Manipulating protons and oxygen vacancies in nickelate oxides via thermochemical dehydration

Abstract

Tuning ionic defects, i.e., protons and oxygen vacancies, in perovskite nickelates can lead to the discovery of new physical properties that cannot be achieved through alternative strategies. However, the existing chemical method used for tuning ionic defects, such as topotactical chemical treatment using metal hydrides, often degrades the crystal quality due to the harsh chemical environment used. To tackle this challenge, we developed a thermochemical dehydration method to induce phase transition from protonated H_xNdNiO₃ (H-NNO) to oxygen-deficient NdNiO_3−δ (NNO-δ) at a low temperature of 300 °C. We systematically investigated the change in physical properties during the dehydration process, including the crystal structure, electrical conductivity and Ni valence state. Importantly, through fine-tuning of the dehydration reaction, we further designed a gradient of oxygen vacancy concentration into a single thin film to establish a quantitative correlation between the oxygen vacancy concentration and the lattice constant, Ni valence state, oxygen content, transport property, and optical properties of NNO-δ. Our work offers a new pathway for converting protonic defects to oxygen vacancies and understanding the effect of ionic defects on physical properties in nickelate perovskite oxides.