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

Abstract

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 (T_c ≈ 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.