Features of increasing the volume concentration of hydrogen in LiNbO$_3$ crystals

The features of increasing the volume concentration of hydrogen in LiNbO$_3$ crystals of congruent composition using a previously proposed method have been studied. After a single treatment of a LiNbO$_3$ crystal of congruent composition in adipic acid and subsequent annealing in humid air at $T$ = 973 K, a satisfactory inhomogeneity of the hydrogen distribution throughout the volume of the sample with dimensions of 5.9 $\times$ 5.8 $\times$ 5.8 mm$^3$, which should significantly improve when the annealing temperature is increased to 1023 K. It has been established that repeated treatments lead to a progressive increase in the volume concentration of hydrogen in the sample, but this process is not linear due to an increase in the reverse diffusion of hydrogen from the surface modified layers into the surrounding air. It has been shown that the value of the volume concentration of hydrogen $c_{\mathrm{Н}}$ affects the structure of the IR absorption spectrum of OH$^-$ groups in LiNbO$_3$ crystals of congruent composition, and with increasing $c_{\mathrm{Н}}$ the probability of localization of H$^+$ ions in undistorted NbNbO$_6$ octahedra increases.