In-situ study of cationic oxidation states in Pr2NiO4+δ using X-ray absorption near-edge spectroscopy

[Display omitted] •The cations oxidation states in Pr2NiO4+d were measured vs. T° for the first time by XANES.•Praseodymium was solely found as Pr3+ whatever the temperature, while Pr4+ was also expected.•The correlated Ni oxidation state and oxygen stoichiometry progressively decrease when T° increases. The mean oxidation state of cationic elements in Pr2NiO4+δ was studied using XANES measurements versus temperature in air. Unexpectedly, praseodymium is solely found as Pr3+ while Pr4+ was also foreseeable. Pr3+ cation is well accommodated in the large rare-earth site with the nine-fold coordination in this K2NiF4 – T-type structure; conversely, the small size of the Pr4+ cation seems not compatible with this site. Moreover, XANES measurements show a progressive decrease of the mean oxidation state of nickel ions when the temperature increases. It is directly correlated to the decrease of the oxygen over-stoichiometry of the material evidenced by TGA measurements.