Structural Aspects of Modulated Superconducting Oxides: Application to Hg1-xTlxSr4-yBayCu2CO3O7-δ

Superconducting oxides of the type Hg_1-xTl_xSr_4-yBa_yCu₂CO₃O_7-δ show complex modulated structures with a modulation vector along 010] and a wavelength varying between 6 and 8 times the basic perovskite unit. High resolution electron microscopy indicates that the modulation is introduced by an ordering of CO₃ groups on the (Hg-Tl) sublattice. Such modulated structures are discussed in general; they are the result of a minimization of the internal stresses due to the differences in ionic sizes of the constituents.