First-order phase transition between orthorhombic phases in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{z}}$

We investigate the effects of elastic interactions and the associated orthorhombic distortion on the structural phase transitions in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{z}}$. Starting from a two-dimensional Ising model previously demonstrated to describe accurately the high-temperature phase boundaries, we show by means of Landau theory that the square-rectangular symmetry breaking may lead to a first-order transition between the orthorhombic single- and double-cell phases at low temperatures. This explains experimental observations of phase separation between orthorhombic phases, while retaining the simplicity and predictive power of the original model.