A new compound Er0.33Sr1.67Ni0.8Cu0.2O4−δ (ErSr5Ni2.4Cu0.6O11) was prepared using the conventional solid state method and annealed at 1423 K in 1 atm of oxygen gas flow. The oxygen non-stoichiometry (δ = 0.47) was determined by iodometric titration. Rietveld refinement using powder X-ray diffraction data confirms that the sample adopts the K2NiF4-type structure (space group I4/mmm (Z = 2), a = 3.760 56(4) and c = 12.3889(1) Ǻ). The final reliability factors were: Rwp = 10.75%, χ2 = 2.51, Rp = 14.80%, RB = 4.77% and RF = 2.73%. Four probe electrical resistivity measurements were performed vs. temperature in the range of 320–540 K. A semiconducting behaviour over the whole range of temperature, with a maximum conductivity of 0.026 S cm−1 is observed at 439 K.

The Nd1.7Ba0.3Ni0.9Cr0.1O4+δ polycrystalline sample was synthesized by the sol–gel process and a subsequent annealing at 1523 K in 1 atm of flowing argon. X-ray diffraction (XRD) analysis and electrical transport properties have been investigated as well. The oxygen non-stoichiometry was determined by iodometric titration. The sample shows adoption of the K2NiF4-type structure based on a tolerance factor calculation. Rietveld refinement of the crystal structure from X-ray powder diffraction data confirmed that Nd1.7Ba0.3Ni0.9Cr0.1O4+δ adopts the tetragonal structure (space group I4/mmm, Z = 2). The room temperature unit-cell parameters are determined to be a = 3.82515(2) and c = 12.47528(6) Å. The reliability factors are: RB = 0.043, Rwp = 0.012 and χ2 = 3.00. The Nd1.7Ba0.3Ni0.9Cr0.1O4+δ compound exhibits a semi-conductive behaviour. The electrical transport mechanism has been investigated and it agrees with the adiabatic small polaron hopping model in the temperature range 313 K ≤ T ≤ 708 K.