Unveiling the Defect Structure of Lithium Niobate with Nuclear Methods

X-ray and neutron diffraction studies succeeded in the 1960s to determine the principal structural properties of congruent lithium niobate. However, the nature of the intrinsic defects related to the non-stoichiometry of this material remained an object of controversial discussion. In addition, the incorporation mechanism for dopants in the crystal lattice, showing a solubility range from about 0.1 mol % for rare earths to 9 mol % for some elements (e.g., Ti and Mg), stayed unresolved. Various different models for the formation of these defect structures were developed and required experimental verification. In this paper, we review the outstanding role of nuclear physics based methods in the process of unveiling the kind of intrinsic defects formed in congruent lithium niobate and the rules governing the incorporation of dopants. Complementary results in the isostructural compound lithium tantalate are reviewed for the case of the ferroelectric-paraelectric phase transition. We focus especially on the use of ion beam analysis under channeling conditions for the direct determination of dopant lattice sites and intrinsic defects and on Perturbed Angular Correlation measurements probing the local environment of dopants in the host lattice yielding independent and complementary information.

INFLUENCE OF A DOPING ION ON THE PROCESSES OF DEFECT FORMATION IN SIMULATED LITHIUM NIOBATE CLUSTERS

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.

Time evolution of Symmetry-forbidden Raman lines activated by photorefractivity

Transmission Raman spectroscopy experiments were performed on iron doped congruent lithium niobate within two –in principle equivalent- configurations, namely Y(ZX)Y and Y(XZ)Y. While the former respects the Raman selection rules, the other configuration gives a time dependent spectrum that, after a transient time of several minutes, finally results in a mixture of expected and forbidden modes. This breaking of Raman selection rules is caused by the spontaneous conversion of a part of the ordinarily polarized pump beam into an extraordinarily polarized beam by photorefractive anisotropic self-scattering. A numerical modelling of the phenomenon is developed and fairly reproduces the time dependence of conversion energy.