Low-temperature ordering in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$ oxides at x>0.5: Computer simulation

Recent experimental data are used to formulate the effective pair interaction model of oxygen ordering in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ oxides at x≳0.5 which is able to predict the main observed features of the low-temperature ordering. The computer simulation which uses this model characterizes the low-temperature secondary and tertiary ordering and describes the microstructure development resulting in formation of the Magneli-type and 2√${\mathit{a}}_0$×2√${\mathit{a}}_0$ phases (${\mathit{a}}_0$ is the perovskite lattice parameter). It is shown that these phases form stable rather than transient states. The stability fields of these phases on the phase diagram are estimated. Physical reasons distinguishing specific features of ordering in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ from the conventional ordering are discussed. The computer simulation results suggest certain heat treatment regimes which may provide formation of well-developed domains of the triple-period O-III and Magneli phases. The latter is important since lack of material with large domains poses a serious difficulty in experimental studies of these phases. © 1996 The American Physical Society.