Effects of Off-Stoichiometry in the Epitaxial NdNiO₃ Film on the Suppression of Its Metal-Insulator-Transition Properties

Neodymium nickelate (NdNiO3: NNO), which is a typical strongly correlated metal oxide, has attracted considerable attention because of its large resistance changes due to its metal-insulator transition (MIT). Since MIT in NNO is quite sensitive to the stoichiometry, we precisely fabricated four NNO thin films with different Nd:Ni ratios and comprehensively studied stoichiometry-dependent MIT properties using a temperature-dependent resistance measurement, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and a first-principles density functional calculation. We found that the resistance change in MIT is strongly enhanced when the Ni2+/(Ni2+ + Ni3+) component and estimated oxygen deficiency are sufficiently reduced by optimizing Ni/Nd. The relative change in resistance was sensitive to the Nd:Ni ratio of the NNO thin films, varying from 1 (no transition) to 42 of the resistance change ratio. XPS studies revealed that the cationic deficiency from stoichiometry in NNO films led to the deviation of the Ni valence from 3+. DFT calculations supported that a lack of Ni causes metallicity due to a reduction in the Ni valence from 3+ to 2+ and the collapse of the band gap, followed by oxygen defect introduction, which causes the degradation or absence of MIT properties.