  Oxygen vacancy is intrinsically coupled with magnetic, electronic and
transport properties of transition-metal oxide materials and directly
determines their multifunctionality. Here, we demonstrate reversible control of
oxygen content by post-annealing at temperature lower than 300 degree
centigrade and realize the reversible metal-insulator transition in epitaxial
NdNiO3 films. Importantly, over six orders of magnitude in the resistance
modulation and a large change in optical band gap are demonstrated at room
temperature without destroying the parent framework and changing the p-type
conductive mechanism. Further study revealed that oxygen vacancies stabilized
the insulating phase at room temperature is universal for perovskite nickelates
films. Acting as electron donors, oxygen vacancies not only stabilize the
insulating phase at room temperature, but also induce a large magnetization of
~50 emu/cm3 due to the formation of strongly correlated Ni2+ t2g6eg2 states.
The band gap opening is an order of magnitude larger than that of the thermally
driven metal-insulator transition and continuously tunable. Potential
application of the newly found insulating phase in photovoltaics has been
demonstrated in the nickelates-based heterojunctions. Our discovery opens up
new possibilities for strongly correlated perovskite nickelates.
