The structural phase diagram and oxygen equilibrium partial pressure of YBa₂CU₃O_6+x studied by neutron powder diffraction and gas volumetry

An experimental technique based on neutron powder diffraction and gas volumetry is presented and used to study the structural phase diagram of YBa2Cu3O6+x under equilibrium conditions in an extended part of (x, T)-phase (0.15 < x < 0.92 and 25-degrees-C < T < 725-degrees-C). Our experimental observations lend strong support to a recent two-dimensional anisotropic next-nearest-neighbour Ising model calculation (the ASYNNNI model) of the basal plane oxygen ordering based of first principle interaction parameters. Simultaneous measurements of the oxygen equilibrium partial pressure show anomalies, one of which proves the thermodynamic stability of the orthorhombic OII double cell structure. Striking similarity with predictions of recent model calculations support that another anomaly may be interpreted to result from local one-dimensional fluctuations in the distribution of oxygen atoms in the basal plane of tetragonal YBCO. Our pressure data also indicate that x = 0.92 is a maximum obtainable oxygen concentration for oxygen pressures below 760 Torr.