Superconductors of perovskite structure type

The crystal structure of many oxide superconductors can be regarded as based on the perovskite structure, which can be modified by atomic replacements, displacements, vacancies, and changes in the order of stacking layers. 

The first-discovered oxide superconductor is La2Cu04 and it has the K2NiF4 structure type, as shown in Fig. 10.4.3(a). This structure can be derived from one B-type unit cell of the perovskite stmcture in combination with two A-type unit cells, each with one layer removed. In this manner, a tetragonal unit cell with c 3a is generated. The resulting structure belongs to space group — I4/mmm, in which the Cu atom has octahedral coordination and the La atom has nine-coordination. The CuC 6 octahedra share vertices and form infinite layers. 

Structures of three oxide superconductors with perovskite structure type (a) La2Cu04, (b) YBayCujOg, (c) YBayCujOy (small white circles, O small black circles, Cu large black and open circles, M). 

Some trivalent metal fluorides, hexavalent metal oxides, and copper(I) nitride adopt the ReC 3 structure  

The structure of WO3 is of the Re03 type, as shown in Fig. 10.4.4(a). The tungsten bronzes of the formula MxW03 (0 x 1) are derived from the three-dimensional network of WO3, in which the W atoms adopt variable valence states, and the large unoccupied interstices conveniently accommodate other cationic species. When a portion of the WfH cations is converted to W5+, the requisite cations M (normally Na or K but also Ca, Sr, Ba, Al, In, Tl, Sn, Pb, Cu, Ag, Cd, lanthanides, H+, and NHj) are incorporated into the structure to maintain electrical neutrality. The additional cations M are partially located at the centers of the unit cells to give the perovskite stmcture type, as shown in Fig. 10.4.4(b). 

---