The correlation between experimental polarized Raman spectra and their density functional theory prediction in the LCAO framework: The R3c LiNbO₃ crystal as a test case

Abstract

In this study, the polarized Raman spectra of the R3c LiNbO3 crystal are used as a benchmark test for density functional theory (DFT) full periodic boundary conditions linear combination of atomic orbitals (LCAO) calculation of the Raman tensors, according to the implementation in the CRYSTAL software. The theoretical approach used proved to provide excellent results regarding wavenumbers and relative intensities predictions for the transverse optical modes of both A1 and E symmetry, considerably improving over previously reported data based on the plane waves approach. Overall, the present investigation demonstrates that the LCAO approach, as implemented in the CRYSTAL software, gives excellent results regarding the calculation of Raman tensors and polarized Raman spectra. The possibility to put in correspondence the individual Raman tensors components and bands intensities in the different back‐scattering experimental configurations revealed that the computed Raman tensors are very accurate, not only considering their average values (tensors invariant in the combination suitable for the description of Raman scattering of isotropic materials) but also when the tensors individual components are considered. Based on the present results, a reassignment of the E (TO) modes of the R3c LiNbO3 crystal is proposed, in particular for the 2 E and 9 E modes, which have been a matter of discussion in the recent years.

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

“…The assignment of the experimental peak wavenumbers of TO phonons has been widely discussed in Nogueira et al [16] Table 2 shows a good correspondence between our predicted wavenumbers and the Raman shifts that we determined experimentally. The quality of the agreement is similar to (in some cases) or better than (in other cases) the one obtained in Hermet et al [20] In Table 2, the differences (Δν) between the theoretical and experimental wavenumbers are reported.…”

“…As in the case of TO modes, already discussed in Nogueira et al, [16] we obtained a very good agreement between predicted and experimental vibrational wavenumbers of the LO phonons. Our bands' assignment of LiNbO 3 supports in general the vibrational assignments proposed by Hermet et al [20] On the contrary, some non-negligible differences from Hermet et al [20] appear in the description of TO/LO modes shifts and overlaps.…”

“…The obtained results confirm the reliability of the theoretical approach here adopted for a thorough investigation of the structure and phonons properties of the LiNbO 3 crystal; together with the analysis reported in our previous study, [16] the procedure here illustrated can be considered a benchmark test for simulations of the phonons of crystalline materials.…”

“…In a recent study, [ 16 ] the density functional theory (DFT) has been used with linear combination of atomic orbitals (LAO) to calculate the Raman spectrum of the ferroelectric R3c LiNbO 3 crystal. The relative Raman intensities of the transverse optical (TO) phonons of A 1 and E symmetry, calculated with the CRYSTAL software, [ 17 ] resulted in a remarkable improvement over previously reported calculations.…”

Vibrational Analysis of Paraelectric–Ferroelectric Transition of LiNbO3: An Ab-Initio Quantum Mechanical Treatment

“…The global hybrid B3LYP calculations reproduce accurately the crystallographic data proposed in the literature, with a small underestimation of the cell parameters and the vibrational features [48]. The R3c system is a non-centro-symmetric structure with a large octahedral distortion due to the Jahn-Teller effect.…”