Magnetic susceptibility data show La 2 CuO 4 to be antiferromagnetic. Oxides of the K 2 NiF 4 structure of the general formula (La 1−y Ln y ) 2−x Sr x (Ba x )CuO 4 show superconductivity in the 15-40K range, the T c becoming maximum at a specific value of x or of the a-parameter. Superconductivity in La 3−x Ba 3+x Cu 6 O 14 and La 4−x Ba 1+x Cu 5 O 13 systems has been investigated besides that in the Y-Ba-Cu-O system. Role of oxygen in the superconductivity of YBa 2 Cu 3 O 7−δ is discussed. The problem of rationalizing macro and microstructures of YBa 2 Cu 3 O 7−δ is indicated. Infrared spectra of the superconducting oxides and their marginal metallicity in the normal state are examined. Based on electron spectroscopic studies, it is shown that holes are likely to be on oxygen rather than on copper, giving rise to peroxide-like species; Copper is present in the 1+ state (besides the 2+ state).
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