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

Conductive nickel-cobaltite (NixCo3−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 NixCo3−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 NixCo3−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 NixCo3−xO4 exhibited a linearity between ln(ρ) and $$T ^{- 1/4}$$ below ~120 K, implying a variable-range hopping.