Observation of an exotic lattice structure in the transparent KTa1-xNbx O3 perovskite supercrystal

We perform redundant X-ray diraction versus temperature experiments in bulk transparent KTN. We nd a violation of the standard perovskite cubic-to-tetragonal symmetry-breaking at the Curie point in the form of an orthorhombic cell distortion. The lattice distortion spans coherently macroscopic volumes of the sample and is characterized by a negative-to-zero thermal volumeexpansion. Dielectric measurements and calorimetry indicate that the anomalous behavior is not associated to a distinct thermodynamic phase. The comparison of linear and nonlinear optical scattering experiments with structural data suggests that the lattice distortion is a consequence of strain relaxation to a 3D superlattice of polarization vortices.