Crystal growth and structure of AlSr2YCu2O7

Millimeter-size crystals of AlSr2YCu2O7 were synthesized by the flux method in alumina crucibles. Different mixtures of SrO:CuO (ranging from 25:75 to 12:88 mol ratio) were used as flux. No aluminum was added to the reagents. Depending on the flux composition and cooling rate, crystals different in morphology and size were obtained. The crystals appear to be tetragonal, sp ace group P4/mmm. However, electron diffraction photographs, in agreement w ith Ramirez-Castellanos et al. (1), reveal that the symmetry is othorhombic with a cell a = 2a(t), b = 4a(t), and c = 2c(t) (the subscript t refers to the YBCO tetragonal cell). The structural refinement was based on X-ray da ta collected with a KCCD Nonius diffractometer equipped with a graphite mon ochromator and AgK alpha radiation. One hundred and ninety-seven independen t reflections with respect to the space symmetry quoted above, with I > 3 s igma, were used. The arrangement is that of a YBCO structure in which the c hain Cu cations have been replaced by Al. This substitution induces the mov ement of the oxygen atoms in the basal layer in such a way as to form, toge ther with the apical oxygen atoms, corner-sharing zig-zag chains of oxygen tetrahedra centered around the Al cations and running parallel to the a-axi s either tetragonally or orthorhombically. The b- and c-axes are quadrupled and doubled, respectively, because the zig-zag chains are out of phase per pendicularly to these directions. The additional distortion results in the shift of Al to xx0 (x approximate to 0.056) from 000, O1 to xxz (x approxim ate to 0.073) from 00z, and O3 to (0.5, y, 0) (y approximate to 0.116) from (0.5, 0, 0). (C) 2000 Academic Press.