Optical materials

Influence of features of localization of hydrogen atoms in lithium niobate crystals on the form of oxygen-octahedral clusters of the structure

N. V. Sidorov, L. A. Bobreva, M. N. Palatnikov

Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials Kola Science Centre of the Russian Academy of Sciences

Abstract: Based on the IR absorption spectra in the region of OH$^-$-group stretching vibrations, it was established that the oxygen-octahedral clusters МеО$_6$ ( Ме – Li, Nb, vacant octahedron V, impurity ion) in the structure of the LiNbO$_3$:Er (3.1 wt.%) crystal and the nominally pure highly perfect LiNbO$_3$ crystal of stoichiometric composition (R = [Li] / [Nb] = 1) have a shape close to the shape of a regular octahedron and at the same time there are practically no deep electron traps – point defects NbLi, responsible for the crystal's resistance to optical damage. In the IR spectrum of these crystals, only one line is observed, corresponding to the stretching vibrations of hydrogen atoms linked by hydrogen bonds with oxygen atoms oscillating along the polar axis of the crystal in the Me–O–Me bridge ( Ме – Li, Nb, vacancy, impurity metal). This vibration changes the dipole moment of the unit cell of the crystal and is active in the IR absorption spectrum. The vibration frequency is determined by the crystal composition. The stretching vibrations of the remaining hydrogen atoms of the hydroxyl groups OH of the highly symmetric oxygen-octahedral clusters МеО$_6$ of LiNbO$_3$ crystals do not change the dipole moment of the unit cell and are not active in the IR absorption spectrum. In the structure of non-stoichiometric LiNbO$_3$ crystals, there are two or more nonequivalent positions of OH$^-$ groups and more than two lines in the frequency range 3450–3550 cm$^{-1}$ are observed in their IR absorption spectrum.

Keywords: lithium niobate single crystal, stoichiometry, erbium doping, point defects, OH$^-$-groups, materials for active nonlinear laser media and laser radiation conversion, IR absorption spectra.

Received: 14.04.2025
Revised: 30.05.2025
Accepted: 02.06.2025

DOI: 10.61011/OS.2025.07.61108.7813-25

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