Synthesis, structure and superconducting properties of metastable Y(Ba _2-xY_x)Cu₃O_7+δ

The oxygen-deficient orthorhombic oxide YBa₂Cu₃O _7-δ^1-3 has been the centre of intense recent interest because of its bulk superconducting properties near 90 K. More recently the closely related oxide, La_1.5Ba_1.5Cu₃O _7+δ, is receiving increased attention^4-7. The yttrium analogue, Y_1.5Ba_1.5Cu₃O _7+δ, has not been successfully synthesized by the usual ceramic routes involving firing at temperatures above 900°C^8,9, because of its intrinsic metastability. Here we report on the synthesis and characterization of a majority (90-95%) phase Y_1.5Ba _1.5Cu₃O_7+δ(referred to for historical reasons8 as Y 3-3-6), prepared by firing an atomically mixed citrate precursor between 810° and 825 °C for 2 h. X-ray powder diffraction data indicate that Y 3-3-6 is isostructural with its tetragonal La analogue^4,7. Electron microprobe and thermogravimetry data yield a unit-cell composition Y(Ba_2-xY_x)Cu₃O_7+δ, with x=0.50 and δ=0.1-0.15 for the parent compound. Specimens of Y 3-3-6 with x = 0.50 and 0.375, which were annealed in O₂ at 1 atm and >3 atm, displayed semiconducting behaviour. Filamentary superconductivity, detected electrically and magnetically, with transition onsets at 80 and 60 K respectively, is observed in specimens annealed in 3 atm O₂ at 520°C.