Intrinsic stacking fault of the ilmenite type in the structure of lithium niobate crystals of various compositions

X-ray diffraction (XRD) patterns of lithium niobate crystals doped with zinc (Zn2+) and crystals double doped with cations (Zn2+:Mg2+) as well as the spectrum of a nominally pure one were studied. Doping was carried out using a method of direct and homogeneous introduction of impurities into a charge. It was found that in all XRD patterns of the studied samples in the range of scattering angles from 11° to 22° and from 25° to 31° weak additional reflections, which are forbidden by the symmetry space group R3c, appeared. We established that the appearance of weak reflections was due to the ordering of intrinsic extended superstructural defects. In the double cell (a' = 2a, c' = c) an ilmenite type defect was formed. The calculated concentration of these defects was about 5% over the entire structure.