Monoclinic phase in the relaxor-based piezoelectric/ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 system

A ferroelectric monoclinic phase of space group Cm (MA type) has been discovered in 0.65 Pb(Mg1/3Nb2/3)O3-0.35 PbTiO3 by means of high-resolution synchrotron x-ray diffraction. It appears at room temperature in a single crystal previously poled under an electric field of 43 kV/cm applied along the pseudocubic [001] direction, in the region of the phase diagram around the morphotropic phase boundary between the rhombohedral (R3m) and the tetragonal (P4mm) phases. The monoclinic phase has lattice parameters a=5.692 A, b=5.679 A, c=4.050 A, and β=90.15°, with the bm axis oriented along the pseudocubic [110] direction. It is similar to the monoclinic phase observed in PbZr1-xTixO3, but different from that recently found in Pb(Zn1/3Nb2/3)O3-PbTiO3, which is of space group Pm (MC type).