Pseudotetragonal and orthorhombic ordered structures in substoichiometric YBa2Cu3O6+x oxides at x

Abstract

The previous analysis [S. Semenovskaya and A.G. Khachaturyan, Phys. Rev. B 46, 6511 (1992); Physica D 66, 205 (1993)] of the structural transformations in nonstoichiometric YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}} oxides is extended to a ``tetragonal`` stoichiometry region, {ital x}{lt}0.4. Available diffraction data [Th. Zeiske, D. Hohlwein, R. Sonntag, F. Kubanek, and G. Collin, Z. Phys. B 86, 11 (1992); Tan Kemin, Hu Meisheng, and W. Yening, J. Phys. Condens. Matter 1, 1049 (1989)] (on the {radical}2 {ital a}{sub 0}{times}2 {radical}2 {ital a}{sub 0} structures formed at small {ital x}{lt}0.4) are analyzed by the concentration-wave method and computer-simulation technique. It is found that at small {ital x}{lt}0.4, oxygen ordering results in a sequence of transformations different from those observed at {ital x}{gt}0.5. It produces the pseudotetragonal and orthorhombic ({l_brace}1/4 1) / 4 0{r_brace} phases formed by the (1/21/20) and ({l_brace}1/4 1) / 4 0{r_brace} concentration waves. These phases form a family of [110] oxygen-atom chain structures, which are different from the [010] Cu(1)-O chain structures formed at {ital x}{gt}0.5. These differences can be explained only if a significant change in oxygen-oxygen (O-O) potential near {ital x}{similar_to}0.4 (where the superconductivity disappears) is assumed. The O-O interaction potential at {ital x}{lt}0.4 wasmore » estimated and used to perform computer simulations of oxygen ordering kinetics at {ital x}=0.25.« less