Investigation of microstructure in ferroelectric lead-free La2Ti2O7 thin film grown on (001)-SrTiO3 substrate

Abstract

(012)-Oriented La₂Ti₂O₇ (LTO) thin films with perovskite layered/monoclinic structure (a = 7.825 Å, b = 5.532 Å, c = 12.938 Å, β = 98.0°) have been deposited on (001)-oriented doped Nb:SrTiO₃ (STO) substrates by a sol–gel method coupled to the spin-coating technique. Rocking curves measurements evidence the existence of four crystallographic domains in these films, the (012)_LTO planes for each domain being slightly disoriented from 2.3° with respect to the plane of the substrate, as confirmed by reciprocal space map performed around the (002) reflection of the substrate. Pole figure measurements permit the conclusion that the crystallographic relationships between the film and substrate are [001]_STO//[100]_LTO, [010]_STO//[01]_LTO and (100)_STO//(012)_LTO. Using high-resolution X-ray diffraction, the (025)_LTO-reflection can be distinguished on 2θ/ω-scan pattern recorded in parallel beam geometry. As highlighted by both rocking-curve measurements and reciprocal space map recording, the (025)_LTO planes for each of the four populations are disoriented from 4.1° with respect to the plane of the substrate. As demonstrated, this angle's value is a signature of the existence of the fourfold disoriented crystalline domain structure. From piezoresponse force microscopy experiments, intermediate directions of the polarization vectors between the normal and the plane of the substrate are revealed. On the other hand, piezoloops recorded on nanoscale unambiguously confirm the ferroelectric behavior of these films.