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

Abstract

In this study, the Y XX ð ÞY and the two equivalent Z XX ð ÞZ/Z YY ð ÞZ polarized Raman spectra of a LiNbO 3 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 A 1 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 LiNbO 3 LO modes. This analysis highlights the role of the suitable determination of the static and highfrequency 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.

Vibrational Spectra and Electro‐Optic Effect of Ferroelectric LiNbO3Crystal

“…After this adjustment, the 4E (LO) may be assigned to 332 cm −1 as proposed in a recently reported joint experimental and theoretical work. [ 39 ] Then, the remaining big difference is the 370 cm −1 peak assigned to both 6E (TO) and 5E (LO), which may also be questionable. For both Raman and IR inactive A 2 modes, three of the five frequencies were detected by inelastic neutron scattering, [ 58 ] they all show satisfactory agreement with those of present calculation (Table 2).…”

“…Measurements and calculations of vibrational spectrum on LN have been carried out for over 50 years. [ 26–39 ] However, until very recently, they are still in dispute for the assignments of individual modes to the peaks in the observed Raman or IR spectra with different methods or treatments, for example, two of the most recent calculations on LN with the same computing method didn't agree with each other, reflecting the sensitivity of choosing subtle parameters which may result in large difference of the computed vibrational spectra. [ 37,38 ]…”

“…Measurements and calculations of vibrational spectrum on LN have been carried out for over 50 years. [26][27][28][29][30][31][32][33][34][35][36][37][38][39] However, until very recently, they are still in dispute for the assignments of individual modes to the peaks in the observed Raman or IR spectra with different methods or treatments, for example, two of the most recent calculations on LN with the same computing method didn't agree with each other, reflecting the sensitivity of choosing subtle parameters which may result in large difference of the computed vibrational spectra. [37,38] Present study uses CRYSTSAL, a linear combination of atomic orbitals (LCAOs)-based ab initio Hartree-Fock (HF) or DFT methods for systems with periodic translational symmetry, [40] which was proved to be successful in a series of tests on vibrational spectra of different type of minerals or man-made materials.…”

Raman Spectroscopy of Crystalline Materials and Nanostructures

“…The non-destructive nature of Raman spectroscopy further enhances its effectiveness in the characterization of delicate crystalline samples, making it an indispensable tool in material science research. Raman measurements in polarized light provide invaluable information on a crystal's and symmetry when single crystals are available and state-of-the-art modeling of the Raman response is performed [6].…”

Ultrafast Multistage Lattice Strain via Laser‐Excited Phonons in Lithium Niobate

“…[35,36] However, the complex multistage physical processes of LiNbO 3 require theoretical models with increased degrees of freedom and precision to accurately describe its structural properties. [37] To address this, time-dependent ab initio simulation based on real-time timedependent density functional theory (rt-TDDFT) with momentum resolution has been employed to demonstrate the evolution behavior of electrons and atoms. [38,39] This technique has been successfully applied in laser-induced phase-change materials.…”