Synthesis of bulk superconducting YBa₂Cu₄O₈ at one atmosphere oxygen pressure

Abstract

THE well-known 90-K superconductor Y₂Ba₂Cu₃O₇ ('123') is the first (n = 0) member of a homologous series of compounds with the general formula Y₂Ba₄Cu_6+nO_14+n. These compounds combine layers of copper–oxygen pyramids with single and/or double copper–oxygen chains. The n =2 member, Y₂Ba₂Cu₇O₈ ('124'), with double Cu–O chains, and the n = 1 ytterbium analogue, Yb₂Ba₄Cu₇O₁₅ ('247'), which mixes single and double chains, were first observed as intergrowths in bulk 123 by electron microscopy^1,11. The 124 phase was then synthesized as a majority phase in thin films^2,3,12, and its crystal structure was determined⁴ and found to be in agreement with the model proposed from microscopy. An important advance in the synthesis of bulk materials, the result of extensive pressure–temperature phase equilibria studies, was the isolation of 124 and 247 at oxygen pressures of >200 atm, and detailed determinations of their crystal structures (see, for example, refs 5–9). High-pressure studies have also shown that the 124 phase could be made with many rare-earth elements¹³. Here we report the synthesis of the 124 phase in pure bulk form by a novel synthetic route in a flowing oxygen stream at 1 atm pressure. This technique allows YBa₂Cu₄O₈ to be synthesized without specialized equipment, will make it generally available for study of its normal and superconducting properties, and will make possible more extensive comparisons with YBa₂Cu₃O₇ and other high-T_c superconductors. Our results suggest that 124 is the thermodynamically stable phase at low temperatures and 1 atm pressure, under oxidizing conditions; the usual inability to synthesize it in these conditions is probably due to the limitations of reaction kinetics.