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

CeNiO3 has been reported in the literature in the last few years as anovel LnNiO3 compound with promising applications in different catalyticfields, but its structure has not been correctly reported so far. Inthis research, CeNiO3 (RB1), CeO2 and NiO have been synthesized in ananocrystalline form using a modified citrate aqueous sol-gel route. Adirect comparison between the equimolar physical mixture (n(CeO2) :n(NiO) = 1 : 1) and compound RB1 was made. Their structural differenceswere investigated by laboratory powder X-ray diffraction (PXRD),selected area electron diffraction (SAED), transmission electronmicroscopy (TEM) with an energy-dispersive X-ray spectroscopy (EDS)detector, and Raman spectroscopy. The surface of the compounds wasanalyzed by X-ray photoelectron spectroscopy (XPS), while the thermalbehaviour was explored by thermogravimetric analysis (TGA). Theirmagnetic properties were also investigated with the aim of exploring thedifferences between these two compounds. There were clear differencesbetween the physical mixture of CeO2 + NiO and RB1 presented by all ofthese employed methods. Synchrotron methods, such as atomic pairdistribution function analysis (PDF), X-ray absorption near edgestructure (XANES) and extended X-ray absorption fine structure (EXAFS),were used to explore the structure of RB1 in more detail. Threedifferent models for the structural solution of RB1 were proposed. Onestructural solution proposes that RB1 is a single-phase pyrochlorecompound (Ce2Ni2O7) while the other two solutions suggest that RB1 is atwo-phase system of either CeO2 + NiO or Ce1-xNixO2 and NiO.Preparation of CeNiO3 in order to elucidate the actual crystal structurewith the aim of establishing a structure-property relationship. Threedifferent structural models were proposed accompanied by detailedstructural analysis.