Role of bond-length mismatch in ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ (L=lanthanide)

The electron-doped ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ (L=lanthanide) superconductors have intergrowth structures in which ${\mathrm{CuO}}_2$ sheets alternate with (L,Ce${)}_2$ ${\mathrm{O}}_2$ fluorite layers along the c axis. Stabilization of such intergrowth structures requires bond-length matching between Cu-O and (L,Ce)-O bonds. Any bond-length mismatch will result in the buildup of compressive or tensile stresses in the Cu-O and (L,Ce)-O bonds. The consequences of such internal stresses in ${\mathit{L}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ are investigated by a systematic variation through ${\mathit{L}}^{3+}$ size of the lattice parameter a. A decrease in the degree of bond-length mismatch or internal stresses with decreasing ${\mathit{L}}^{3+}$ size causes a systematic decrease in the Ce solubility limit and in the ease with which oxygen vacancies can be created. The concentration of oxygen vacancies decreases—or the oxygen content increases—with decreasing ${\mathit{L}}^{3+}$ size for a given ${\mathrm{N}}_2$-annealing temperature and Ce content; it also decreases with increasing Ce content for a given ${\mathit{L}}^{3+}$ ion. The decreasing oxygen-vacancy concentration with decreasing size of ${\mathit{L}}^{3+}$ causes an apparent increase in the critical Ce concentration ${\mathit{x}}_{\mathit{c}}$ required to induce the antiferromagnetic semiconductor to superconductor transition as the size of ${\mathit{L}}^{3+}$ decreases, although the transition seems to occur at a fixed critical electron concentration ${\mathit{n}}_{\mathit{c}}$=0.175±0.005 irrespective of the ${\mathit{L}}^{3+}$ size.