Synthesis and nonstoichiometry of magnetoplumbite

Magnetoplumbite has been synthesized by flux-growth from an initial temperature of 1200degreesC 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 PbFe(12)O(19) formula. 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, Pb(2)Fe(11)O(19-delta)square(delta), 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 (Me(2+)) ions, not charge-balanced by Ti(4+) or Sb(5+), increases the degree of nonstoichiometry in magnetoplumbite. This observation is best explained by a substitution of the type Pb(2+) + Me(2+) + square = 2 Fe(3+) + O(2-), which is probably driven by Pb(2+) lone-pair electrons.