Potassium-based perovskites: The key role of polar nanoregions in their cubic-to-tetragonal phase transition

An experimental study on different samples of disordered potassium-based perovskite compounds, a class of single crystals hosting polar nanoregions (PNRs), is carried out. By combining Raman spectroscopy, dielectric response, and Fröhlich entropy estimation, we investigate the specific behavior straddling the nominal cubic-to-tetragonal long-range phase transition. Our results demonstrate that the arising of the phase transition is driven by the temperature evolution of the PNRs, whose behavior is correlated to several macroscopic features. In particular, the onset of the phase transition occurs when PNRs display a percolation process, which is highlighted by the sudden arising of a specific Raman mode.