Vibrational Spectra and Electro-Optic Effect of Ferroelectric LiNbO3 Crystal

Though being vitally important in telecommunications and modern technology, the basic optic and especially electro-optic (EO) properties of ferroelectric LiNbO3 crystal (LN) studied from high-accuracy first-principles calculation are still not satisfactory. Herein, the electronic structure and vibrational spectra of LN are calculated by the CRYSTAL program. With a hybrid B3LYP functional, the bandgap of 4.8473 eV, refractive indices of n(o) = 2.2339, n(e) = 2.1440, and nonlinear optical coefficients of d(11) = 2.1 pm V-1, d(31) = -4.35 pm V-1, and d(33) = -27.2 pm V-1 are obtained. It is found that the phonon vibrations are asymmetric along especially the polar axis and the numerically calculated frequencies are strongly dependent on displacement parameters chosen. By averaging displacements to both the positive and negative directions, the obtained vibrational frequencies show excellent agreement to the measured ones except in the range of 320-370 cm(-1) where the assignments of the observed ones are in dispute due to extreme low spectral intensities. Based on the calculated vibrational spectra, the clamped dielectric constants of epsilon 11S=38.62 and epsilon 33S=24.17 and clamped EO coefficients of gamma 33S( )= 27.08 pm V-1, gamma 13S = 9.60 pm V-1, gamma 11S = -5.14 pm V-1, and gamma 51S = 16.90 pm V-1 are obtained.