Tuning of oxidation states in the LaNiO3-δ perovskite around the insulator-metal transition

The LaNiO3-delta perovskite has been studied during its reoxygenation process at the reversible insulator-metal transition by spectroscopic ellipsometry for different pressures and temperatures conditions. First, it was demonstrated that the reoxygenation dynamics increases with both increasing pressure and temperature. Considering the temperature dependent experiments, two regimes of kinetics have been identified: a slow reoxygenation dynamics at low temperature (below 523 K) and fast dynamics above 623 K. Second, contrary to our expectations, the reoxygenation process of a preliminary reduced sample is completed after a sufficient time delay even for the smallest investigated temperature of 473 K or oxygen pressure of 0.03 mu bar, respectively. Modeling the change in extinction coefficient as a function of time during the reoxygenation, it was found that the oxygen diffusion coefficient varies from 4.2x10(-14) to 1.1x10(-13) cm(2)/s depending on the temperature (473 to 523 K). This oxygen diffusion constant is similar in magnitude order to those observed in the YBCO superconductor. At the light of these investigations, a pulsed oxygen injection was used, allowing a control of the injected oxygen quantity and by consequence, it was possible to precisely tune the oxidation state for a LaNiO3-delta film between the reversible reduced sample and the stoichiometric compound. In situ ellipsometry measurements are simultaneously performed to follow changes in optical constants.