CRYSTAL-STRUCTURE AND T-C OF 1212-TYPE CUPRATE (TI,CR) SR-2 (CA,TL) CU2O7

The crystal structure of the 1212-type cuprate (T1,Cr)Sr-2(Ca,Tl)Cu2O7 was investigated using X-ray diffraction data and the Rietveld refine ment technique. The refinement was performed using the tetragonal symm etry of space group P4/mmm. It was found that Cr atoms entered the T1 site while partial T1 atoms occupied the Ca site. (TI,Cr) atoms displa ced from the origin to (0.101(1), 0, 0), and 0(3) shifted from (0.5, 0 .5, 0) to (0.626(8), 0.5, 0). The refined lattice parameters were a = 3.8155(1) Angstrom and c = 12.0222(5) Angstrom. The refined compositio n was (Tl0.812Cr0.100Ca0.088)Sr-2(Ca0.842Tl0.158)Cu2O7. Cr atoms were most likely to be at a high valence state. The enhancement of T-c in t he (T1,Cr)-1212 cuprate is briefly discussed based on the structure pa rameters by comparison with the un-doped TlSr2CaCu2O7.