Polarization Rotation via a Monoclinic Phase in the Piezoelectric 92% ${\mathrm{PbZn}}_{1/3}{\mathrm{Nb}}_{2/3}{O}_3$-8% ${\mathrm{PbTiO}}_3$

The origin of ultrahigh piezoelectricity in the relaxor ferroelectric ${\mathrm{PbZn}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3-{\mathrm{PbTiO}}_3$ was studied with an electric field applied along the [001] direction. The zero-field rhombohedral $R$ phase starts to follow the direct polarization path to tetragonal symmetry via an intermediate monoclinic $M$ phase, but then jumps irreversibly to an alternate path involving a different type of monoclinic distortion. Details of the structure and domain configuration of this novel phase are described. This result suggests that there is a nearby $R-M$ phase boundary as found in the $\mathrm{Pb}(\mathrm{Ti},\mathrm{Zr}){\mathrm{O}}_3$ system.