Colossal Magnetoresistance Without Mn ³⁺ /Mn ⁴⁺ Double Exchange in the Stoichiometric Pyrochlore Tl ₂ Mn ₂ O ₇

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl 2 Mn 2 O 7 pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstroms) observed in phases based on LaMnO 3 perovskites that exhibit CMR. Both results in Tl 2 Mn 2 O 7 indicate oxidation states very close to Tl 3+ 2 Mn 4+ 2 O 7 . Thus, Tl 2 Mn 2 O 7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn 3+ , both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl 2 Mn 2 O 7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.