Nanocrystalline nickel ferrite and copper nickel ferrite materials were synthesized through surfactant Tween 80-assisted hydrothermal reaction at low temperature using metallic copper, nickel, and iron as raw materials. Thermogravimetric (TG) and differential thermal analysis (DTA) were used to determine the decomposition of the precursors. Phase composition, microstructural characterization, and distribution of cations in tetrahedral and octahedral sites of crystal structure properties of synthesized ferrite materials were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) absorption, and Mössbauer spectroscopy. The impedance spectroscopic studies of copper nickel ferrite sintered ceramics were carried out at room temperature. TG and DTA of precursors revealed that the crystallization temperature for spinel ferrites phases formation was around 650 °C which were being synthesized through Tween 80-assisted hydrothermal process in highly basic reaction at 180–200 °C for 11–13 h in PTFE-lined stainless steel autoclave. XRD analysis and Rietveld refinement study confirm the formation of single-phase cubic spinel nickel ferrite (NiFe2O4) and copper nickel ferrite (Ni0.5Cu0.5Fe2O4) with cell parameters 8.3385, 8.3595 Å and space group Fd3 m, respectively. The synthesized ferrite products showed extensive XRD line broadening, and the average crystallite sizes of NiFe2O4 and Cu0.5Ni0.5Fe2O4 ferrites were in range of 21–29 nm and 18–23 nm, respectively. Mössbauer parameters are characteristic of substituted Cu-Ni-Fe2O4 ferrite material. Isothermal shrinkage characteristic and coefficient of thermal expansion were determined by dilatometry. The ferrite specimens showed excellent densification at 1,050 °C temperature, and uniformly fine grain-sintered ceramics with submicron grain size (10–18 μm) were obtained after sintering at 850 and 1,050 °C. Sintering at 1,050 °C, results in the formation of depletion layer at grain boundaries which act as trapping centers for the carriers and an increase in the impedance values are conferred.
