Research of physicochemical properties and structure of strongly doped LiNbO₃:ZnO ([ZnO] ~ 4.02–8.91 mol %) crystals

In LiNbO₃:ZnО crystals grown from melts containing ~4.0–9.0 mol % ZnO, the evolution of structure was studied, the lattice periods were detected, and the models of atomic structure were analyzed. Raman spectroscopy and full-profile analysis of XRD patterns were used to obtain the results. The rise in ZnO concentration in the melt from 4.0 to ~6.99 mol % leads to the decrease in the unit cell volume of the crystals. The further rise in ZnO concentration in the melt to ~7.8 mol % results in the increase in the unit cell volume of the crystals. Refinement of the profile characteristics of XRD patterns and structural characteristics of the researched crystal samples was performed by the Rietveld method using the PDWin software package. All samples were considered single-phase, taking into consideration the absence of intense bands coinciding with the lithium niobate bands. Zn²⁺ cations were detected to occupy Li sites (which are vacant in the congruent LiNbO₃ crystal). In this case, Nb vacancies are absent. Zinc displaces all excess Nb atoms in Li sites. In the sample grown from melt with ~6.12 mol % ZnO. In this case, Li vacancies and electroneutrality stay the same in the sample. Thus, NbLi antistructure defects are absent in the LiNbO₃:ZnО crystal (~6.12 mol % ZnО in the melt). In LiNbO₃:ZnО crystals with higher ZnO concentration, NbLi antistructure defects appear again. In this case, the concentration of Zn in Li sites almost coincides with the one in the LiNbO₃:ZnО crystal. Unit cell parameters are equal in crystals grown from melts with ~7.8 and ~6.76 mol %. The biggest changes in the Raman spectra of these crystals are observed in the regions of vibrations of cations (200–300 cm^–1) located in oxygen octahedra ВО6 (В: Nb⁵⁺, Li⁺, Zn²⁺) and in the region of vibrations of oxygen atoms in oxygen octahedra (500–900 cm^–1). This indicates a change in order of alteration of intrinsic, doping cations and vacancies along the polar axis and deformation of oxygen octahedra during doping. This corresponds to anisotropic expansion of oxygen octahedra along the polar axis in LiNbO₃:ZnО crystals.