Tailoring the breathing-mode distortions in nickelate/ferroelectric heterostructures

In transition metal oxides, electron–electron interaction and lattice degree of freedom are basic ingredients of emergent phenomena, such as metal-to-insulator transition (MIT) and superconductivity. Perovskite rare-earth nickelates are largely studied for their temperature-driven MIT, which is accompanied by a breathing-mode distortion and associated with a bond-disproportionation of the expanded (3d8L0) and compressed (3d8L2) NiO6 octahedra. Steric effects control the onset temperature of the MIT, the latter being concomitant or not with a complex antiferromagnetic spin arrangement depending upon the choice of the rare-earth ion (TMIT ≥ TNéel). Interface engineering of oxygen octahedra tilting, as imposed by the symmetry and orientation of the substrate, has resulted in an efficient pathway to modify both TMIT and TNéel, hence suggesting a key role of the electron–phonon coupling for both transport and magnetic properties in nickelate thin films. Here, via a combination of resonant elastic x-ray scattering and transport experiments, we show control over both TMIT and TNéel in heteroepitaxial PbZr0.2Ti0.8O3(d)/NdNiO3(7 nm)//SrTiO3 heterostructures, which are characterized by different strains and polarization states of the PbZr0.2Ti0.8O3 layer grown at different thicknesses d. We found the expected NdNiO3 bulk behavior (TMIT = TNéel), for a fully relaxed PbZr0.2Ti0.8O3 layer showing a monodomain polarization state. On the other side, an almost 30 K difference (TMIT > TNéel), is found for a fully strained PbZr0.2Ti0.8O3 characterized by a multidomain texture of the polarization state. We discuss our results in terms of an altered breathing distortion pattern of the underlying nickelate layer as supported by x-ray absorption spectroscopy measurements. We infer that locally different polar distortions controlled by a combination of polarization direction and strength of the strain state play the main role in the observed TMIT and TNéel variations.