Module 1: Structure of Ceramics
  Orthogonal Structures
 


      1.9 Orthogonal Structures

  • Many superconductors follow the structures which are perovskite based i.e., the structure contains the perovskite structured units stacked along c-axis or [001]-direction in most cases. The examples are superconductors such as YBa2Cu3O7, ferroelectrics such as Bi4Ti3O12 etc. In some other compounds such as La-Sr-Cu-O, the structure is composed of alternating perovskite and rocksalt structure units. Such a representation makes it easy to understand them.

  • Here we will take examples of Y-Ba-Cu-O and La-Sr-Cu-O and discuss them very briefly.

       1.9.1 Yttrium Barium Copper Oxide or YBCO (YBa2Cu3O7)

  • Parent compound is Y3Cu3+3O9 (see Fig. 1.35) which also contains perovskite units.
  • Doping of Y by Ba leads to structure modification (step 1) as well as reduction of Cu3+ to Cu2+ state (step 2) and thus resulting in the reduction in the number of required oxygen ions and hence creates oxygen vacancies in the structure. This gives a transition temperature of ~92 K below which the compound has zero electrical resistance i.e. is a superconductor.
Y3Cu3+3O9→YBa2Cu3+3O8→ YBa2Cu2+2Cu3+O7-x

Figure 1.35 Origin of the structure of YBa2Cu3O7-x as a triple-perovskite unit. (D.M. Smyth, PP.1-10 in ceramic superconductors II. Research Update 1988, M.F.Yan, Ed. The American Ceramic Society, 1988.)

  • Here Cu coordination is of interest:

    • Cu2+ atoms have four-fold coordination along Cu-O chains.

    • Cu3+ atoms have five-fold coordination in the Cu-O planes.

Figure 1.36 Atomic coordination in YBCO