Crystal chemistry of superconductors: A guide to the tailoring of new compounds

Abstract

The crystal structures of the known superconducting copper oxides can be described in terms of two basic structural types. The series La 2Ca n−1Cu nO 2n+2, (Tl,Bi) 2(Ba,Sr) 2Ca n−1Cu nO 2n+4 and TlBa 2Ca n−1Cu nO 2n+3 can be viewed as made of alternating slices having the rock salt and perovskite structure. The compounds Ba 2YCu 4O 8 and Ba 4Y 2Cu 7O 14+x , on the other hand, comprise of perovskite blocks alternating with blocks in which a crystallographic shear is present. The effect of this shear is that of forming double chains of edge sharing squares with oxygen atoms at the corners and copper atoms at the center. The superconductor Ba 2YCu 3O 7 can be described in terms of both structural types and may be considered as an intermediate type between the other two. The basic building blocks of these superconducting materials can be further broken down into constituent nets (or meshes). This description allows one to envisage new structures built from these meshes containing the key structural elements present in the currently known superconductors. As such, the structural schemes used in this description may be used as a guide in the preparation of new materials with interesting electronic properties.