Zhenmian Shao

Evidence of ferroelectricity in metastable Sm2Ti2O7 thin film

The synthesis by the sol–gel route of metastable Sm2Ti2O7 thin film is reported. While for standard conditions of pressure and temperature, the Sm2Ti2O7 compound crystallizes in a cubic/pyrochlore structure, in the form of a thin film (grown on (110)-oriented SrTiO3 substrate), a monoclinic/layered perovskite structure is stabilized by strain effects induced by the substrate. Under these conditions, (00l)-oriented Sm2Ti2O7 oxide is isostructural to ferroelectric Ln2Ti2O7 (with Ln = La, Ce, Pr and Nd) phases, which is already well known. Studies by Selected Area Electron Diffraction (SAED) reveal the existence of a systematically twinned structure and the presence of a fault in the stacking of the perovskite layers. Piezoresponse Force Microscopy (PFM) measurements confirm undoubtedly the ferroelectricity in this metastable phase.

Ferroelectricity in La2Zr2O7 thin films with a frustrated pyrochlore-type structure

Thin films of lanthanum zirconate (La2Zr2O7) have been grown by a sol–gel route on (110)-oriented SrTiO3 substrates. Electrical measurements, locally performed by piezoresponse force microscopy, evidence unambiguously the ferroelectric state of the films at the nanoscale level. In the La2Zr2O7 bulk material, ferroelectricity is absent due to the centro-symmetric cubic pyrochlore structure. In thin films, the extensive study carried out by high resolution X-ray diffraction highlights a lowering of the cubic symmetry that may explain the emergence of ferroelectricity. This slight structural modification is interpreted as a geometrical frustration induced by the substrates strains during the film growth. In addition, pole figure experiments are used to give epitaxial relationships between the La2Zr2O7 film and the SrTiO3 substrate. Finally, high resolution transmission electron microscopy images obtained on a cross-section of the film are given.

Stability limit of the layered-perovskite structure in Ln2Ti2O7 (Ln = lanthanide) thin films grown on (110)-oriented SrTiO3 substrates by the sol–gel route

The stability limit between the layered-perovskite and pyrochlore structures was investigated for Ln2Ti2O7 (Ln = lanthanide) thin films grown on (110)-oriented SrTiO3 substrate by the sol–gel route. In bulk, for standard conditions of pressure and temperature, these oxides adopt the layered-perovskite structure for Ln = La to Nd while they crystallize in the pyrochlore one for Sm to Lu. When grown in thin films on (110)-oriented SrTiO3 substrates, the frontier separating these two structures is shifted. High-resolution X-ray diffraction measurements evidence the stabilization of a metastable (00l) oriented layered-perovskite structure for Sm2Ti2O7, Eu2Ti2O7 and Gd2Ti2O7. Such stabilization is explained by a combination of both the strains induced by the substrate in the film and the good lattice match between the substrate and the metastable layered-perovskite oxide. The stabilization of layered-perovskite structures in thin films opens interesting perspectives in terms of development of new ferroelectric/piezoelectric devices, keeping in mind that the isostructural La2Ti2O7 and Nd2Ti2O7 possess a very high Curie temperature (Tc ≈ 1500 °C). Finally, the role played by the lanthanide precursor (nitrate or chloride) in the stabilization of the layered-perovskite structure is highlighted and prediction for stabilization of other new layered-perovskite compounds in the Ln2B2O7 (B = Zr, Mn) system for thin films deposited on the (110)-oriented SrTiO3 substrate is reported.

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

(012)-Oriented La2Ti2O7 (LTO) thin films with perovskite layered/monoclinic structure (a = 7.825 A, b = 5.532 A, c = 12.938 A, 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 angles 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.

Fabrication of La 2 Ti 2 O 7 nanostructures by focused Ga 3+ ion beam and characterization by piezoresponse force microscopy

(012)-oriented lead-free La₂Ti₂O₇ thin films with the monoclinic/perovskite layered structure have been grown by a solgel route on (100)-oriented doped Nb:SrTiO₃ substrates. On nanoscale, both poling experiments performed via the tip of atomic force microscope and the existence of local piezoloops within the domains confirm the piezo-/ferro-electric behaviour of the films. Islands in the lateral range 300-500 nm have been fabricated by focused Ga³⁺ ion beam etching on platinum top electrode. As measured on piezoloops, electromechanical activity within the islands is shown to be similar to the one obtained for the virgin film; no piezoelectric degradation for La₂Ti₂O₇ islands is highlighted. These results confirm that La₂Ti₂O₇ is a highly resistant oxide to ion-beam irradiation. La₂Ti₂O₇ could be considered as a material of choice for the realization of lead-free piezoelectric nanostructures.