Role of bond-length mismatch in L_2-xCe_xCuO₄ (L=lanthanide)

The electron-doped L_2-xCe_xCuO₄ (L=lanthanide) superconductors have intergrowth structures in which CuO₂ sheets alternate with (L,Ce)₂O₂ 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 L_2-xCe_xCuO₄ are investigated by a systematic variation through L³⁺ size of the lattice parameter a. A decrease in the degree of bond-length mismatch or internal stresses with decreasing L³⁺ 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 L³⁺ size for a given N₂-annealing temperature and Ce content; it also decreases with increasing Ce content for a given L³⁺ ion. The decreasing oxygen-vacancy concentration with decreasing size of L³⁺ causes an apparent increase in the critical Ce concentration x_c required to induce the antiferromagnetic semiconductor to superconductor transition as the size of L³⁺ decreases, although the transition seems to occur at a fixed critical electron concentration n_c=0.175±0.005 irrespective of the L³⁺ size. © 1994 The American Physical Society.