Stability of the monoclinic phase in the ferroelectric perovskite PbZr1-xTixO3

Recent structural studies of ferroelectric PbZr1-xTixO3 (PZT) with x=0.48, have revealed a monoclinic phase in the vicinity of the morphotropic phase boundary (MPB), previously regarded as the boundary separating the rhombohedral and tetragonal regions of the PZT phase diagram. In the present paper, the stability region of all three phases has been established from high-resolution synchrotron x-ray powder-diffraction measurements on a series of highly homogenous samples with 0.42 less than or equal tox less than or equal to0.52. At 20 K, the monoclinic phase is stable in the range 0.46 less than or equal tox less than or equal to0.51, and this range narrows as the temperature is increased. A first-order phase transition from tetragonal to rhombohedral symmetry is observed only for x=0.45. The MPB, therefore, corresponds not to the tegragonal-rhombohedral phase boundary, but instead to the boundary between the tetragonal and monoclinic phases for 0.46 less than or equal tox less than or equal to0.51. This result provides important insight into the close relationship between the monoclinic phase and the striking piezoelectric properties of PZT; in particular, investigations of poled samples have shown that the monoclinic distortion is the origin of the unusually high piezoelectric response of PZT.