Microsc. Microanal. Microstruct. 7, 107-141 (1996)
DOI: 10.1051/mmm:1996109

Mixed Layers in Copper Based Superconducting Materials

Maryvonne Hervieu¹, Gustaaf Van Tendeloo², Claude Michel¹, Denis Pelloquin¹ et Bernard Raveau<sup>1

1</sup>  Laboratoire de Cristallographie et Sciences des Matériaux, ISMRA/Université de Caen, Boulevard du Maréchal Juin, 14050 Caen Cedex, France
²  EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, 2020, Antwerp, Belgium

Abstract
Recently discovered series of high Tc superconductors, characterized by the existence of two types of cations within the same layer, are presented. The first family concerns the mercury based cuprates, , with A = Ba and/or Sr, which exhibit structures closely related to that of the thallium cuprates . They differ from the thallium cuprates by a high oxygen deficiency at the level of the mercury layer. It is shown that cations such as M = Cu, Pb, Tl, Bi, Ce, Pr, Cr, V, Mo, W, Ti, Sr, Ca,... can partially substitute for mercury ions, stabilizing the structures. The cationic composition of the layer depends indeed on the nature of the M cation but also on that of the alkaline earth A. For given A and M cations, the x value remains unchanged even when the number of copper layers varies. M and Hg cations are either statistically distributed over the same site or ordered. Different types of ordering have been detected. Another way of generating mixed layers is to shear periodically the structure, leading to the formation of the so called "collapsed phase". In the collapsed bismuth cuprates, bismuth and copper segments, a few octahedra long, alternate in strongly waving layers. In the collapsed oxycarbonates, carbonate groups and M cations are ordered within the intermediate layer so that they can be simply described from a partial and ordered substitution of carbon for Hg,Tl, Bi and other M cations building the intermediate layer. The oxycarbonitrates can also be described as an ordered substitution of carbon for copper in the 123 matrix. The different families of superconducting materials which are generated by such mechanisms are described as well as the way the different species are distributed within the mixed layers. Their influence on the physical properties are discussed.

PACS
7460M - Material effects on T/sub c, K, critical currents in type II superconductors.
7470V - Perovskite phase superconductors.
7470J - Superconducting layer structures and intercalation compounds.
6170R - Crystal impurities: general.
0130R - Reviews and tutorial papers: resource letters.

Key words
barium compounds -- calcium compounds -- high temperature superconductors -- impurities -- mercury compounds -- reviews -- stoichiometry -- strontium compounds -- yttrium compounds -- high T sub c superconductors -- Hg sub 1 x M sub x Ba sub 2 Ca sub m 1 Cu sub m O sub 2m+2+ delta -- Hg sub 1 x M sub x Sr sub 2 Ca sub m 1 Cu sub m O sub 2m+2+ delta -- cationic composition -- collapsed phase -- partial substitution -- ordered substitution -- Y sub 1 x Ca sub x sub n Ba sub 2n Cu sub 3n 1 C,NO sub 3 O sub 7n 3 -- mixed layers -- YCaBaCuCNO


© EDP Sciences 1996