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

摘要: We have prepared a family (${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$)(${\mathrm{Ba}}_{1.75\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{La}}_{0.25+\mathit{x}}$)${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ within the 1:2:3 high-temperature-superconductor system in which Ca occupies solely Y site while La partly and Ba site. This is characterized by constant charge (7.25) of noncopper cations via compensating substitutions. It tetragonal throughout single-phase-existence range 0.0\ensuremath{\le}x\ensuremath{\le}0.5. Attempts to prepare ${\mathrm{CaLaBaCu}}_{3}$${\mathrm{O}}_{7}$ were unsuccessful yielding x=0.5 member as major phase impurity phases. determined atomic positions occupancies Rietveld refinement x-ray powder-diffraction data, oxygen stoichiometry iodometric titration. A double-plateau behavior transition temperature (${\mathit{T}}_{\mathit{c}}$) vs cation concentration (x), very similar ${\mathit{T}}_{\mathit{c}}$ \ensuremath{\delta} variation ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, has been observed.Albeit, almost (i.e., y\ensuremath{\simeq}7.1) between both plateaus present family. Besides, width decreases with increasing x. examined several known models that attempt correlate chemical structural parameters, including deal ordinary transfer, bond-valence sums, interlayer distance, oxygen-ordered domains. propose materials substitution takes place, additional transfer responsible for increase hole planes, Specifically, besides electron from planes ``chains'' at x=0, there exists (at xg0) an Ca. At same time, (on site) does not fully compensate since some electrons go ``chains.'' The believed play important role structure determination, band-structure changes through its tendency attract plane chains.