Synthesis and transport properties of substituted Tl2Mn2O7 pyrochlore

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.