M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

5 4 H I G H L I G H T S 2 0 2 3 I

X-rays and neutrons unearth the origin of local atomic disorder in energy materials

The cubic thermoelectric GeTe is claimed to be disordered. X-ray and neutron pair distribution function experiments show that all disorder is dynamic (i.e., not symmetry-breaking), and that GeTe is perfectly crystalline. The X-ray data are reinterpreted with a theory of anisotropic strains, explaining how high electrical and low thermal conductivity are needed for thermoelectricity to co- exist.

The most promising photovoltaics, thermoelectrics and ferroelectrics are thought to be highly disordered, which improves their performance. However, the nanometre length-scale is hard to characterise, making it difficult to link the atomic structure to macroscopic properties. So- called total scattering measurements, which produce the pair distribution function (PDF), are one solution. The PDF is a real-space histogram of interatomic distances, produced by Fourier transforming high-energy X-ray powder scattering data. PDFs for many energy materials are consistent with local disorder, due to the presence of peak splittings and asymmetry in the first few coordination shells.

The IV-VI semiconductor GeTe has potential thermoelectric and phase-change memory applications. At high temperature, X-ray diffraction data suggest a highly crystalline cubic structure [1]. However, local probes such as extended X-ray absorption fine structure (EXAFS) spectroscopy and PDF claim symmetry-breaking disorder instead [2].

In this work, temperature-dependent PDF data were collected at beamline ID15B, using 87 keV X-rays and a high dynamic range Mar345 image plate. Until the highest temperatures (Figure 37), the nearest-neighbour Ge-Te peak at 3 Å remains split/asymmetric. This is incompatible with the average cubic symmetry and can only be fit by disordering the Ge atomic positions (Figure 37, inset). Other features, such as an anomalous sharpening of the <100>c Ge-Ge/Te-Te peak at 6 Å, cannot even be fit using the disordered model.

Fig. 37: Fit to the high-temperature X-ray PDF of GeTe using a disordered model. The split Ge sites are

shown in the inset.

Using spallation neutrons, it was possible to develop an energy-sensitive neutron PDF technique. Although of limited resolution, this showed that the anomalous 3 and 6 Å features in the PDF arise from dynamics, and hence are only symmetry-breaking in the most trivial definition. Ab initio molecular dynamics simulations were in complete agreement.

How then to reconcile the conflicting observations for GeTe? The full, temperature-dependent X-ray PDF dataset shows that the 6 Å peak (representing <100>c correlations) stiffens on heating, while correlations along the <111>c direction rapidly weaken. A theoretical model that links local ferroelectric fluctuations to lattice strain was thus consulted. At lower temperatures, GeTe undergoes a rhombohedral distortion. The PDF results show that fluctuations of this effect are present at high