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

Abstract

The mean oxidation state of cationic elements in Pr 2 NiO 4+δ was studied using XANES measurements versus temperature in air. Unexpectedly, praseodymium is solely found as Pr 3+ while Pr 4+ was also foreseeable. Pr 3+ cation is well accommodated in the large rare-earth site with the nine-fold coordination in this K 2 NiF 4 -T-type structure; conversely, the small size of the Pr 4+ 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 overstoichiometry of the material evidenced by TGA measurements.

Evaluation of the effect of site substitution of Pr doping in the lithium garnet system Li5La3Nb2O12

“…Each phase has a strong absorption peak noted at 5970 eV, characteristic of Pr 3+ , which matches both the Pr 2 O 3 reference and other Pr 3+ references used elsewhere. 52 When the Pr content is increased to x = 1, only increased absorbance occurs, as expected, but the peak characteristics remain unchanged. Hence, the characterisation data support the conclusion that only Pr 3+ is present, and the different characteristics of the x = 0.5 phase prepared in air are unique to the synthesis method, as seen in the XANES data.…”

“…3) and can be assigned to Pr 4+ . Further support for the assignment of Pr 4+ in the Li 5.5 La 3 Nb 1.5 Pr 0.5 O 12 samples is provided by comparison with Pr 0.5 Ce 0.5 O 2 , a Pr 4+ reference used elsewhere, 52,53 which shows peaks at 5970 and 5980 eV.…”

Using applied pressure to guide materials design: a neutron diffraction study of La2NiO4+δand Pr2NiO4+δ

“…The latter was assumed to be 2+ in the present bond valence sum calculations (but potentially could also be higher as a result of potential small quantities of oxide impurities below the detection limits of the techniques used). 31 In the case of the Ni-O bond valence for both materials we note the difference from 2+ is quite high and suggests that the oxidation state is on average greater than 2+ (as would be suggested by the oxygen excess described above). However, we are unable to confirm the exact value of the oxidation state of nickel, but no change in oxidation state would be expected to be induced with increasing pressure and the overall trends in the bond valence sum should be unaffected.…”

Temperature Programmed Oxygen Desorption and Sorption Processes on Pr2-XLaxNiO4+δ Nickelates

“…According to the available data in literature (6,20), the peaks observed in the temperature range 210 located in Pr2O2 layers (NaCl-type structure) but without any link with the possible lowering of the mean praseodymium charge (which would be between 3+ and 4+), as supposed by Sadykov et al (20). Indeed, the charge is only 3+ in Pr2NiO whatever the oxygen over-stoichiometry in air at T = 25 by XANES studies (21). Combination of experimental and modelling data concerning interstitialcy / interstitial diffusion mechanisms from (20, 22 25) allows the following suggestions: the peaks 1 3, 5, 6, depicted in Fig.…”