Recently, a new orthorhombic phase has been discovered in the ferroelectric system $\left(1\mathrm{}-x\right)\mathrm{Pb}\left({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3-x{\mathrm{PbTiO}}_3$ $(\mathrm{PZN}-x\mathrm{PT})$ 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 $\mathrm{Pb}\left({\mathrm{Zr}}_{1-x}{\mathrm{Ti}}_x\right){\mathrm{O}}_3$ (PZT) system over a triangle-shaped region of the phase diagram in the range $x=0.46\mathrm{}–\mathrm{0.52.}$ To further characterize this relaxor-type system, neutron inelastic scattering measurements have also been performed on a crystal of $\mathrm{PZN}-x\mathrm{PT}$ with $x=15\%.\mathrm{}$ 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.

• Received 17 August 2001

DOI:https://doi.org/10.1103/PhysRevB.65.144101