CATION ORDERING IN [(TL, M)O] LAYERS OF 1201-BASED CUPRATES - SIMILARITY TO ORDERING IN FCC-BASED ALLOYS

''1201'' Tl-based substituted cuprates of the type (Tl1-xMx) Sr2CuO5 h ave been synthesized for M = Nb, Ta, or W. These materials do not supe rconduct due to a statistical distribution of some of the M for Cu. Th e remarkable feature of these materials is the ordering observed betwe en Tl and M in the (Tl1-xMx-epsilon)O plane. The type of ordering depe nds on the composition and shows remarkable similarities with the orde ring in Ni-Mo or other so-called 1 1/2 0 type fcc-based alloys or with the ordering in rocksalt oxides TiOx. The short-range order, for M = W, can be readily interpreted in terms of a mixing of nano-clusters wi th two different compositions. These observations of two-dimensional o rdering confirm recent ideas about ordering in three-dimensional fcc-b ased alloys. (C) 1997 Academic Press.