Sol-Gel Synthesis, Crystal Structure and Electronic Properties in Bi2+xAlxNi4-3xO7+δ (0.0 ≤ x ≤ 0.75) Composite Oxides

Sol-gel synthesis of Bi2+xAlxNi4-3xO7+δ (A1-A4: x = 0.0, 0.25, 0.50, 0.75) composite oxides were performed via nitrate-citrate route. Analysis of the powder XRD patterns by Fullprof shows cubic unit cell with lattice parameters in A1-A4: a = 10.2678, 10.1197, 10.1183, 10.1134 Å; Z= 4 and space group Pm3n. Average crystallite sizes in A1-A4 determined by Scherrer’s relation are found to be in the range ~ 42-61 nm. The differential thermal analysis (DTA)/ thermogravimetric (TG) results show no phase transitions in the range 25-800 ºC. The morphology of the samples characterized by scanning electron microscopy (SEM) show similar results with respect to the particle sizes and shapes. Rietveld refinement of the unit cell structure shows appreciably lowered agreement factors Rp, Rwp and Rexp for all the samples. The Bi-O and Ni-O bonds of different sites show marginal variations in the samples. Circular electron density contours around Ni and O due to valence orbitals shows the partial ionic character of Ni-O bonds. The plane-wave Density Functional Theory (DFT) calculations on crystal of structure of A1-A4 for band structure and density of states (DOS) using a CASTEP programme package show the valence band (VB) located at -8.0 to ~ 0.0 eV (Fermi level) predominantly comprised of O 2p orbitals overlapping with in-phase with Ni t2g, and the Bi 6s orbitals to form Ni-O-Bi bonds, and O 2p orbitals overlapping with Ni t2g or Bi 6s forming O-Ni-O or Bi-O-Bi bonds. The conduction band (CB) is composed of Ni eg orbitals, which narrows down with complete smearing of ~ 2.0 to 3.0 eV CB as in A1 in A2-A4 due to the absence of Al3+ d orbitals. This result shows the transition of semiconductor-like as in A1 to metallic character with the Ni eg band predominantly crossing the Fermi level, in A2-A4.