Dynamic crystallography reveals spontaneous anisotropy in thermoelectric GeTe

Peak performance in energy materials is associated with high symmetry and atomic fluctuations. Indeed cubic thermoelectrics, phase change memories, photovoltaics etc are often claimed to be highly disordered or 'intrinsically nanostructured'. In GeTe and related IV-VI compounds, this is said to provide the low thermal and high electronic conductivities needed for thermoelectric applications. However, a general theory linking these empirical observations to transport properties is lacking. Since conventional crystallography cannot distinguish between disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function (vsPDF) technique. We show that the atomic lattice of GeTe is nearly perfectly crystalline at all temperatures, but hosts remarkably large and anisotropic dynamics. Along the <111>_c direction, motion is almost uncorrelated, however, interactions are strengthened along the <100>c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model which couples polarisation fluctuations through long-range elastic interactions. We propose that the liquid-like optical phonons screen charge carriers from scattering, as found in the lead halide hybrid perovksites. This explains the contradictory properties of c-GeTe, which conducts heat like a glass, yet electricity like a crystal. Coupling of the resultant ferroelectric large polarons to elastic anisotropy is likely ubiquitous in the IV-VI materials, ferroelectrics and photovoltaics, allowing strain engineering of novel optoelectronic properties