Crystal structure of a new high-$T_c$ Tl${\mathrm{Ba}}_2$${\mathrm{Ca}}_3$${\mathrm{Cu}}_4$ ${\mathrm{O}}_{11}$ superconductor by high-resolution electron microscopy

The crystal structure of a new high-$T_c$ Tl${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_3$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_{11}$ (1:2:3:4) superconductor was studied by a high-resolution electron microscopic measurement. The electron diffraction patterns give a simple tetragonal lattice with the lattice constants of $a=b=3.9\mathrm{}$ Å and $c=19\mathrm{}$ Å. The lattice image viewed along [010] showed clearly the single Tl-O layer alternating with four Cu-O sheets. The $A$-site layers of Ba or Ca atoms also alternate with the $B$-site layers of Tl or Cu atoms as in the layered perovskite compound ${\mathrm{ABO}}_x$. Since oxygen sites in the Ca layer are vacant, the structure is one of the oxygen-deficient perovskite. This phase has a new space group of $\frac{P4\mathrm{}}{\mathrm{mmm}}$ different from the previous cuprate oxide superconductors and it has a large new superconductor family of $\mathrm{Tl}{\mathrm{Ba}}_2{\mathrm{Ca}}_{n-1\mathrm{}}{\mathrm{Cu}}_n{\mathrm{O}}_{2n+3} (n=1,2,3,4,5,\dots )$.