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 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.