Strain-Driven Disproportionation at a Correlated Oxide Metal-Insulator Transition

TL;DR

This study reveals that epitaxial strain induces local disproportionation and oxygen vacancy formation in NdNiO3 thin films, which directly influence the metal-insulator transition by modifying bond angles, valence states, and electronic structure.

Contribution

It demonstrates how epitaxial strain affects oxygen stoichiometry and local structural changes, providing new insights into controlling phase transitions in correlated oxide thin films.

Findings

Increased tensile strain leads to higher resistivity and unit-cell volume.

Epitaxial strain causes reduction in Ni valence and oxygen stoichiometry changes.

Theoretical predictions of bond angle decrease and band gap opening under strain.

Abstract

Metal-to-insulator phase transitions in complex oxide thin films are exciting phenomena which may be useful for device applications, but in many cases the physical mechanism responsible for the transition is not fully understood. Here we demonstrate that epitaxial strain generates local disproportionation of the NiO6 octahedra, driven through changes in the oxygen stoichiometry, and that this directly modifies the metal-to-insulator phase transition in epitaxial (001) NdNiO3 thin films. Theoretically, we predict that the Ni-O-Ni bond angle decreases, while octahedral tilt and local disproportionation of the NiO6 octahedra increases resulting in a small band gap in otherwise metallic system. This is driven by an increase in oxygen vacancy concentration in the rare-earth nickelates with increasing in-plane biaxial tensile strain. Experimentally, we find an increase in pseudocubic unit-cell volume and resistivity with increasing biaxial tensile strain, corroborating our theoretical predictions. With electron energy loss spectroscopy and x-ray absorption, we find a reduction of the Ni valence with increasing tensile strain. These results indicate that epitaxial strain modifies the oxygen stoichiometry of rare-earth perovskite thin films and through this mechanism affect the metal-to-insulator phase transition in these compounds.