Microstructure, crystal structure and electrical properties of Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics obtained at different sintering conditions

Details of the formation of Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics under different sintering conditions have been studied by optical microscopy, scanning electron microscopy (SEM), electron probe and energy dispersive spectroscopy (EDX) microanalyses, X-ray diffraction (XRD) and electrical resistivity measurements. Microstructure studies of samples sintered at 1170 deg. C for 1 h indicated the presence of a secondary phase besides the main spinel phase with modified composition. XRD measurements showed that the spinel phase exhibits a tetragonally distorted spinel structure (space group I41/amd, a=5.9410(5) A, c=8.4196(15) A). The secondary phase (solid solution based on NiO) crystallizes with the NaCl-type structure (space group Fm3-bar m, a=4.1872(3) A). The content of the secondary phase in ceramics is 10.61 mass%. For NiMn2O4 ceramics, prepared under the same sintering conditions, the decomposition with Ni1-xMnxO solid solution (NaCl-type structure) and spinel phase formation have been observed. The tetragonal modification of the spinel phase for NiMn2O4 ceramics is more preferable (space group I41/amd, a=5.9764(5) A, c=8.4201(8) A). The distribution of atoms in the structure has been proposed for both ceramics. According to XRD results the Cu0.1Ni0.8Co0.2Mn1.9O4 ceramic samples, sintered at 920 deg. C for 8 h (program 1), at 920 deg. C for 8 h and at 750 deg. C for 24 h (program 2), at 920 deg. C for 8 h, at 1200 deg. C for 1 h and at 920 deg. C for 24 h (program 3) and at 920 deg. C for 8 h, at 1200 deg. C for 1 h, at 920 deg. C for 24 h and at 750 deg. C for 48 h (program 4), contain a single phase with the cubic spinel structure (space group Fd3-bar m). Small residuals of the secondary phase for the ceramics, prepared via programs 3 and 4, have been observed by SEM investigations. The structure transformations of the spinel phase for Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics sintered at 1170 deg. C are attributed to a Jahn-Teller-type distortion due to a compositional change as a result of the secondary phase separation