Oxygen intercalation in the perovskite superconductor YBa2Cu3O6+x

We have studied superconductivity and the thermodynamics of oxygen insertion in the oxygen intercalation system YBa2Cu3O6+x. The superconducting transition temperature decreases as x decreases, and the transition is sharp at Tc=90 K near x=1 and at 55 K near x=0.5. The thermodynamics of oxygen intercalation at high temperatures (both isotherms and isobars), and the orthorhombic-tetragonal transition, can be explained in terms of a lattice-gas model that includes a large energy of repulsion between oxygen atoms on nearest-neighbor sites. The high-temperature behavior is also consistent with smaller interactions, some attractive, between second neighbors. Such energies are needed to explain the behavior at low temperatures.