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PRINCIPAL PUBLICATION AND AUTHORS

Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe, S.A.J Kimber (a), J. Zhang (b), C.H Liang (c,d), G.G Guzmán-Verri (e,f), P.B. Littlewood (c,g), Y. Cheng (b), D.L. Abernathy (b), J.M. Hudspeth (h), Z-Z. Luo (i), M.G. Kanatzidis (i), T. Chatterji (j), A.J. Ramirez-Cuesta (b), S.J.L Billinge (k,l), Nat. Mater. 22, 311-315 (2023); https:/doi.org/10.1038/s41563-023-01483-7 (a) Université Bourgogne Franche-Comté, Université de Bourgogne, Nanosciences Department, ICB-Laboratoire Interdisciplinaire Carnot de Bourgogne, Bâtiment Sciences Mirande, Dijon (France) (b) Neutron Scattering Division, Oak Ridge National Laboratory, Tennessee (USA) (c) James Franck Institute, University of Chicago, Illinois (USA) (d) Pritzker School of Molecular Engineering, University of Chicago, Illinois (USA) (e) Centro de Investigación en Ciencia e Ingeniería de Materiales (CICIMA), Universidad de Costa Rica, San José (Costa Rica) (f) Escuela de Física, Universidad de Costa Rica, San José (Costa Rica) (g) Materials Science Division, Argonne National Laboratory, Argonne, Illinois (USA) (h) ESRF (i) Department of Chemistry, Northwestern University, Evanston, Illinois (USA) (j) Institut Laue-Langevin, Grenoble (France) (k) Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, New York (USA) (l) Department of Applied Physics and Applied Mathematics, Columbia University, New York (USA)

REFERENCES

[1] T. Chattopadhyay et al., J. Phys. C: Solid State Phys. 20, 1431- 1440 (1987). [2] P. Fons et al., Phys. Rev. B 82, 155209 (2010).

Fig. 38: a) Schematic of anisotropic strain in ferroelastic GeTe. b) Conduction band of GeTe, made of <100>c p-orbitals.

temperature. A 2D sketch is shown in Figure 38a, where distorting the central unit cell is shown to induce strong anisotropy. Along the x and y directions, long-range distortions are found, whereas along the xy direction, they rapidly vanish. This is exactly the effect seen in the data.

Why does this matter for thermoelectric and other energy applications? Thermoelectrics require low thermal and high electrical conductivity, the so-called phonon glass, electron crystal paradigm. This is hard to achieve, as the former usually requires disorder, while the latter is favoured by high crystallinity. These results show that, viewed along <111>c, GeTe looks instantaneously disordered, due to highly anharmonic atomic motions. However, at the same time, bonding is strengthened along the <100>c direction. As shown in Figure 38b, this corresponds to the direction of maximum overlap for the valence p-orbitals. Thermal and electrical conductivity are hence decoupled by the emergent anisotropy in GeTe, an effect that is highly general for energy materials.