Structural Distortions and the Metal Insulator Transition in (111) LaNiO₃ Ultrathin Films

LaNiO₃ (LNO) is unique among rare-earth nickelates in that it doesn’t exhibit a temperature dependent metal-insulator transition (MIT) in bulk. A thickness dependent MIT can be seen in (001)-oriented LNO films at ~8 Å thick and has been ascribed to ligand-holes, oxygen vacancies and charge disproportionation. We observe a similar MIT in (111)-oriented LNO on LaAlO₃ in transport measurements, but at a greater critical thickness than in (001) LNO on LAO. Synchrotron x-ray diffraction data and dynamical analysis show an elongation of the out-of-plane lattice parameter to 2.4 Å near the (111) interface. As the film gets thicker, the lattice constant shrinks to 2.25 Å and the film exhibits bulk metallic behavior. This distortion in the [111] direction in the first 8 unit cells is distinct from strain accommodation in (001) films, where oxygen octahedra can more easily rotate to relieve strain. We correlate the insulating behavior to this distortion and an increase in Ni²⁺, measured via x-ray absorption spectroscopy. Together these results imply that metallicity in LNO films is a balance among the charge, lattice and orbital degrees of freedom.