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

In this work, the analysis of both experimental and theoretical angular dispersion of the Raman frequencies of the optical polar phonons in LiNbO3 (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. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim