REDOX MECHANISM FOR THE GENERATION OF HOLES IN TLBA1-XSRXLACUO5+DELTA - A NEUTRON-DIFFRACTION STUDY
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Our neutron-diffraction study of the single T1-O layered T1Ba(1) (-) xSrxLaCuO5 (+) (delta) system (x = 0.0, 0.4, 0.7, and 1.0) shows that the in-plane Cu-O(2) bond length decreases monotonically with increase of x. This is consistent with the introduction of holes in CuO2 layers or the appearance of superconductivity in the sample with x = 0.4 (T-c = 22 K) and increase of T-c with x, T-c = 25 and 30 K for samples with x = 0.7 and 1.0, respectively. Further, the valence of copper, derived from Cu-O bond length, increases with increase of x despite the fact that the O content decreases as x increases. This clearly suggests that the internal redox mechanism due to the overlap of the empty T1 6s band with the filled Cu 3d(x2-y2) band is responsible for the generation of holes, and therefore superconductivity, in these materials.
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