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

Abstract

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 substrate's 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.