MODULATED ORDERING OF BISMUTH AND STRONTIUM IN THE 1201-CUPRATE (TL2 3BI1/3)(0.97)SR1.86BI0.14CUO5-DELTA/

The structure of the as-synthesised ''1201'' cuprate (Tl2/3Bi1/3)(0.97 )Sr1.86Bi0.14CuO5-delta (delta = 0.04) has been studied by powder X-ra y, neutron diffraction and high-resolution electron microscopy. It con sists of slices of the normal ''1201'' structure, Tl2/3Bi1/3Sr2CuO5-de lta (a congruent to b congruent to a(p), c = c(1201)) intergrown with slices of a new ''1201'' superstructure, Tl2.67Bi1.33Sr6Bi2Cu4O20 (d = 4a congruent to 4a(p), b' = 2b congruent to 2a(p) and c' = c(1201)). The originality of the latter deals with the substitution of bismuth f or strontium in the rock-salt layers, leading to [Sr6Bi2O8](infinity) layers characterized by long-range ordering of bismuth and strontium, whereas the stacking of these layers along c obeys only a short-range ordering forming triple layers [Sr6Bi2Cu4O16](infinity). Relationships between structure, charge distribution and superconductivity in the o xides Tl1-xBixSr2CuO5-delta are discussed.