We perform redundant x-ray diffraction versus temperature experiments in bulk transparent KTN. We find 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 volume expansion. 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.
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Acknowledgments. This research was supported in part by the Israel Science Foundation (Grant No. 1960/16). F.D. and E.D. were supported through the Attract project funded by the EC under Grant Agreement No. 777222, the Sapienza Ricerca di Ateneo 2018 project, and the PRIN 2017 PELM (Grant No. 20177PSCKT). L.L.P. was supported by the Italian Ministry for University and Research (MIUR) through the grant FFABR 2017. L.L.P., L.L., and R.S. thank P. Colombo for technical assistance in single-crystal x-ray diffraction experiments.
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