Determination of Stoichiometry, Concentration of OH Groups, and Point Defects in Lithium Niobate Crystals from Their IR
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Determination of Stoichiometry, Concentration of OH Groups, and Point Defects in Lithium Niobate Crystals from Their IR Absorption Spectra N. A. Teplyakovaa,*, N. V. Sidorova, and M. N. Palatnikova a Institute
of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Russian Academy of Sciences, Kola Science Center, Apatity, Murmansk oblast 184209 Russia *e-mail: [email protected] Received March 20, 2020; revised April 21, 2020; accepted April 28, 2020
4+ − Abstract—The concentration of OH impurity groups, the Li/Nb ratio, the concentration of Nb Li and V Li point defects in stoichiometric and congruent LiNbO3 crystals, as well as in crystals doped with magnesium and zinc at concentrations close to threshold concentrations, have been calculated from IR absorption spectra in the range of stretching vibrations of OH groups. The behavior of bands in the IR absorption spectra of heavily doped LiNbO3:Mg and LiNbO3:Zn crystals has been revealed to correlate with the behavior of the Raman spectrum line that corresponds to stretching bridge vibrations of oxygen atoms in the NbO6 octahedron along the polar axis.
Keywords: lithium niobate crystal, doping, IR absorption spectroscopy, Raman scattering DOI: 10.1134/S0030400X20080378
INTRODUCTION The nonlinear optical, electrical, and photorefractive properties of a lithium niobate single crystal (LiNbO3) are largely determined by the presence in its structure of point as well as complex defects due to hydrogen bonds. In this case, the state of the defective crystal substantially depends on stoichiometry (Li/Nb ratio) and the presence of dopants. Many of the physical properties of a LiNbO3 crystal (coercive field, position of the fundamental optical absorption edge, resistance to damage by optical radiation, etc.) can be improved by changing the state of defects of its structure. In addition, it is believed that some properties of lithium niobate crystals heavily doped with magnesium, when the concentration of the dopant is close to the threshold value, are shifted to the properties of stoichiometric crystals (Li/Nb = 1); i.e., they are also determined by the Li/Nb ratio [1, 2]. This fact is important, since stoichiometric crystals have a lower coercive field compared to congruent crystals (Li/Nb = 0.946), which makes them promising for creating materials for the laser radiation conversion that are based on periodically polarized micron and submicron domains [3]. In accordance with the Li-vacancy compensation model [2], the crystal lattice of a congruent LiNbO3 4+ crystal contains ~1 mol % of NbLi point defects 5+ (excess Nb cations located in positions of Li+ cations
of an ideal stoichiometric structure (Li/Nb = 1)) and ~4 mol % of VLi− point defects. The structure of an ideal 4+ stoichiometric crystal does not contain NbLi point − defects and, correspondingly, VLi defects. The presence of NbLi point defects, which are deep electron traps, significantly affects the photorefractive properties of both nominally pure and doped LiNbO
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