Showing papers by "T. N. Guru Row published in 1991"

An X-ray and neutron diffraction study of cation substituted TlSr2CuO5

Abstract: The structures of TlSr (2− x ) La x CuO (5+δ) , with x =0.5, 0.75 and 1, and Tl .5 Pb 0.5 Sr 2 CuO (5+δ) have been examined with X-ray and neutron powder Rietveld refinement. They are isostructural (P4/mmm) with the corresponding thallium-barium cuprate having one Cu-O layer with Cu 3+ ions in octahedral coordination with oxygen (structure type 1201). The influence of cation substitution and disorder on the structure and superconducting properties of these phases have been investigated. La 3+ substitution for Sr 2+ stabilises the structure and reduces Cu 3+ , permitting superconductivity, while Pb 2+ substitution for Tl 3+ only stabilises the structure, without reducing Cu 3+ .

Structure and superconducting properties of Tl1-yPbyY1-xCaxSr2Cu2O7

Abstract: Structure and superconducting properties of Tl 1− y Pb y Y 1− x Ca x Sr 2 Cu 2 O 7 ( y =0.0, 0.1, 0.25 and 0.5) , derived from the parent insulator TlYSr 2 Cu 2 O 7 , have been investigated for different values of x . XANES studies show Pb to be in the 4+ state while Tl is in the 3+ state, suggesting thereby that in this series, ( x − y ) approximately corresponds to the hole concentration. The in-plane CuO distance decreases with increase in x for all values of y . The apical CuO distance as well as the Tl(Pb)O(2) distance show anomalies at x =0.5 in the series with y =0.25; at this composition, T c also reaches a maximum. In general, the composition (value of x ) at which the T c of Tl 1− y Pb y Y 1− x Ca x Sr 2 Cu 2 O 7 reaches a maximum depends on the Pb content and the maximum T c itself increases with increase in y , reaching a value of 105 K at y =0.5. More interestingly, a maximum T c occurs at an ( x − y ) value of ∼0.25, which is close to the hole concentration at maximum T c in other cuprate superconductors containing two CuO 2 layers.