Raman Scattering in Non-Stoichiometric Lithium Niobate Crystals with a Low Photorefractive Effect

• Main Authors: Nikolay Sidorov, Mikhail Palatnikov, Alexandra Kadetova
• Format: Article
• Language: English
• Published: MDPI AG 2019-10-01
• Series: Crystals
• Subjects: raman scattering; lithium niobate single crystal; doping; structure disorder; cluster; extra bands; pseudo scalar vibrations; two-particle states of acoustic phonons
• Online Access: https://www.mdpi.com/2073-4352/9/10/535

Raman spectra of lithium niobate single crystals strongly doped by zinc and magnesium, it has been established, contain low-intense bands with frequencies 209, 230, 298, 694, and 880 cm^−1. Ab ignition calculations fail to attribute these bands to fundamental vibrations of A₂ symmetry type unambiguously. Such vibrations are prohibited by the selection rules in the space group C_3V⁶ (R3c). Ab initio calculations also proved that low-intense “extra” bands with frequencies 104 and 119 cm^−1 definitely do not correspond to vibrations of A₂ symmetry type. We have paid special attention to these extra bands that appear in LiNbO₃ single crystals Raman spectra despite the fact that they are prohibited by the selection rules. In order to do so, we have studied a number of lithium niobate single crystals, both nominally pure and doped, by Raman spectroscopy. We have assumed that some “extra” bands correspond to two-particle states of acoustic phonons with a total wave vector equal to zero. We have also detected a Zn concentration area (0.05−0.94 mol.% ZnO in a crystal) where doped crystal structure is more ordered: The order of alternation of the main, doping cations, and vacancies along the polar axis is increased, and oxygen octahedra are less distorted.