Sol–gel synthesis, crystal structure, electronic properties and magnetic studies of B_2+xAs_xCo_4−3xO₇ (0.0 ≤ x ≤ 0.75) composites

Synthesis of B_2+xAs_xCo_4−3xO₇ (S1–S4: x = 0.0, 0.25, 0.50 and 0.75) composite oxides were performed by sol–gel method. The powder X-ray diffraction pattern and Reitveld refinement results revels that the samples are formed monoclinic phase with Z = 2 and space group P₂₁/m. Average crystallite size of the samples determined by Scherrer’s relation are found to be ~28–50 nm. The observed and calculated density values are determined and compared. Thermogram shows no phase transition in the range of 50–1,000 °C. The scanning electron micrographs show the morphology of the samples which are observed, the crystallites are rod like shape. The purity and the quantitative analysis were examined by the energy dispersive X-ray. The B–O and Co–O bonds of different sites show marginal variation in the samples, the circular valence charge density contour map of the Co and O in S1–S4 show partial covalent nature of Co–O. Based on the plane-wave density functional theory calculations on crystal structure for band structure and density of states of sample S1–S4 using CASTEP programme package show all the samples are conductor with no band gap. The magnetic moment plot in the range ±10 kG indicates the weak ferromagnetic behavior of the samples. The electron paramagnetic resonance line shapes of all four (S1–S4) samples are isotropic, Diffuse reflectance spectra of sample S1–S4 at room temperature show the band around 273 nm is ligand to metal (O²⁻ → Co²⁺) charge transfer transition and d–d transition around 570 nm, respectively.