STRUCTURAL AND PHYSICAL-PROPERTIES OF THE SINGLE AND DOUBLE CHAIN Y-BA-CU-O PHASES 124 AND 123.5

A review is given of the up to now experimentally determined structural and physical properties of the double chain compounds YBa2Cu4O8, in pure (T(c) = 80 K) and Ca-substituted form (T(c) = 90 K), as well as Y2Ba4Cu7O15 +/- x (T(c) = 94-14 K). These compounds belong together with 123 to the Y2Ba4Cu6+nO14+n family of HTc superconductors. The structure of the first compound (124) is very similar to that of 123 but contains only double chains. These are thermodynamically stable up to 850-degrees-C and stoichiometric, therefore, both O -> T phase transition and a-b twinning are supressed. The structure of the second compound (123.5 or 247) is also derived from that of 123, but it contains alternating single and double chains which make the properties more complex than those of 124. All three up to now known HTc-compounds of the Y-Ba-Cu-O system are classical examples of structure sensitive superconductors, so that in depth structural characterization is necessary for the understanding of the physical properties. Most sensitiv structural feature is the apex of the pyramid (Cu2-O1). Structural investigations show that its contraction can be correlated with the increase of T(c). Some properties of these compounds are not yet completely understood. Two examples are discussed here : a) The increase of Tc of 123.5 up to 94K, and b) the site of incorporation of Ca, in Ca-124, whose increase of T(c) does not seem to be due to the substitution of Y by Ca, but of Ba by Ca. Evidence from various methods is presented. Last but not least, evidence is presented in this review, indicating that also other Y-Ba-Cu-O compounds, some with much higher T(c), may exist.