Atomic force microscopy studies of lithium niobate single crystals containing heterogeneously distributed lanthanide (Gd) admixture and a regular domain structure of 100 nm to 1 µm steps obtained under conditions of severe thermal instability have revealed fractal structures of the size of 10 to 100 nm within regions of the regular domain structures. A super-structure of clustered defects with 1-2 nm steps explaining results of Raman spectra analysis is supposed to exist in the cation sub-lattice and formation of periodic fractal structures of the size of ∼1 nm-100 µm is suggested to take place in lithium niobate single crystals containing lanthanide admixture.