The effect of stoichiometry on the structural, thermal and electronic properties of thermally decomposed nickel oxide

Abstract

A thermal decomposition route with different sintering temperatures was employed to prepare non-stoichiometric nickel oxide (Ni_1−δO) from Ni(NO₃)₂·6H₂O as a precursor. The non-stoichiometry of samples was then studied chemically by iodometric titration, wherein the concentration of Ni³⁺ determined by chemical analysis, which is increasing with increasing excess of oxygen or reducing the sintering temperature from the stoichiometric NiO; it decreases as sintering temperature increases. These results were corroborated by the excess oxygen obtained from the thermo-gravimetric analysis (TGA). X-ray diffraction (XRD) and Fourier transformed infrared (FTIR) techniques indicate the crystalline nature, Ni–O bond vibrations and cubic structural phase of Ni_1−δO. The change in oxidation state of nickel from Ni³⁺ to Ni²⁺ were seen in the X-ray photoelectron spectroscopy (XPS) analysis and found to be completely saturated in Ni²⁺ as the sintering temperature reaches 700 °C. This analysis accounts for the implication of non-stoichiometric on the magnetization data, which indicate a shift in antiferromagnetic ordering temperature (T_N) due to associated increased magnetic disorder. A sharp transition in the specific heat capacity at T_N and a shift towards lower temperature are also evidenced with respect to the non-stoichiometry of the system.