Key role of polar nanoregions in the cubic-to-tetragonal phase transition of potassium-based perovskites

We investigate samples of compositionally disordered potassium-based perovskite single crystals with different composition and stoichiometry. The dielectric and Raman response is inspected over the nominal cubic-to-tetragonal long-range phase transition. The comparison between results shows that the occurrence of the phase transition is controlled by the temperature evolution of polar nanoregions (PNRs). We are able to correlate PNR order, formation, percolation, and freezing to the characteristic temperatures of each macroscopic crystal. The onset of the phase transition is found to occur when PNRs undergo a percolative ordering process, in correlation to the sudden arising of a specific high-frequency Raman mode.