Raman activity of the longitudinal optical phonons of the LiNbO3 crystal: Experimental determination and quantum mechanical simulation

• Published in: Journal of Raman spectroscopy 2022-11, Vol.53 (11), p.1904-1914
• Main Authors: A. Nogueira, Bernardo, Rérat, Michel, Fausto, Rui, Castiglioni, Chiara, Dovesi, Roberto
• Format: Article
• Language: English
• Subjects: Boundary conditions; CRYSTAL software; Density functional theory; DFT simulations; Lithium niobates; LO modes; Phonons; polarized Raman; Quantum mechanics; Raman activity; Raman spectra; Raman spectroscopy; Single crystals; Tensors
• ISSN: 0377-0486; 1097-4555
• DOI: 10.1002/jrs.6426

In this study, the YXXY¯ and the two equivalent ZXXZ¯/ ZYYZ¯ polarized Raman spectra of a LiNbO3 single crystal have been recorded and used as a benchmark test for the density functional theory (DFT) calculation of the longitudinal modes and of their Raman activity. The theoretical approach, based on periodic boundary conditions and a linear combination of atomic orbitals (LCAO), provides excellent predictions of phonon wavenumbers and relative bands intensities for both A1 and E Longitudinal Optical (LO) modes and complements a previous paper limited to the study of Transverse Optical (TO) modes. Overall, the present investigation demonstrates that the LCAO approach, as implemented in the CRYSTAL software, gives results of similar accuracy for the TO and the LO phonons features. By means of a band deconvolution scheme applied to the experimental spectra, we present, for the first time, a quantitative comparison between experimental and theoretically predicted polarized Raman band intensities of LiNbO3 LO modes. This analysis highlights the role of the suitable determination of the static and high‐frequency dielectric matrices that are needed for the prediction of the TO/LO frequency split but also the first nonlinear electric susceptibility tensor for an accurate description of the Raman intensity pattern. A joint theoretical and experimental study of the Optical modes of the R3c LiNbO3 crystal demonstrates that an excellent agreement can be achieved of both the Transverse Optical (TO) and the Longitudinal Optical (LO) wavenumbers and intensities. The results highlight the role of the static and high‐frequency dielectric matrices, in the prediction of the TO/LO frequency split, and of the first nonlinear electric susceptibility tensor, for an accurate description of the Raman intensity pattern.