Colossal magnetoresistance (CMR) in doped lanthanum manganite thin tilms (La_1-xM_xMnO_3, where M is a divalent ion) has been shown to result in a factor of 10~6 suppression of the resistance. The driving force for the CMR transition is thought to be the double-exchange (DE) interaction. Many studies of both the crystal structure and the local structure of the La_1-xM_xMnO-3 (wikth M=Ca, Sr and Ba, as well as Pb) system have now been carried out. As expected, these systems all show a strong coupling of the lattice to the CMR transition. On the other hand, neutron diffraction data and x-ray absorption studies for the Tl_2Mn_2O_7 pyrochlore, which also exhibits CMR,shows no deviations from ideal stoichiometry. mixed valency, or Jahn-Teller distortions of the MnO_6 octahedron. We present results of cruystalographic and local structural studies of these two important classes of CMR materials. compare the differences in structural response, and discuss the implications of these finding to our understanding of these materials.
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