  Recently, a new orthorhombic phase has been discovered in the ferroelectric
system Pb(Zn1/3Nb2/3)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 (MC-type) is in contrast to the
monoclinic MA-type symmetry recently identified at low temperatures in the
PbZr(1-x)TixO3 (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 behaviour (''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.
