An Electron and X-Ray Powder Diffraction Study of the Defect Fluorite Structure of Mn0.6Ta0.4O1.65

The oxide Mn0.6Ta0.4O1.65 has been synthesized in air by rapidly cooling a melt from 1400°C, and it has been studied by X-ray powder diffraction (XRPD), selected-area electron diffraction (SAED), high-resolution electron microscopy (HREM), thermal analysis, and measurements of magnetic susceptibility and electrical conductivity. The average structure is that of a cubic fluorite with a=4.9826(3) ? and 18% vacancies on the O atom sites. Prominent diffuse scattering is present in its electron diffraction patterns (EDPʹs) and a structural model for this scattering is proposed. It is based on ca. 10 ? large microdomains with a pyrochlore type structure which are separated by antiphase boundaries. The magnetic susceptibility shows an antiferromagnetic interaction between the Mn2+ ions and an effective number of Bohr magnetons that increases with temperature. When heated in air at 500°C, the compound is oxidized to the composition Mn0.6Ta0.4O1.75, while retaining a cubic fluorite structure with a=4.8380(4) ?. This latter phase is a semiconductor with an activation energy of 0.64 eV.