THE SUPERCONDUCTING COPPER OXYCHROMATE TL-3(CRO4)SR8CU4O16 - LONG-RANGE ORDERING BETWEEN THALLIUM AND CHROMIUM

Long-range ordering between chromium and thallium has been evidenced f or the first time in the copper oxychromate Tl-3(CrO4)Sr8Cu4O16 using HREM and XRD. This compound, which crystallizes in an orthorhombic cel l with a = 3.778 (1) Angstrom, b = 15.244 (1) Angstrom, and c = 17.677 (1) Angstrom, derives from the ''1201'' TlSr2CuO5 structure, by repla cing one row of Tl atoms out of four by one row of chromate groups in an ordered way. This superconductor, whose T-c(onset) can be raised to 34 K after argon-hydrogen annealing, exhibits a rather high supercond ucting volume fraction (>40%) compared to other ''1201'' thallium cupr ates. Doping with lanthanum destroys the long-range ordering and weake ns superconductivity. The electron microscopy study of this phase also shows the existence of other local superstructures that are interpret ed in terms of various modes of ordering of Tl atoms and CrO4 groups.