Bismuth cuprate high-Tc superconductors using cationic substitution.

Abstract

The ${\mathrm{Bi}}_{4}$${\mathrm{Sr}}_{4}$${\mathrm{Ca}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{R}}_{\mathit{x}}$${\mathrm{Cu}}_{4}$${\mathrm{BO}}_{\mathit{y}}$ materials (R is a rare-earth element) were studied to determine their structural and physical properties. For most of the rare-earth elements, a complete solid solution exists up to x=2. Below x=0.5,${T}_{c}$ is not affected and for each added rare-earth element we find that about 0.5 oxygen atom is added to the structure. However, the structural modulation observed along the b axis for the undoped material persists and remains of the same amplitude for the rare-earth-doped samples. When more than one R(x\ensuremath{\ge}1) is substituted, ${T}_{c}$ is depressed and the compound becomes semiconducting beyond x=1.5. The depression in the ${T}_{c}$ from 85 K (x=0) to less than 4.2 K (x=1.5) correlates to a decrease in the formal valence of copper and is independent whether the rare-earth element is magnetic or nonmagnetic. No evidence for magnetic ordering over the range of temperature 1.7--400 K has been observed in all the substituted compounds. The substitution for Cu by 3d metals or for Sr by rare-earth elements fails for the 85-K Bi phase but succeeds for the 10-K Bi phase. Consequently, the following series ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${\mathrm{O}}_{\mathrm{y}}$ (M=Fe, Co) and ${\mathrm{Bi}}_{2}$${\mathrm{RCaCuO}}_{\mathrm{y}}$ (R=La, Pr, Nd, Sm) were made for study. These substitutions result in an uptake of oxygen (0.5 for each substituted element). But the materials become semiconducting even though the formal valence of Cu remains greater than 2. An antiferromagnetic transition at 140 K has been found for the Co sample for which Co is found to be in the +3 state.