Correlated perovskite nickelates with valence variable rare-earth compositions

While the metal to insulator transitions (MIT) of d-band correlated perovskite nickelates (RENiO3) are widely adjustable via their rare-earth composition, the roles of potential valence variabilities associated with the rare-earth elements were rarely concerned. Herein, we demonstrate the material synthesis and MIT properties of RENiO3 containing valence variable rare-earth compositions, such as Ce, Pr, Sm, Eu and Tb. The metastable perovskite structure of SmNiO3 and EuNiO3 with a rare-earth valence states variable towards +2 can be effectively synthesized under high oxygen pressures as it is necessary to reduce their formation free energies. This is in contrast to Ce and Tb, in which situations the variable rare-earth valence state towards +4 reduces their ionic radius and prohibits their occupation or co-occupation of the rare-earth site within the perovskite structured RENiO3. Nevertheless, PrNiO3 with MIT properties can be effectively synthesized at lower oxygen pressures, owing to the higher stability to form a fully occupied 6s orbit associated Pr3+ compared to the half-filled one related to Pr4+. The present work provides guidance for regulating the MIT properties of RENiO3.