Synthesis and nonstoichiometry of magnetoplumbite

Abstract

Magnetoplumbite has been synthesized by flux-growth from an initial temperature of 1200°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 PbFe 1 2 O 1 9 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 1 1 O 1 9 - Φ □ Φ , 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 + + □ = 2 Fe 3 + + O 2 - , which is probably driven by Pb 2 + lone-pair electrons.