Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe

TL;DR

This study uses a novel energy-resolved pair distribution function technique to reveal that cubic GeTe exhibits anisotropic atomic dynamics and ferroelectric fluctuations, challenging previous notions of static disorder in such materials.

Contribution

The paper introduces a new method to distinguish static disorder from atomic motions, revealing anisotropic dynamics in cubic GeTe that were previously misinterpreted.

Findings

GeTe's structure is crystalline at all temperatures.

Anisotropic anharmonic dynamics increase with temperature.

Spontaneous anisotropy is common in cubic IV-VI materials.

Abstract

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the $<$100$>$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.