Single valence and structural order in the colossal-magnetoresistant pyrochlore ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$

Without ${\mathrm{Mn}}^{3+}$ /${\mathrm{Mn}}^{4+}$ mixed-valence or Jahn-Teller distortions in the ${\mathrm{MnO}}_6$ octahedra, the origin of colossal magnetoresistance (CMR) in the pyrochlore ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ is thought to arise instead from superexchange with large incoherent scattering near the ferromagnetic transition. Using x-ray-absorption spectroscopy, we have shown that the electronic state of Tl is nearly identical to that in ${\mathrm{Tl}}_2$ ${\mathrm{O}}_3$ , indicating doping into the Tl 6s bands, that there is no appreciable Mn mixed valence, and that the ${\mathrm{MnO}}_6$ octahedra are well ordered. This further supports the proposed mechanism for CMR in ${\mathrm{Tl}}_2$ ${\mathrm{Mn}}_2$ ${\mathrm{O}}_7$ based on our earlier structural studies.