Structural Types of Superconductors

Superconductors can be described and compared in a compact and logical way by deriving their structures from the basic atomic arrangement of perovskite. As we have mentioned earlier, the (BX2) and (AX) layers of perovskite form a regular succession according to the scheme  

Such regularity, however, can be altered by modifying the sequence in appropriate ways. 

Let us consider first a layer (BX2). Immediately before and after it, there must be a layer of type (AX), as two consecutive layers (BX2) would generate an unreasonable atomic configuration with abnormally short oxygen-oxygen distances. This means that we may substitute a layer (BX2)o c only with a sequence 

On the other hand, before and/or after a layer (AX) there may be a layer of type (BX2) or one of type (AX). This last case is possible because (AX) is characteristic of both the perovskite and the rock salt structures and, therefore, is structurally coherent with both (BX2) and (AX). A sequence. .(AX)c o(AX)o c... can consequently substitute the single layer (AX)co, thus increasing the thickness of the rock salt monolayers (AX) present in the structure of perovskite. In this way, we may derive the structure of A2BX4 (K2NiF4 - type) from that of perovskite by substituting each (AX)C 0 in expression (6) with bilayers 

Following the above discussion, it is now possible to define a general structural type built with alternating blocks having the perovskite and the rock salt structures, according to the scheme  

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