Chemical and physical characterizations of the n = 1 Ruddlesden–Popper phases: Nd2 − y Sr y Ni1 − x Co x O4 ± δ (y = 1 and 0.1 ≤ x ≤ 0.9)

Abstract

The structural stability and physical properties of NdSrNi1 − xCoxO4 ± δ (0.1 ≤ x ≤ 0.9) mixed oxides, elaborated by conventional sol–gel process, have been investigated and obtained results show that substitution of nickel by cobalt at x = 0.5 enhances conductivity at room temperature; σ = 17.24 Ω−1 cm−1 coinciding with minimum activation energy (Ea = 0.05 eV). Rietveld refinements of X-ray powder diffraction patterns at room temperature indicate that all compositions crystallize in a tetragonal system with I4/mmm space group and exhibit K2NiF4-type structure. Variations of a and c parameters display various behavior with increasing cobalt content. Changes in cell parameters are discussed in terms of crystal field theory. In addition, transition metal oxidation state is investigated on the basis of the Brown bond valence calculation. The deduced Global Instability Index (GII) value decreases when cobalt substance increases, indicating that the structure becomes more stable once cobalt is introduced. Oxygen stoichiometry of these compounds was determined from thermogravimetric analyses (TGA) followed by reduction in 5% H2 in N2 gas. Conductivity of NdSrNi1 − xCoxO4 ± δ (0.1 ≤ x ≤ 0.9) oxides was measured by an ac four-probe method. Oxygen vacancies are the possible ionic charge carriers. Specimens exhibit a semiconducting behavior in the whole range of temperature. The electrical transport mechanism agrees with an adiabatic small polaron hopping (ASPH) model. [Figure not available: see fulltext.]. © 2017, Springer-Verlag Berlin Heidelberg.