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 degrees C, and it has been studied by X-ray powder diffracti on (XRPD), selected-area electron diffraction (SAED), high-resolution elect ron microscopy (HREM), thermal analysis, and measurements of magnetic susce ptibility and electrical conductivity. The average structure is that of a c ubic fluorite with a = 4.9826(3) Angstrom and 18% vacancies on the O atom s ites. Prominent diffuse scattering is present in its electron diffraction p atterns (EDP's) and a structural model for this scattering is proposed. It is based on ca. 10 Angstrom large microdomains with a pyrochlore type struc ture which are separated by antiphase boundaries. The magnetic susceptibili ty shows an antiferromagnetic interaction between the Mn2+ ions and an effe ctive number of Bohr magnetons that increases with temperature. When heated in air at 500 degrees C, the compound is oxidized to the composition Mn0.6 Ta0.4O1.75, while retaining a cubic fluorite structure with a = 4.8380(4) A ngstrom. This latter phase is a semiconductor with an activation energy of 0.64 eV. (C) 1999 Academic Press.