Phase diagram of the relaxor ferroelectric (1-x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃

Recently, a new orthorhombic phase has been discovered in the ferroelectric system (1-x)Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃ (PZN-xPT) for x=9%, and for x=8% after the application of an electric field. In the present work, synchrotron x-ray measurements have been extended to higher concentrations 10%≤x≤15%. The orthorhombic phase was observed for x=10%, but, surprisingly, for x≥11% only a tetragonal phase was found down to 20 K. The orthorhombic phase thus exists only in a narrow concentration range with near-vertical phase boundaries on both sides. This orthorhombic symmetry (M_C type) is in contrast to the monoclinic M_A-type symmetry recently identified at low temperatures in the Pb(Zr_1-xTi_x)O₃ (PZT) system over a triangle-shaped region of the phase diagram in the range x=0.46-0.52. To further characterize this relaxor-type system, neutron inelastic scattering measurements have also been performed on a crystal of PZN-xPT with x=15%. The anomalous soft-phonon behavior ("waterfall" effect) previously observed for x=0% and 8% is clearly observed for the 15% crystal, which indicates that the presence of polar nanoregions extends to large values of x.