Synthesis and nonstoichiometry of magnetoplumbite
published: Feb 7, 2003
Magnetoplumbite has been synthesized by flux-growth from an initial temperature of 1200Â°C using PbO as a solvent. Characterization was made by electron-microprobe analyses, optical reflectance spectroscopy and X-ray diffractometry. The product has a higher Pb/Fe ratio than indicated by the ideal PbFe12O19formula. A structural refinement carried out shows that the deviation from stoichiometry is related to partial replacement by Pb at the trigonal bipyramidal Fe sites. This corresponds to a shift in composition towards plumboferrite, Pb2Fe11O19-Î´□Î´, which is accompanied by an increase in unit-cell volume; the most pronounced change is in the c parameter. Partial substitution of Fe by Mn, Zn, Ti, and Sb also influences the lattice dimensions of magnetoplumbite significantly, and a marked depression of the specular reflectance in the visible spectral region is observed. The incorporation of divalent (Me2+) ions, not charge-balanced by Ti4+ or Sb5+, increases the degree of nonstoichiometry in magnetoplumbite. This observation is best explained by a substitution of the type Pb2+ + Me2+ + □ = 2 Fe3+ + O2-, which is probably driven by Pb2+ lone-pair electrons.