A nearly linear decrease in the spectral vision of the UV absorption edge of the gradient \(LiNbO_3:Er^{3+}:Yb^{3+}\) crystal along the ferroelectric axis was detected. The second-order IR absorption spectra of the gradient \(LiNbO_3:Er^{3+}:Yb^{3+}\) crystal in the wavenumber range of 2000–3000 cm−1 have been recorded for the first time. The appearance of absorption lines in the specified spectral range is typical of defective and highly anharmonic crystals. Based on the IR absorption spectra in the region of stretching vibrations of the \(OH^-\) -groups, it has been found that the oxygen-octahedral clusters \(MeO_6\) (Me– \(Li^+\) , \(Nb^{5+}\) , vacant octahedron V, impurity ions \(Er^{3+}\) and \(Yb^{3+}\) ) of the structure of the gradient \(LiNbO_3:Er^{3+}:Yb^{3+}\) crystal are practically not distorted. It has been shown that the volume concentration of \(OH^-\) -groups in the gradient \(LiNbO_3:Er^{3+}:Yb^{3+}\) crystal is several times less than in compositionally uniform congruent and stoichiometric lithium niobate crystals. The obtained results are significant for the complex analysis of the nonlinear materials.
