Oxygen intercalation in the perovskite superconductorYBa2Cu3O6+x

We have studied superconductivity and the thermodynamics of oxygen insertion in the oxygen intercalation system $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. The superconducting transition temperature decreases as $x$ decreases, and the transition is sharp at $T_c=90\mathrm{}$ K near $x=1\mathrm{}$ and at 55 K near $x=0.5\mathrm{}$. 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.