Classification of superconducting oxide structures search for new compounds

Abstract

The structures of superconducting oxides are analysed as symmetrical combinations ofu oru′ (Ca, Sr)CuO₂,v CaTiO₃ andw VH or NaCl structural subunits. The structures with a maximum of 8 subunits can be classified according to the different hole donating elementsu′ (9 structures),w′ (21 structures) or both (20 structures). Two hole donating elements likeu′ CuO andw′ PbO can destroy metallic conductivity by internal redoxreaction between both elements as is outlined for Pb₂Sr₂-YCu₃O₉. The unit cell constants and the appropriate size of metal atoms of theoretical structures can be estimated froma=385 pm,c=160u+200v+265w pm. The alternative description of crystal structures by the sequence of coordination numbers of metal atoms might support the search for new compounds. The metal atoms Cu, Ni or Ti, Tl, Ag, Pb etc. with 20–30% reduction of radii at increased valence are favourable for hole donation in 4 or 6′ fold coordination ofu′ orw′ subunits.