Fig. 19.

View of the structure of the 94 K superconductor Ba₂YCu₃O₇ (right side) and of the configuration produced by stacking along c three unit cells of the ideal structure of perovskite ABO₃ (left side). The atomic arrangement of the superconductor differs from that of perovskite because (i) the Y and Ba atoms are ordered; (ii) the Y layers and the sites O(5) are not occupied by oxygen atoms; (iii) the CuO₂ layers are buckled. In Ba₂YCu₃O₇, the Cu(l) atoms are in four-fold planar coordination, resulting in the characteristic chains which propagate along the b-axis of orthorhombic unit cell.