Anomalies in magnetic susceptibility of nonstoichiometric Nd2NiO4+δ (δ=0.049, 0.065, 0.077, 0.234)

We have investigated the influence of oxygen excess on structural and physical properties of the Nd₂NiO_4+δ compounds. Using the citrate method and subsequent annealing in air and in a reducing atmosphere a various oxygen-doped compounds were prepared. X-ray diffraction at room temperature shows that structure is strongly oxygen excess dependent. Thus, by increasing δ by up to 0.077, the compounds adopt a tetragonal structure gradually with a biphasic domain between orthorhombic and tetragonal structures. And at higher δ values, the structure becomes orthorhombic. Moreover, Rietveld analysis shows that for δ<0.077 the presence of two crystalline phases with different oxygen excess: it should be the signature of interstitial oxygen, which is distributed in heterogeneous way. The biphasic products are composed of a stoichiometric Nd₂NiO₄ phase (orthorhombic structure) and a tetragonal Nd₂NiO_4.077 phase. Magnetic susceptibility shows a deviation from Curie-Weiss law for lower oxygen excess (δ?0.077). Moreover, some anomalies in dc magnetic susceptibility curves was observed at 45, 95 and 130 K for δ<0.077. These transitions are connected to the tetragonal phase, and were attributed, respectively, to an antiferromagnetic transition, possible charge ordering and structural transition.