1ST-ORDER AND 2ND-ORDER PHASE-TRANSITIONS AND OXYGEN REARRANGEMENT OVER THE CU1-O4 CHAINS IN ORTHORHOMBIC YBA2CU3O6+X AT LOW-TEMPERATURES

The thermodynamics of a system of oxygen ions ordered along the O4 chains in the Cu1-O4 basal phase of orthorhombic YBa2Cu3O6 + x is theoretically studied using a modified mean field approximation. This allows the "intrachain" thermodynamics to be treated exactly, although interchain oxygen-oxygen correlations are ignored. Such an approach is justified only when interchain correlations are much weaker than those within the chains. Four different types of thermodynamically stable oxygen ordering are predicted with this model. They are featured by 1a0, 2a0 and 3a0 periodicity along the crystallographic a-direction. The first- and second-order phase transitions between these phases can occur at low temperatures and high oxygen content. The T, x-phase diagram calculated serves as a basis for the explanation of the experimental findings concerning elastic modulus temperature hysteresis and the anomaly in the acoustic loss spectra at T ≈ 300 K in YBa2Cu3O6 + x samples with various oxygen contents. © 1990.