Low-temperature phase transformations in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ by anelastic relaxation measurements and possible formation of ferroelectric and antiferroelectric domains

We report on three phase transformations occurring between 120 and 170 K in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ for x>0.2. It is proposed that one or two of them are due to the ordering of the O atoms among the off-center positions in the Cu-O zigzag chains. Since an electric dipole is associated to ${\mathrm{O}}^{2\mathrm{-}}$ ions in such off-center positions, ferroelectric (oxygens on the same side of the chain) and antiferroelectric (oxygens on opposite sides in zigzag fashion) domains should also form. The proposed interpretation correlates well with the overall phenomenology, including an anelastic relaxation process previously interpreted in terms of the nearly uncorrelated jumps of such oxygens. The phase transition around 170 K may correspond to a ferroelectric displacive transition appearing in the dielectric function.