  Relaxor ferroelectrics are applied in electronic devices such as actuators
and sonars. Morphotrophic phase boundaries (MPBs) with monoclinic structures
are known for their high piezoelectricity and electromechanical coupling
factors in solid solutions of PbTiO$_{3}$ and relaxor ferroelectrics
(Pb(Mg$_{1/3}$, Nb$_{2/3}$)O$_{3}$ or Pb(Zn$_{1/3}$, Nb$_{1/3}$)O$_{3}$).
However, the monoclinic structures related to polar nanosize domains (polar
nanoregions) exhibiting the relaxor properties of dielectric dispersion have
not been reported. Using transmission electron microscopy and synchrotron x-ray
scattering, we present the first observations of coexisting monoclinic
structures and polar nanoregions near the MPB in
Pb(Yb$_{1/2}$Nb$_{1/2}$)O$_{3}$-PbTiO$_{3}$. The polar nanoregions in this
material are randomly shaped, unlike the ferroelectric nanodomains of the
canonical relaxor Pb(Mg$_{1/3}$, Nb$_{1/3}$)O$_{3}$-PbTiO$_{1/3}$. Furthermore,
in situ observations reveal that the monoclinic polar nanoregions grow as the
temperature decreases. A pair-distribution function analysis reveals a mixture
of monoclinic Pm and Cm structures in the polar nanoregions without the
rhombohedral structure of other Pb-based relaxor solid solutions. Owing to the
peculiar nature of the coexistence of the relaxor property (polar nanoregions)
and high piezoelectricity (monoclinic structure), this material is expected as
a new platform for understanding relaxor ferroelectricity.
