Crossover from First-Order to Continuous Phase Transition Induced by Symmetry-Breaking Fields

The crossover from first order to continuous transition induced by a symmetry-breaking field $g$ in an $n=2\mathrm{}$ cubic model, for which a stable fixed point is not accessible, is studied. It is shown that unlike previously studied cases, the ($g$,$T$) phase diagram is rather complicated, exhibiting critical end points, tricritical, and fourth-order critical points. It is suggested that this phase diagram be studied experimentally in ferroelectric ${\mathrm{Tb}}_2$(${\mathrm{Mo}{\mathrm{O}}_4)}_3$ by applying uniaxial and shear stresses.