Orthorhombic distortion and cation ordering in ${\mathrm{Y}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_8$${\mathrm{O}}_{16}$ and ${\mathrm{Er}}_2$${\mathrm{Ba}}_4$${\mathrm{Cu}}_8$${\mathrm{O}}_{16}$ high-$T_c$ thin films

Detailed examination of a well-ordered, high-$T_c$ ${\mathrm{Y}}_2$ ${\mathrm{Ba}}_4$ ${\mathrm{Cu}}_8$ ${\mathrm{O}}_{16}$ (2:4:8) oriented thin-film sample revealed directly that the three crystallographic axes were unequal in length. Each layer of this high-$T_c$ superconductor, like the closely related superconducting $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-y}$ (1:2:3) phase, has structural anisotropy in the $a-b$ plane. The difference between $a$ (3.859 ± 0.002 Å) and $b$ (3.874 ± 0.002 Å) in 2:4:8 is significantly smaller than in 1:2:3, so many associated strains such as those occurring during oxidation or epitaxial growth will also be smaller. Well-ordered, high-$T_c$ thin films of 2:4:8 using Er in place of Y have been produced. By comparing the x-ray-diffraction patterns of the Y and Er 2:4:8 samples, a one-dimensional electron density map has been produced directly, without structure refinement. The result demonstrates that relative to the 1:2:3 structure, 2:4:8 contains an additional -CuO- layer. Together, the results indicate, without appeal to indirect chemical analogy, that 2:4:8 contains double -CuO- layers with interlayer vacancy ordering.