Annealing study on (Tl1-xHgx)(2)Sr2Ca2Cu3Oy and determination of its crystal structure by X-ray powder diffractometry

Nearly single-phase superconducting (Tl1-xHgx)(2)Sr2Ca2Cu3Oy samples were p repared by the high-pressure technique, As-prepared samples with x = 0.35 a nd 0.65, with optimally nominal oxygen contents, were overdoped and show T- c of 103 and 80 K, respectively. Reducing treatments in Ar/H-2 were found t o be effective in increasing T-c up to 116 and 118 K for x = 0.35 and 0.65, respectively. The increase in T-c is also accompanied by a small weight lo ss and an increase in the room temperature c-axis length, with both phenome na resulting from oxygen loss. The crystal structure of the sample with x = 0.35 was determined by X-ray Rietveld refinements. This crystal belongs to a tetragonal system and has a space group of I4/mmm with a = 3.8130(5) and c = 34.7230(4) Angstrom. According to the present refinement, the T1 site must be partially occupied by a lighter element, whereas the Ca site is occ upied by a heavier one and the O(4) atom must shift from its ideal position 4e site (1/2,1/2,z) to the 16m site (x,x,z). (C) 2000 Elsevier Science Ltd . All rights reserved.