THE well-known 90-K superconductor Y2Ba2Cu3O7 ('123') is the first (n = 0) member of a homologous series of compounds with the general formula Y2Ba4Cu6+nO14+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 microscopy1,11. The 124 phase was then synthesized as a majority phase in thin films2,3,12, 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-Tc 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.
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