Model for the order-disorder transition inYBa2Cu3Oy

Abstract

We present a model for the tetragonal-to-orthorhombic structural phase transition in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$. We assume the transition involves only the oxygen atoms which form the Cu-O chains in this compound and use an Ising-like Hamiltonian on a square lattice with nearest- and next-nearest-neighbor interactions to describe the system. Comparison of theoretical predictions with experimental results for site occupancy as a function of temperature and oxygen partial pressure shows good agreement.