Abstract
Pyrochlore-type Tl 2 Mn 2 O 7 with various levels of substitution by Ru, Bi and Sb has been prepared by high-pressure synthesis at 6 GPa and 1150 °C. The ruthenium system shows a complete solid solution from Tl 2 Mn 2 O 7 to Tl 2 Ru 2 O 7 . All three substituents induce an increase in cell parameter; in the case of Sb, this indicates that the substitution involves rather Sb 5+ on Mn sites than Sb 3+ on Tl sites. These pyrochlores start releasing thallium oxide when annealed at temperatures as low as 150 °C under 1 atm oxygen pressure; the pyrochlore phase, however, is still present at 750 °C. The partial replacement of Mn by Ru or of Tl by Bi in Tl 2 Mn 2 O 7 induces a metal-semiconductor transition, in both cases at low levels of substitution (x<0.2). In the ruthenium case, transport measurements in magnetic fields up to 8 T yielded a magnetoresistance (R 0 –R H )/1v\R 0 =96% in the vicinity of the metal-semiconductor transition.
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