Crystal structure of Tl 2 Ba 2 Ca 2 Cu 3 O 10 , a 125 K superconductor

There is now a new series of high-temperature superconductors that may be represented as (A III O) 2 A 2 II Ca n-1 Cu n O 2+2n where A III is Bi or Tl, A II is Ba or Sr, and n is the number of Cu-O sheets stacked consecutively. There is a general trend toward higher transition temperatures as n increases. The highest n value for a bulk phase is three and is found when A III is Tl. This compound, Tl 2 Ba 2 Ca 2 Cu 3 O 10 , has the highest transition temperature( ~ 125 K) of any presently known bulk superconductor. The structure of Tl 2 Ba 2 Ca 2 Cu 3 O 10 has been determined from single-crystal x-ray diffraction data and is tetragonal, with a = 3.85 A and c = 35.9 A. No superstructure is observed, and the material is essentially twin-free. Electron microscopy in the Tl/Ba/Ca/Cu/O system has revealed intergrowths where n = 5; such regions may well be responsible for the superconducting onset behavior observed in this system at about 140 K.