<p>The existence and stability of phases in ferroelectric solid solutions under hydrostatic pressure are studied by using a phenomenological theory. Phase diagrams, with the consideration of phase coexistence, are calculated based on the criterion of the Hessian matrix at various temperatures. Results have shown that rhombohedral (R) phase and tetragonal (T) phase at morphotropic phase boundary (MPB) exhibit the highest stability in resisting the depolarization caused by hydrostatic pressure. Enhanced piezoelectricity exists near the phase boundaries, where ferroelectric–paraelectric phase transition occurs. Besides, average piezoelectric coefficient <i>d</i>₃₃ of domains with the coexistence of R phase and T phase is relative high at MPB due to the sharp increase of piezoelectricity close to the phase transition.</p>