Compounds with the Rocksalt-Perovskite Structure

Substitution of Ba with Sr slightly increases the value of the critical temperature (T 40K for x 0.15) (23). Structural analyses of (Lax 8BSr01B)Cu04 have shown that this compound has a phase transition at -200K and that its structure is isomorphic with that of (Lax 8BBa01B)Cu04 at room temperature as well as at low temperature (24)-(26). The lattice parameters for this composition are given in 

Neither the calcium nor the strontium compound is a superconductor. A possible reason may be that the oxidation state of copper is equal, or very close, to 2+. 

From a structural point of view, Pb2Sr2YCu808 can be oxidized to Pb2Sr2YCu809 with the layers (Cu) becoming (CuO). The formation of the oxidized compound, with a layer (CuO) in its structure, would make possible the existence of compounds with crystallographic shears of the type discussed before in the case of Ba CugOy. No oxidation experiments have been carried out so far, however. No other possible compounds of this homologous series have been reported so far. 

No tenperature and occupancy factors are given in the original paper. 

The basic difference between the structures with different values of n is in the number of consecutive (Cu02) planes in the perovskite blocks. The value of the critical temperature is a function of n. For example, in the Tl-Ba system Tc is 85K, 110K, and 125K for n = 1, 2,3, respectively. The compound with n = 5 has not been prepared as a bulk phase, but the value of Tc determined for this material on a microscopic level appears to be of the order of 140K (35). 

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