X-RAY-ABSORPTION SPECTROSCOPIC EVIDENCE FOR THE ABSENCE OF MN3+ MN4+ DOUBLE EXCHANGE IN THE CMR PYROCHLORE TL2MN2O7/

The X-ray absorption near edge spectra (Mn K edge) for Tl2Mn2O7 pyroch lore exhibiting colossal magnetoresistance (CMR) has been studied at r oom temperature. The observed Mn K absorption edge coincides with the edges observed for MnO2 and Er2Mn2O7, both containing only Mn4+. In ad dition, the observed edge for Tl2Mn2O7 is markedly shifted from the Mn K edge observed in CMR perovskite La0.7Sr0.3MnO3, with an average Mn valence of +3.3, as well as Mn2O3, (containing only Mn3+). The above r esults further substantiate our earlier conclusion, drawn from the cry stal structure study, which indicated that the Mn3+/Mn4+ double-exchan ge mechanism responsible for the CMR effect in perovskite is not opera tive in Tl2Mn2O7. (C) 1996 Academic Press,Inc.