Redox mechanism for the generation of holes in TlBa1-xSrxLaCuO5+ delta : A neutron-diffraction study.

Abstract

Our neutron-diffraction study of the single Tl-O layered TlBa{sub 1{minus}{ital x}}Sr{sub {ital x}}LaCuO{sub 5+{delta}} system ({ital 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 {ital x}. This is consistent with the introduction of holes in CuO{sub 2} layers or the appearance of superconductivity in the sample with {ital x}=0.4 ({ital T}{sub {ital c}}=22 K) and increase of {ital T}{sub {ital c}} with {ital x}, {ital T}{sub {ital c}}=25 and 30 K for samples with {ital x}=0.7 and 1.0, respectively. Further, the valence of copper, derived from Cu-O bond length, increases with increase of {ital x} despite the fact that the O content decreases as {ital x} increases. This clearly suggests that the internal redox mechanism due to the overlap of the empty Tl 6{ital s} band with the filled Cu 3{ital d}{sub {ital x}}{sup 2}{minus}{ital y}{sup 2} band is responsible for the generation of holes, and therefore superconductivity, in these materials.