Superstructure along the $c$ axis in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ single crystals having $T_c=92\mathrm{}$ K

We have grown by flux methods $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ single crystals, having critical temperature $T_c=92\mathrm{}$ K, and orthorhombic distortion not detectable either by x-ray or by electron diffraction (difference between $a$ and $b$ lattice constants smaller than 7 × ${10}^{-3\mathrm{}}$ Å). In such crystals, transmission-electron-microscopy investigations have shown the presence of a domain structure, and electron-diffraction measurements have revealed the occurrence of a new superstructure along the $c$ axis. This superstructure consists in a systematic crossing of the Cu-O chains in alternate (001) planes, which causes a doubling of the $c$ lattice parameter and a strong decrease of the orthorhombic distortion. The occurrence of this superstructure is probably induced by tiny dopant impurities, and it can explain the discrepancies existing among the reports by several authors about the amount of orthorhombic distortion of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ single crystals.