  We report the finding of unusual scale-dependent symmetry below the
ferroelectric Curie temperature in the perovskite Pb(Zr1-xTix)O3 single
crystals of morphotropic phase boundary compositions. The crystals of
tetragonal symmetry (from x-ray diffraction experiments) on sub-micrometer
scale exhibit a macroscopic (optically determined) cubic symmetry. This
peculiar optical isotropy is explained by the anomalously small size of
tetragonal ferroelectric domains. Upon further cooling the crystals transform
to the phase consisting of micrometer-sized domains of monoclinic Cm symmetry.
