Structural and magnetotransport properties of the colossal magnetoresistance material ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$s

The crystal structure and its variation with temperature of the spin-charge coupled material ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ pyrochlore is investigated. Structure refinement from the neutron powder-diffraction data reveals that ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ has nearly stoichiometric composition and is crystallized in the cubic-pyrochlore-type structure with a characteristic Mn-O-Mn bond angle of 133°. There is no structural anomaly associated with the change in magnetotransport properties at 142 K, suggesting weak spin-lattice and charge-lattice correlations. Despite the similarity of magnetotransport properties observed in the perovskite and the pyrochlore colossal magnetoresistance manganese oxides, the double-exchange mechanism, which is usually applied to the perovskite materials, does not seem to explain the behavior in the pyrochlore ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ compound. A superexchange-type interaction may stabilize the ferromagnetic ordering of Mn spins and cause the metallic conduction at low temperatures in ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ .