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

The physicochemical characteristics, optical uniformity and photorefractive properties of a series of four LiNbO3:Er:Zn single crystals have been studied. Single crystals have been obtained by the Czochralski method. The erbium content in the crystals was 0.5 mol

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.

Single crystals LiNbO3cong., LiNbO3stex., LiNbO3stex.(6.0 wt. % K2O), LiNbO3 : Mg (5.29 mol % MgО), LiNbO3 : Gd(0.003) : Mg(0.65 wt. %) were studied by infrared absorption spectroscopy in the area of valent vibrations of OH-groups. It was found that the absorption bands on the infrared spectrum can be referred to two groups. The first group includes absorption bands in the 3465-3488 cm–1 frequency range related to stoichiometry disturbances in the crystal due to the presence of point defects (VLI, NbLi) and complex defects VLi-OH, NbLi-OH formed with them. The second group includes absorption bands in the frequency range 3490–3590 cm–1 related to changes in the mechanism of dopant entry into the crystal structure and formation of complex defects (MeNb-OH, MeLi-OH-MeNb).

Методом фотолюминесценции в видимой области спектра (λ= 380-700 нм) исследованы оптические свойства номинально чистых кристаллов LiNbO, близкого по составу к стехиометрическому LiNbO (6,0 мас.% KO), LiNbO и номинально чистых кристаллов LiNbO:B(0,55 и 0,83 мол. % BO в шихте), полученных методом Чохральского по технологии прямого твёрдофазного легирования шихты конгруэнтного состава оксидом бора (BO). Обнаружено, что интенсивность фотолюминесценции растет в ряду кристаллов LiNbO, LiNbO (6,0 мас.% KO), LiNbO и определяется изменением концентрации глубоких ловушек электронов (NB - катион ниобия, локализованный в литиевом октаэдре) при изменении стехиометрии этих кристаллов. В кристаллах LiNbO:B (0,55 и 0,83 мол. % BO в шихте) интенсивность фотолюминесценции близка к таковой для кристалла LiNbO, что обусловлено приближением состава и структуры кристаллов LiNbO:B (0,55 и 0,83 мол. % BO в шихте) к составу и структуре стехиометрического кристалла. The optical properties of nominally pure LiNbO, near-stoichiometric LiNbO (6,0 wt% KO), LiNbO and LiNbO:B(0,55 and 0,83 mol% BO in the charge) crystals in the visible region of the spectrum (λ = 380-700 nm) were studied by photoluminescence. LiNbO:B crystals were grown by Czochralski using the technology of a direct solid-phase doping of the congruent charge by boron oxide (BO). The photoluminescence intensity increases in the series of crystals: LiNbO, LiNbO (6,0 wt% KO), and LiNbO. The photoluminescence intensity is determined by the concentration of deep electron traps (Nb - «niobium antisite») and the stoichiometry of these crystals. The photoluminescence intensity of LiNbO:B (0,55 and 0,83 mol% BO in the charge) crystals is close to the photoluminescence intensity of LiNbO crystal. This can be explained by the fact that the composition and structure of LiNbO:B (0,55 and 0,83 mol% BO in the charge) crystals approach the composition and structure of the stoichiometric crystal.

Specific features of the defect structure of nominally pure LiNbO3:B crystals were studied by the X-ray diffraction analysis. Nominally pure LiNbO3:B crystals were grown by Czochralski using the technology of the direct solid-phase doping of the congruent charge by orthoboric acid (H3BO3). The bonds lengths of Me-O in MeO6 clusters (Me-Li, Nb) determine the ferroelectric and nonlinear optical properties of the lithium niobate crystal. The values of these bonds in LiNbO3:B crystals differ significantly from the bonds lengths of the nominally pure congruent crystal LiNbO3cong. The differences in the bonds lengths are caused by a change in the properties of the boron-containing melt, technological parameters of the growth of LiNbO3:B crystals, and the localization of a trace amounts of boron in tetrahedral voids of the lithium niobate crystal structure. The results of the study of LiNbO3:B crystals were compared with those for nominally pure LiNbO3cong crystals and near-stoichiometric LiNbO3stoich (5,5 wt% K2O) ones.

Показано, что ион В3+ в следовых количествах может встраиваться либо в грани кислородных тетраэдров кристаллической структуры LiNbO3, граничащие или с литиевым, или с вакантными кислородными октаэдрами, либо в кислородную плоскость, разделяющую кислородно-октаэдрические слои. При этом ион В3+, взаимодействуя с атомами кислорода октаэдров О6, заметно искажает анионный каркас структуры кристалла и изменяет поляризуемость октаэдров, определяющую нелинейно-оптические свойства кристалла. Кроме того, в кристаллах с В3+ происходит упорядочение структурных единиц катионной подрешетки вдоль полярной оси и увеличение отношения Li/Nb за счет уменьшения концентрации точечных дефектов NbLi, являющихся глубокими электронными ловушками. Показано, что флюс В2О3 (как активный комплексообразователь) заметно изменяет свойства шихты и расплава, тем самым определенным образом структурирует расплав и увеличивает температуру Кюри кристаллов.

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.

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 Nb-Li 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.

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.

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.

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.

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.

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.

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 LiNbO 3 : B crystals have been determined. It has been shown that the diffusion field governing the concentration of shallow electron traps in LiNbO 3 : 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 LiNbO 3 : B crystals is the same as in the stoichiometric crystal but the optical homogeneity of LiNbO 3 : B crystals is closer to that of the congruent crystal. In addition, the concentration of OH groups in LiNbO 3 : B crystals is lower and their arrangement in the structure is more regular than in the congruent crystal.

Показано, что применение для выращивания номинально чистых кристаллов ниобата лития расплава, структурированного неметаллическим элементом бором, позволяет регулировать особенности вторичной структуры, оптическую однородность, величины фотоэлектрических полей и ширину запрещенной зоны. По характеристикам фотоиндуцированного рассеяния света определены напряженности фотовольтаического и диффузионного полей в номинально чистых кристаллах LiNbO3:В. Показано, что величина диффузионного поля, определяющая концентрацию мелких электронных ловушек, для кристаллов LiNbO3:В имеет промежуточное значение между кристаллами конгруэнтного и стехиометрического составов и зависит от концентрации бора в шихте. При этом ширина запрещенной зоны в кристаллах LiNbO3:В соответствует значению для стехиометрического кристалла, но оптическая однородность кристаллов LiNbO3:В близка к оптической однородности конгруэнтного кристалла, концентрация ОН-групп в кристаллах LiNbO3:В меньше, а их расположение в структуре более упорядочено, чем в конгруэнтном кристалле. Ключевые слова: ниобат лития, расплав, фотоэлектрические поля, ИК-спектроскопия, оптическая спектроскопия.