Investigations on the structural, electrical, and magnetic properties of Nd_2-xSr_xNiO_4+δ

Solid-solutions of the type Nd_2-xSr_xNiO_4+δ (0 ≤ x ≤ 1) were prepared by codecomposition of the metal nitrates and were characterized by means of powder X-ray diffraction, TGA, electrical resistivity, and magnetic susceptibility measurements. Nd₂NiO_4+δ possesses orthorhombic symmetry at ambient temperatures and exhibits a phase transition to pseudo-tetragonal symmetry upon doping with Sr²⁺ (x ≈ 0.2). The tetragonality ratio c a_t measured as a function of x shows an anomalous increase at x ≈ 0.2 and a maximum near x = 0.6. Considerable deviation from oxygen stoichiometries was observed in the orthorhombically distorted members, while for the samples with tetragonal geometry the oxygen contents were near the ideal value. All the samples, except NdSrNiO₄, were semiconducting from room temperature down to 10 K. NdSrNiO₄ showed a metal-semiconductor transition at ∼ 190 K. The magnetic susceptibility shows Curie-Weiss behavior with an onset of long range antiferromagnetic ordering (T_N = 17 K) in some of the intermediate members of the solid-solution series. The effect of Sr²⁺ substitution on the structural and electronic properties of Nd_2-xSr_xNiO_4+δ are discussed in terms of the mixed valence character of Ni²⁺ Ni³⁺, distortions in the local coordination of NiO octahedra, and correlation effects of d_x²-y² bands in the NiO basal plane.