We report the finding of unusual scale-dependent symmetry below the ferroelectric Curie temperature in theperovskite Pb(Zr-xTix)03 single crystals of morphotropic phase boundary compositions. The crystals of te-tragonal symmetry on submicrometer scale exhibit a macroscopic (optically determined) cubic symmetry. Thispeculiar optical isotropy is explained by the anomalously small size of tetragonal ferroelectric domains. Uponfurther cooling the crystals transform to the phase consisting of micrometer-sized domains of monoclinicsymmetry.
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