Synthesis and transport properties of substituted Tl2Mn2O7 pyrochlore

Pyrochlore-type Tl2Mn2O7 with various levels of substitution by Ru, Bi and Sb has been prepared by high-pressure synthesis at 6 GPa and 1150 degrees C . The ruthenium system shows a complete solid solution from Tl2Mn2O7 to Tl2 Ru2O7. All three substituents induce an increase in cell parameter; in the case of Sb, this indicates that the substitution involves rather Sb5+ on Mn sites than Sb3+ on Tl sites. These pyrochlores start releasing thallium ox ide when annealed at temperatures as low as 150 degrees C under 1 atm oxyge n pressure; the pyrochlore phase, however, is still present at 750 degrees C. The partial replacement of Mn by Ru or of Tl by Bi in Tl2Mn2O7 induces a metal-semiconductor transition, in both cases at low levels of substitutio n (x < 0.2). In the ruthenium case, transport measurements in magnetic fiel ds up to 8 T yielded a magnetoresistance (R-o - R-H)/R-o = 96% in the vicin ity of the metal-semiconductor transition.