Resolving a structural issue in cerium-nickel-based oxide: a single compound or a two-phase system?

Abstract

CeNiO₃ has been reported in the literature in the last few years as a novel LnNiO₃ compound with promising applications in different catalytic fields, but its structure has not been correctly reported so far. In this research, CeNiO₃ (RB1), CeO₂ and NiO have been synthesized in a nanocrystalline form using a modified citrate aqueous sol–gel route. A direct comparison between the equimolar physical mixture (n(CeO₂) : n(NiO) = 1 : 1) and compound RB1 was made. Their structural differences were investigated by laboratory powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS) detector, and Raman spectroscopy. The surface of the compounds was analyzed by X-ray photoelectron spectroscopy (XPS), while the thermal behaviour was explored by thermogravimetric analysis (TGA). Their magnetic properties were also investigated with the aim of exploring the differences between these two compounds. There were clear differences between the physical mixture of CeO₂ + NiO and RB1 presented by all of these employed methods. Synchrotron methods, such as atomic pair distribution function analysis (PDF), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were used to explore the structure of RB1 in more detail. Three different models for the structural solution of RB1 were proposed. One structural solution proposes that RB1 is a single-phase pyrochlore compound (Ce₂Ni₂O₇) while the other two solutions suggest that RB1 is a two-phase system of either CeO₂ + NiO or Ce_1−xNi_xO₂ and NiO.