  A first-principles-derived approach is developed to study finite-temperature
properties of PbZr{1-x}Ti{x}O3 (PZT) solid solutions near the morphotropic
phase boundary (MPB). Structural and piezoelectric predictions are in excellent
agreement with experimental data and direct first-principles results. A
low-temperature monoclinic phase is confirmed to exist, and is demonstrated to
act as a bridge between the well-known tetragonal and rhombohedral phases
delimiting the MPB. A successful explanation for the large piezoelectricity
found in PZT ceramics is also provided.
