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.

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.

Разработанный нами подход к расчету кластеров в структуре кристалла ниобата лития на основе не элементарных ячеек, а кислородных кластеров, позволяет не допустить разорванности структур на границах кластера и сохранить электронейтральность модельного кластера. Обнаружены оптимальный размер кластера, имеющий минимум энергии, и структура, стремящаяся к структуре конгруэнтного кристалла ниобата лития, подтверждающая энергетическую оптимальность конгруэнтного кристалла. При введении легирующего иона 3 + во всех трёх случаях концентраций наблюдается энергетический оптимум именно вблизи конгруэнтного соотношения Li/Nb. Также наблюдается понижение энергии после оптимизации модельного кластера, что согласуется с данными спектрального анализа об образовании внутри кристалла микрокластеров с локальным упорядочением структуры. An approach developed by us to calculation of clusters in the structure of a lithium niobate crystal based not on unit cells, but on oxygen clusters, makes it possible to prevent disruption of structures at the boundaries of the cluster and to preserve the electroneutrality of the model cluster. An optimal cluster size with a minimum energy and a structure tending to the structure of a congruent lithium niobate crystal, confirming the energy optimality of a congruent crystal, are found. With the introduction of the dopant ion 3 + , in all three cases of concentrations, an energy optimum is observed precisely near the congruent ratio Li/Nb . A decrease in energy is also observed after optimization of the model cluster, which is consistent with the data of spectral analysis on the formation of microclusters with local ordering of the structure inside the crystal.

We have performed a computer simulation of processes taking place at formation of energetic equilibrium oxygen-octahedral clusters in a ferroelectric phase of a lithium niobate crystal (LN, LiNbO3). In order to do so we used a semiclassical atomistic model with a qualitative approach. For the first time we have modeled lithium niobate structure on the basis of oxygen octahedra, while unit cells are usually used. We have established that there is an energy efficient cluster size with a specific structure that approaches a congruent LN structure. We have also proved that stoichiometric composition of LN leads to loss of electrical neutrality; thus achievement of demanded properties including photorefractive ones is possible by doping. We have chosen an ion with a charge 3+ to build a model.

The processes occurring upon the formation of energetically equilibrium oxygen-octahedral clusters in the ferroelectric phase of a stoichiometric lithium niobate (LiNbO3) crystal have been investigated by the computer modeling method within the semiclassical atomistic model. An energetically favorable cluster size (at which a structure similar to that of a congruent crystal is organized) is shown to exist. A stoichiometric cluster cannot exist because of the electroneutrality loss. The most energetically favorable cluster is that with a Li/Nb ratio of about 0.945, a value close to the lithium-to-niobium ratio for a congruent crystal.

Modeling of processes occurring during the formation of the energy equilibrium oxygen octahedral clusters of a crystal of lithium niobate (LiNbO3). It is shown that because of the loss of electrical neutrality cluster strictly stoichiometric composition cannot exist. It is shown that there is energetically favorable, having a minimum of energy, the size of the cluster, which i s organized in a certain structure, tending to structure congruent crystal. The most profitable energy is a cluster ratio Li / Nb = 0.94.

The processes occurring during the formation of energetically equilibrium oxygen-octahedral clusters in the ferroelectric phase of lithium niobate (LiNbO3) crystal, have been qualitatively modeled in dependence of the phase composition. The modeling results are compared with the data obtained within vacancy models. It is shown that the cluster structure constructed along the crystallographic Y axis is most ordered, while that constructed along the polar Z axis is least ordered. The largest spread in the ratio R = Li/Nb is observed in the direction of the Z axis.

The cation disorder in stoichiometric and congruent lithium niobate crystals and also in congruent crystals doped with Y3+ or Gd3+ has been studied by X-ray diffraction and Raman scattering spectroscopy using vacancy models. The results indicate that the structural disordering induced by doping with Y3+, revealed by both X-ray diffraction and Raman spectroscopy, can be understood in terms of the mechanism of yttrium incorporation into the cation sublattice of the crystal. Yttrium substitution for Nb5+ on its normal lattice site causes Nb5+ to occupy vacant octahedra, thereby increasing the cation and vacancy disorder along the polar axis and distorting the octahedra because the ionic radius of Y3+ exceeds that of Nb5+.