Bi,Sn and Sb valence states in highT
c
(Bi,Pb/Sn,Sb)2Sr2Ca2Cu3Oy |
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Authors: | R A Brand P Sen Ch Sauer J -P Sanchez A Rojek W Zinn |
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Institution: | (1) IFF der KFA Jülich, Postfach 1913, D-5170 Jülich, FRG;(2) Laboratoire de Chimie Nucléaire, Centre de Recherch Nucléaires, B.P. 20, F-67037 Strasbourg, France;(3) Present address: Institute for Low Temperature and Structural Research, Polish Academy of Sciences, Wroclaw, Poland |
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Abstract: | The valence of Bi and its substituents Sn and Sb are investigated in the high temperature superconductor (Bi0.8X0.15Sb0.05)2Sr2Ca2Cu3Oy (often denoted 2223) where X=Pb or Sn. Pb and Sn are isoelectronic with the valence states 2+ and 4+; Bi and Sb are as well with valence states 3+ and 5+. The valence state of Sn and Sb has been obtained from Mössbauer spectroscopy: they exist in the high charge states 4+ and 5+ respectively. Sn, Sb, and presumably Pb, take on the high valence state, and so furnish electrons to the conduction band. This is probably the reason why, although the (Pb, Sb)-doping aids in stabilizing the 2223 crystal structure, it is detremental to the electronic properties which lead to the superconducting phase. The valence state of Bi has been studied using X-ray photoelectron spectroscopy (XPS). These results show clearly that the Bi-based superconductor has a metal-like density of states at the Fermi level, and that the valence in the (Pb, Sb)-doped compounds is less than 3. This very surprising change in valence will have a profound effect on the superconducting properties, and is probably associated with the high valent states of Pb, Sn and Sb. |
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