Experimental and theoretical directional dependence of optical polar phonons in the LiNbO₃ single crystal: New and complete assignment of the normal mode frequencies

Abstract

, Phone/Fax: 00237699491389In this work, the analysis of both experimental and theoretical angular dispersion of the Raman frequencies of the optical polar phonons in LiNbO 3 (LN) single crystal has been determined with the emphasis of the determination without ambiguity of a new and complete set of the normal mode assignments that clarify long-standing debates. The coherency of such couple assignments is assessed by the computed oscillator strengths that are revealed to be in satisfactory agreement with others on the one hand and by the comparison with the oblique frequencies measured from the compilation of the Raman spectra acquired under different geometrical configurations in the 908 scheme on the other hand. Finally, the directional behavior of the quasinormal vectors has been established and discussed with regard to the symmetry mixing of modes or the mode with oblique polarization.

Feasible and Clean Solid-Phase Synthesis of LiNbO3 by Microwave-Induced Combustion and Its Application as Catalyst for Low-Temperature Aniline Oxidation

“…Figure b presents the respective Raman spectrum, which is constituted by a set of relatively wide bands in the frequency range 100–900 cm –1 (at least 16 modes could be visualized). For this material, many previous works studied the vibrational features besides group-theory calculations. − Using the site symmetry and the site occupation factor of each atom of LiNbO 3 , the nuclear-site group method of Rousseau et al., was applied to determine the active phonons at the Brillouin-zone center (Γ). At room temperature, the structure of LiNbO 3 belongs to the R 3 c (#161) space group and rhombohedral 3 m point group, with two molecules per unit cell.…”

“…Accordingly, 18 vibrational modes at zero wavevector are decomposed into 4A 1 + 9E + 5A 2 . Whereas the A 2 phonons are Raman and infrared inactive (silent modes), A 1 and the doubly degenerate E modes are both Raman and infrared active (a total of 13 Raman phonons are expected). − …”

“…Table 3 presents the results from fitting, in which 26 bands could be identified and assigned by using the excellent study of Mai ̈mounatou et al 39 In that work, the authors made a rigorous analysis of both experimental and theoretical angular dispersion of the Raman frequencies of the optical polar phonons in LiNbO 3 single crystal with the emphasis of the determination without ambiguity of a new and complete set of the normal mode assignments that clarify long-standing debates. Our results showed that all the transversal (TO) and longitudinal (LO) modes could be determined from the experimental data, in perfect agreement with the theory calculations, which can help future studies in similar ceramics.…”

Determination of the Chemical Composition of Lithium Niobate Powders

“…The incident radiation then propagates in a plane containing the extraordinary axis, inducing in this way short-range atomic forces (extraordinary refractive index) that compete to long-range atomic forces behind the splitting of longitudinal optic (LO) and transverse optic (TO) phonons [42]. Significant changes in the Raman spectra of LN single crystals, especially in the position of the bands located at 153 cm −1 and 578 cm −1 (red and blue shifts), have already been identified and addressed to the overlapping of the LO and TO lattice vibrations [42][43][44]. Such an overlapping is clearly a drawback for band resolution and it might be the reason behind the discrepancy between predicted and measured values; interestingly, this is only relevant in single crystals of ST composition.…”

Origins of THz Modes and Their IR Intensities in LiNbO3

“…Note that all the following discussions refer to the transverse optical phonon modes; the longitudinal phonon modes are out of the scope because they do not interact with THz radiation in the generation process. The complete set of E and A 1 modes have been extensively studied by Raman spectroscopy, − ,− far-infrared reflective spectroscopy, , and THz spectroscopy. , …”