Investigation of structural perfection of lithium niobate single crystals of different composition and genesis by IR spectroscopy in the area of valent vibrations of hydrogen bonds

We have analyzed complex defects due to the presence of hydrogen bonds in the crystal structure in nominally pure lithium niobate crystals with different Li/Nb ratio, in crystals alloyed with magnesium and zinc in a wide concentration range (LiNbO$_3$ : Mg (0.19–5.91 mol.% MgO) and LiNbO$_3$ : Zn (0.04–6.5 mol.% ZnO)) and in the double-alloyed crystals (LiNbO$_3$ : Y(0.24) : Mg(0.63 wt.%) and LiNbO$_3$ : Gd( 0.25) : Mg(0.75 wt.%)), obtained by technology of direct melt alloying, and also in the double-alloyed crystal (LiNbO$_3$ : Mg(5.05 mol.% MgO) : Fe(0.009 mol.% Fe$_2$O$_3$)) grown from a charge synthesized using the technology of homogeneous alloying with magnesium and iron Nb$_2$O$_5$. We revealed the influence of doping impurities on the concentration of OH-groups, the type and localization of complex defects in the crystal structure. The change in the number of hydrogen atom positions in the structure of the LiNbO$_3$ crystal allow us to judge with sufficient accuracy whether the crystal composition is stoichiometric or congruent. For doped crystals of different compositions data were obtained testifying to changes in the character of complexation of OH-groups with point defects of the cationic sublattice with formation of defects: Mе$_{\mathrm{Li}}$–OH$^-$, Mе$_{\mathrm{Li}}$–Mе$_{\mathrm{Nb}}$–OН. A change in the mechanism of entry of the dopant cation into the structure dramatically affects the change in the properties of the crystal.The difference in the frequencies (and, correspondingly, in the values of the quasi-elastic constants of the O–H bonds) in the spectrum of a congruent crystal and doped crystals can also be contributed by differences in the electronegativity and ionic radii of the principal and doping cations.