGdAP host. The related acceleration of the transfer stage in the scintillation mechanism due to diminishing electron trap depths is proven by thermally stimulated luminescence (TSL). Furthermore, we quantitatively characterize and model the energy transfer processes that are responsible for the change in the photoluminescence and scintillation decay kinetics of Ce3+ in the LuGdAP matrix. Such an innovative (Lu, Gd)AP:Ce scintillator will become competitive for use in applications that require heavy, fast, and high light yield bulk scintillators.

Особенности структуры и оптические свойства номинально чистых кристаллов LiNbO-=SUB=-3-=/SUB=-, выращенных из шихты, содержащей B-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

It is shown that the use of B2O3 as a flux allows us to obtain the nominally pure LiNbO3 crystals possessing high compositionally uniformity. The LiNbO3:B crystals have both the increased ordering of the structural units of the cation sublattice, close to stoichiometric crystals, and high optical damage resistance. According to the results of computer simulation it was found that the boron element can incorporate into the faces of oxygen tetrahedra of the LiNbO3 crystal structure. Trace amounts of boron (10⋅10-4 wt. %) in the LiNbO3:B structure prevent the formation of the point defects (NbLi). At the same time, B3+ noticeably deforms the oxygen sublattice of the LiNbO3 crystal structure thereby it changes the polarizability of the oxygen octahedra, which determines the nonlinear optical properties of the crystal.

TEM and DFT study of basal-plane inversion boundaries in SnO2-doped ZnO

In our recent study (Ribic et al. 2020) we reported the structure of inversion boundaries (IBs) in Sb2O3-doped ZnO. Here, we focus on IBs that form in SnO2-doped ZnO. Using atomic resolution scanning transmission electron microscopy (STEM) methods we confirm that in SnO2-doped ZnO the IBs form in head-to-head configuration, where ZnO4 tetrahedra in both ZnO domains point towards the IB plane composed of a close-packed layer of octahedrally coordinated Sn and Zn atoms. The in-plane composition is driven by the local charge balance, following Pauling's principle of electroneutrality for ionic crystals, according to which the average oxidation state of cations is 3+. To satisfy this condition, the cation ratio in the IB-layer is Sn4+: Zn2+=1:1. This was confirmed by concentric electron probe analysis employing energy dispersive spectroscopy (EDS) showing that Sn atoms occupy 0.504 ? 0.039 of the IB layer, while the rest of the octahedral sites are occupied by Zn. IBs in SnO2-doped ZnO occur in the lowest energy, IB3 translation state with the cation sublattice expansion of ?IB(Zn-Zn) of +91 pm with corresponding O-sublattice contraction ?IB(O-O) of -46 pm. Based on quantitative HRTEM and HAADF-STEM analysis of in-plane ordering of Sn and Zn atoms, we identified two types of short-range distributions, (i) zigzag and (ii) stripe. Our density functional theory (DFT) calculations showed that the energy difference between the two arrangements is small (~6 meV) giving rise to their alternation within the octahedral IB layer. As a result, cation ordering intermittently changes its type and the direction to maximize intrinsic entropy of the IB layer driven by the in-plane electroneutrality and 6-fold symmetry restrictions. A long-range in-plane disorder, as shown by our work would enhance quantum well effect to phonon scattering, while Zn2+ located in the IB octahedral sites, would modify the bandgap, and enhance the in-plane conductivity and concentration of carriers. Keywords

CALCULATION OF THE POINT DEFECT CONCENTRATION OF THE CATION SUBLATTICE AND HYDROXYL GROUPS IN LITHIUM NIOBATE CRYSTALS OF DIFFERENT COMPOSITION

Исследованы ИК-спектры поглощения в области валентных колебаний OH - групп кристаллов ниобата лития стехиометрического и конгруэнтного составов. Рассчитана концентрация OH - групп, отношение Li / Nb, а также концентрация точечных дефектов Nb и V. Результаты расчетов совпадают с литературными данными и подтверждаются фазовой диаграммой ниобата лития. Показано, что увеличение количества свободных протонов (вносящих вклад в проводимость) может обуславливать более высокие электропроводность, скорость термической фиксации голограмм и снижение эффекта фоторефракции. The IR absorption spectra in the region of stretching vibrations of OH - groups of lithium niobate crystals of stoichiometric and congruent compositions are studied. The results of calculations of the OH - groups concentration, the Li / Nb ratio and the concentration of point defects Nb and V coincide with the literature data. The phase diagram of lithium niobate confirms these results. It is shown that an increase in the number of free protons that contribute to conductivity can lead to reducing the optical damage and higher electrical conductivity and the rate of thermal fixation of holograms.

SIMULATION OF STRUCTURE OF LINBO CRYSTALS GROWN WITH USING OF BO FLUX

Показано, что элемент B в следовых количествах (≈10 масс.%) может встраиваться в составе группы [BO ] в грани кислородных тетраэдров кристаллической структуры LiNbO. При этом бор заметно искажает анионный каркас кристалла, изменяя длины < O - O > связей, улучшает упорядочение структурных единиц катионной подрешетки, изменяет поляризуемость кислородно-октаэдрических кластеров MeO (Me - Li, Nb), определяющую сегнетоэлектрические и нелинейнооптические свойства кристалла. It is shown that the B element is able to incorporate into the facets of oxygen tetrahedra of LiNbO, crystal structure [BO ] in a trace amounts (≈10 wt. %). In this case, boron noticeably distorts the anion sublattice of the crystal, changing the lengths of the < O - O > bonds, increasing the ordering of structural units of the cation sublattice. At the same time, boron changes the polarizability of the oxygen-octahedral MeO clusters (Me - Li, Nb) which determines the ferroelectric and nonlinear optical properties of the crystal.

Structural particularities and optical properties of lithium niobate single crystals grown from the charge doped by boron.

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.

Structural Features of Nominally Pure Lithium Niobate Crystals Grown from Boron-Doped Charge

The features of the structure of single crystals LiNbO3:B3+ (0.12 and 0.18 wt %) grown by the Czochralski method from the mixture of different genesis were studied. It was found that boron is able to incorporate into the crystal structure of lithium niobate in a trace amounts (~ 10–4–10–5 wt %), decreasing the concentration of structural defects NbLi. Thus, ordering of structural units of the cation sublattice of lithium niobate crystals grown from a congruent composition melt approach in that of stoichiometric crystals.