Investigations on the structural, electrical and magnetic properties of Sr substituted Ln₂NiO₄ (Ln = Pr, Sm, Gd)

Solid solutions of Ln_2-x Sr_xNiO_4±δ (Ln = Pr, Sm) and GdSrNiO₄ were synthesized in air using conventional solid state methods and were characterized by powder X-ray diffraction, TGA, electrical resistivity, DSC, Seebeck coefficient, and magnetic susceptibility measurements. A structural transition from orthorhombic to tetragonal was observed at x ∼ 0.2 for Pr and 0.8 for Sm. For both series of compounds (Ln = Pr, Sm), the tetragonal unit cell parameters a_t and c as functions of x display sharp minima and maxima, respectively, at x ∼ 0.6. The tetragonality ratio, c/a_t, shows a maximum at x ∼ 0.6. All the compounds are semiconducting from room temperature down to 25 K, except SmSrNiO₄ and GdSrNiO₄, which exhibit a degenerate semiconductor to semiconductor transition at ∼292 K and ∼260 K, respectively. Seebeck measurements indicate that electrons are the dominant carriers of conductivity at ambient temperature in all the compounds. The magnetic susceptibility data for all the samples, except Sm_2-xSr_xNiO_4±δ for x = 0.8 and 1.0, follow Curie-Weiss behavior in the temperature range 100-300 K with no evidence of long range magnetic ordering. The effect of Sr²⁺ substitution on the structural, electrical, and magnetic properties of Ln_2-x Sr_xNiO_4±δ (Ln = Pr, Sm) is discussed in terms of the mixed-valent character of Ni^2+/3+ ions, the Ln-O vs Ni-O bond competition along the crystallographic c-axis, the size effect, the local distortion of NiO₆ octahedra, and a configurational transition in the d-manifold or nickel ions.