The shift of t h o UV-absorption edge was applied to determine the real LiNbO, crystal cornposition. The accuracy of the method is about lo-* iiiole% iisiiig a siriiple comrnercia1 UV-sprctrophotometer. IiNhO, is a typical nonstoichionietric compound, the maximum of the phase diagram curve is shifted from the stoichiornetric 1,i:Nb = 1 ratio to the Kb rich direction (LRRNER et al.). A relatively wide homogeneity range, however, allows to produce crystal sections out of the congruent composition. The crystal quality as well as various physical parameters highly depend on the crystal composition. Both scientific investigations as well as the extended practical application demand controlled high crystal quality. Claim arose therefore to determine the real cr>stal composition in some cases. Chemical analysis and some physical methods were found to/be more or less suitable for this purpose.The chemical analysis presents an absolute method but generaly is not sensitive enough for any of the main components of the host material ( -10-l mol",,). Besides, the chemical processing of LiNbO, is rather complicated (POLGAR et al.).The practical physical methods are more sensitive ( -lop2 rnol(;b) though generally relative and are strongly affected by various impurities. Only some of then1 can be calibrated by sintered powder mixtures. (e. g. : Curie temperature measurement, phase matching temperature measurement.) I n addition these physical methods are rather inconvenient. Curie temperature measurement can be performed over thousand degrees centrigrade (BERGMANN et al.). The phase matching temperature of the second harmonic generation can be measured only by a precision laser optical system. The installation of this method therefore is too expensive. The measurement of the extraordinary refraction index with acceptable accuracy demands very high level of crystal processing (MIDWINTER). The influence of impurities on refractivity has been pointed out recently (BODNAR, YARUNICHEV).A remarkable shift of the intrinsic UV absorption edge was observed between the crystals grown from stoichiometric and congruent melt compositions ( FOLDVLRI et al.; RBDFIELD, BRUKE). This effect can easily be used to determine the crystal conlposition in the range from congruent to stoichiometric cases. Figure 1 shows the shape of the absorption curves of various crystal compositions after correcting for the reflected light from the surfaces. Relatively high absorbance is suggested t o take as reference point for the absorption edge analysis (abs. coeff. = 20 em-l). In this

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