Investigation on the structural and electrical properties of NdSrNi1-xCrxO4+δ (0.1≤x≤0.9) system

The phases NdSrNi1-xCrxO4+δ (0.1≤x≤0.9) have been synthesized by modified sol-gel method and subsequent annealing at 1250 deg. C in 1 atm of flowing argon. X-ray diffraction (XRD) analysis and electrical resistivity have been measured at room temperature. Rietveld refinement shows that all compositions with x>0.1 were found to crystallize in the tetragonal K2NiF4 type structure in the space group I4/mmm, while for x=0.1, a mixture of two phases with the tetragonal space group I4/mmm and the orthorhombic space group Fmmm. Variations of a and c parameters show a complex behavior with increasing chromium content. It was established that compounds with chromium content less then x≤0.5 are oxygen-deficient, while for x>0.5 the sample are oxygen-overstoichiometric. The NdSrNi0.5Cr0.5O4+δ compound exhibits semiconductive behavior and the electrical transport mechanism agrees with the non-adiabatic small polaron hopping model in the temperature ranges 298-493, 493-573 and 573-703 K separately. - Graphical abstract: The X-ray pattern of the x=0.1 sample showed a shift of some Bragg reflections that was indicative of a unit cell of lower symmetry. Two distortions of the I4/mmm K2NiF4 aristotype cell to orthorhombic symmetry are known, Fmmm and I mmm, depending on which of the <1 0 0> and <1 1 0> sets of mirror planes and twofold axes are lost together with the fourfold axis when the symmetry is lowered