Cationic structure and charge transport in sol–gel-derived nickel-cobaltite thin films

Abstract

Conductive nickel-cobaltite (Ni_xCo_3−xO₄) thin films were synthesized on Al₂O₃ substrates by using a sol–gel process for varying Ni composition x (≤ 1.0). X-ray diffraction data indicated that the polycrystalline Ni_xCo_3−xO₄ samples have cubic spinel (Fd$\bar{3}$m) structure with the lattice constant increasing slightly with increasing Ni composition (by 0.5 % for x = 1.0 compared to that of Co₃O₄). X-ray photoelectron spectroscopy data indicated that the Ni ions in nickel-cobaltite have multi-valence (Ni²⁺ and Ni³⁺). The temperature-dependent electrical resistivity (ρ) data of the nickel-cobaltite samples were measured in the 4–300 K range in comparison with Co₃O₄. At all temperatures (T), the resistivity of Ni_xCo_3−xO₄ decreased gradually with increasing x down to ρ = 1.9 × 10⁻² Ω cm for x = 1.0 at 300 K (reduction by a factor of ~10⁻³ from that of Co₃O₄). The decrease in ρ by the Ni doping in Co₃O₄ can be understood in terms of the polaronic d-electron hopping between octahedral Ni²⁺ and Ni³⁺ ions in the spinel lattice. The analysis on ρ(T) for Ni_xCo_3−xO₄ exhibited a linearity between ln(ρ) and $T^{ - 1/4}$ below ~120 K, implying a variable-range hopping.

Graphical Abstract