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

Nikolay Sidorov,Alexandra Kadetova,Mikhail Palatnikov

doi:10.3390/cryst9100535

Abstract

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 C3V6 (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.

Highlights

The widespread use of lithium niobate (LiNbO3, LN) as a non-linear optical material necessitates a detailed study of its vibrational spectrum
Theseusprocesses relate different types of defective structure fractal-type nanostructures in. These processes relate to different types of defective regardless of the specific physical mechanisms that generate it
Raman spectra that we studied clearly show that doped LN crystals have super-structure lattice of cluster defects several unit cell in diameter

Summary

Introduction

The widespread use of lithium niobate (LiNbO3 , LN) as a non-linear optical material necessitates a detailed study of its vibrational spectrum. The non-centrosymmetric LN single-crystal is one of the most widely sought-after nonlinear-optical, acousto-optical, and laser materials at the present time [1,2,3,4,5,6]. This ferroelectric is characterized by a deeply defective crystal structure with very high Curie temperature (≈1410 K) and spontaneous polarization (Ps = 5 × 10−5 Cl/cm2 ) [1,2]. Special attention towards semiconductor magnonics is paid in papers [7,8] The defects have a significant effect on many physical properties of a crystal [2,5,9]. LN is a non-stoichiometric phase of variable composition and differs in a wide area of homogeneity on the phase diagram

Methods

Results

Conclusion