ELECTRON AND POWDER X-RAY DIFFRACTION STUDY OF THE DEFECT FLUORITE STRUCTURE OF Mn0.6Ta0.4O1.65

摘要

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 powder XRD (XRPD), selected-area electron diffraction (SAED), 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) angstroms and 18% vacancies on the O atom sites. Prominent diffuse scattering is present in its electron diffraction patterns (EDP's) and a structural model for this scattering is proposed. It is based on approx 10 angstrom large microdomains with a pyrochlore type structure that 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) angstroms. This latter phase is a semiconductor with an activation energy of 0.64 eV. 36 refs.