Whereas low-temperature ferroelectrics have a well understood ordered spatial dipole arrangement, the fate of these dipoles in paraelectric phases remains poorly understood. This is studied here as an energy minimization problem using both static and
molecular dynamic (MD) density functional theory (DFT). We find that considering the non-thermal internal energy already reveals the formation of a distribution of static local displacements that (i) mimic the symmetries of the low temperature phases, while (ii) being the precursors of what high temperature DFT MD finds as thermal motifs.
The prototypical phase change material GeTe shows an enigmatic phase transition at Tc ca. 650 K from rhombohedral (R3m) to cubic (Fm-3m) symmetry. While local probes see little change in bonding, in contrast, average structure probes imply a displaci
ve transition. Here we use high energy X-ray scattering to develop a model consistent with both the local and average structure pictures. We detect a correlation length for domains of the R3m structure which shows power law decay upon heating. Unlike a classical soft mode, it saturates at ca. 20 {AA} above Tc. These nanoclusters are too small to be observed by standard diffraction techniques, yet contain the same local motif as the room temperature structure, explaining previous discrepancies. Finally, a careful analysis of the pair distribution functions implies that the 0.6 % negative thermal expansion (NTE) at the R3m -Fm-3m transition is associated with the loss of coherence between these domains.
