Investigation of the Intrinsic Defects of LiTaO 3 Crystals by NMR Spectroscopy

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STRUCTURE OF CRYSTALS

Investigation of the Intrinsic Defects of LiTaO3 Crystals by NMR Spectroscopy A. V. Yatsenkoa,*, M. N. Palatnikovb, and N. V. Sidorovb a Crimean

b Institute

Federal University, Simferopol, 295007 Russia of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Scientific Center, Russian Academy of Science, Apatity, Murmansk oblast, 184209 Russia *e-mail: [email protected] Received January 16, 2018; revised January 18, 2018; accepted January 23, 2018

Abstract—The nuclear magnetic resonance (NMR) of 7Li and 6Li nuclei in a LiTaO3 single crystal sample of congruent composition has been experimentally investigated. It is found that there are weak side lines in the NMR spectra of 7Li nuclei, which have not been observed before; these lines are due to cation sublattice defects. A computer simulation of NMR spectra has revealed the most likely configuration of (Ta 5Li+ + 4VLi ) defect complexes that are present in nonstoichiometric LiTaO3 crystals. DOI: 10.1134/S1063774519010310

INTRODUCTION Lithium niobate (LN) LiNbO3 and lithium tantalate (LT) LiTaO3 crystals are ferroelectric materials. They are widely used in acoustoelectric, electro-optic, acousto-optic, and nonlinear optical devices [1, 2]. At present, there is ample evidence to suggest that the real composition of nominally impurity-free nonstoichiometric LN and LT crystals is described by the so-called model of lithium vacancies [3]. In accordance with this model, the structural formula of LT has the form [Li1 – 5xTaxV4x][Ta]O3, where х ∼ 0.0083 for an LT crystal of congruent composition [4]. Most of researchers tend to believe that the intrinsic defects in LN crystals form (Nb5Li+ + 4VLi ) complexes, where Nb5Li+ is an niobium ion occupying a Li site, and VLi are lithium vacancies, placed near Nb5Li+ . Based on the analysis of the shape of NMR spectra [5, 6] and ab initio simulation of the structure [7–12], several versions of preferable configuration of these complexes were proposed for LN crystals. Since the structure of LT crystals at room temperature is very similar to the LN structure, the problem of configuration of similar complexes in LT was not put before. The nuclear magnetic resonance (NMR) of quadrupole nuclei is an efficient method of studying defects in solids [13]. The 7Li NMR studies of LT crystals have been previously performed by various research teams [14–20], however, the known results do not contain any information concerning the nature of intrinsic

defects of LT crystals. At the same time, 7Li and 6Li NMR studies of LN crystals of congruent composition [5, 21] gave an impetus to analysis of the nature of intrinsic defects in LN. The main objective of this paper is to establish a likely configuration of (Ta 5Li+ + 4VLi ) complexes by experimental 7Li and 6Li NMR study of a nominally impurity-free LT single crystal of congruent composition and simulation of NMR spectra with allowance for defects in the Li sublattice. EXPERIMENTAL RESULTS Experiments were performed on a single-crystal LT sample 7 ×

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