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The assignments of fundamental mode frequencies in LiNbO 3  determined by IR-and Raman scattering-experiments show some characteristic differences. Comparative frustrated total reflection (FTR) -and Raman scattering-studies were carried out in order to find the origin of these discrepancies. The most serious deviation was due to the fact that in one case the ordinary phonon is observed by Raman scattering whereas the extraordinary mode is recorded only by FTR. It has been shown, on the other hand, that the oblique phonon dispersion technique did not provide arguments for the correct assignments. The main limit of the method is due to the fact that the influence of exciton states on polaritons hitherto is taken into account only by a constant high frequency dielectric 'function' (ε ∞ ). The uncertainty in the choice of a suitable ε ∞ -value allows fittings of quite diverging sets of experimentally determined fundamental frequencies.

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Comparative FTR- and Ramanspectroscopic Studies of Fundamental Mode Frequencies in LiNbO 2  and the Present Limit of Oblique Phonon Dispersion Analysis