Thickness Dependent Metal-Insulator Transition in (111) LaNiO₃ Thin Films

LaNiO₃ is the only metallic rare earth nickelate, but a metal-insulator transition can be induced as a function of thickness in LaNiO₃ films. Some proposed mechanisms for this behavior include charge disproportionation, symmetry changes, and order-disorder transition. To better understand this metal-insulator transition, we synthesized (111)-oriented LaNiO₃ films of various thicknesses on (111)-oriented LaAlO₃ substrates in layer-by-layer mode by interval pulsed laser deposition. In contrast to the 2-4 unit cell transition thickness seen in most (001) LaNiO₃ films studied thus far, we have found the onset of insulating behavior in (111) LaNiO₃ to occur at 6 unit cells. Atomic force microscopy shows typical rms roughness of < 0.2 nm and X-ray diffraction confirms excellent crystallinity. X-ray absorption spectroscopy indicates the presence of Ni²⁺ in the thinnest insulating films while thicker metallic films are primarily Ni³⁺. The correlation of Ni²⁺ with insulating behavior in (111) and (001) films suggests charge disproportionation might play a significant role in the metal-insulator transition. The tilts and deformations of the NiO₆ octahedra in the strained (111) films likely also affect the onset of the metal-insulator transition.