Tetragonal superconducting family (CaxLa1−x) (Ba1.75−xLa0.25+x)Cu3Oy: The effect of cosubstitution on the transition temperature

Abstract

We have prepared a family (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oy within the 1:2:3 high-temperature-superconductor system in which Ca occupies solely the Y site while La partly occupies the Y site and partly the Ba site. This family is characterized by constant charge (7.25) of the noncopper cations via compensating substitutions. It is tetragonal throughout the single-phase-existence range 0.0≤x≤0.5. Attempts to prepare CaLaBaCu3O7 were unsuccessful yielding the x=0.5 member as a major phase and impurity phases. We have determined atomic positions and occupancies by Rietveld refinement of x-ray powder-diffraction data, and oxygen stoichiometry by iodometric titration. A double-plateau behavior in transition temperature (Tc) vs cation concentration (x), very similar to the Tc vs δ variation in YBa2Cu3O7−δ, has been observed. Albeit, oxygen stoichiometry is almost constant (i.e., y≃7.1) between both plateaus in the present family. Besides, the transition width decreases with increasing x. We have examined several known models that attempt to correlate Tc with chemical and structural parameters, including models that deal with ordinary charge transfer, bond-valence sums, interlayer distance, and oxygen-ordered domains. We propose that in materials in which compensating substitution takes place, additional charge transfer is responsible for the increase in hole concentration in the planes, yielding increase in Tc with increasing x. Specifically, besides ordinary electron transfer from planes to ‘‘chains’’ at x=0, there exists (at x>0) an additional electron transfer from the planes to the Ca. At the same time, electron transfer from the La (on Ba site) to the planes does not fully compensate for the transfer to the Ca since some of the electrons go from the La to the ‘‘chains.’’ The La (on Ba site) is believed to play an important role in charge transfer, structure determination, and band-structure changes through its tendency to attract oxygen to the plane of the chains.

Source-reported materials — not catalogue approval

Similar papers