M A T T E R A T E X T R E M E S

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

2 6 H I G H L I G H T S 2 0 2 1 I

is considerably more anisotropic than the underlying Sn crystal structure, at the same pressure. Hence on melting, templating to highly anisotropic local polyhedral units hinders quenching of the new isotropic cubic phase. This is in sharp contrast to interdiffusion upon heating purely in the solid state, where the structural environment is less anisotropic. Solid-state reaction is also assisted by the low activation barrier for diffusion of Sn, where diffusion starts as low as 391 K.

Establishing that this Im-3m solid solution is formed without adherence to conventional formation criteria and routes to synthesis creates fertile avenues for new materials development. Unconventional synthetic origins are a source of the structural novelty and electronic appeal of the resulting materials, as exemplified by new Ge-Sn solid solutions. Novel b-GeSn is electronically appealing due to its mixed covalent and metallic bonding sourced from unlike semiconducting c-Ge and metallic t-Sn (Figure 13a). Novel bulk c-GeSn (Figure 13b) formed from b-GeSn is optoelectronically appealing, because replacing Si with Ge and Sn may improve inefficient light-electricity interconversion.

PRINCIPAL PUBLICATIONS AND AUTHORS

An Unexpected Cubic Symmetry in Group IV Alloys Prepared Using Pressure and Temperature, G. Serghiou (a), H.J. Reichmann (b), N. Odling (c), K. Spektor (d), A. Pakhomova (e), W.A. Crichton (d), Z. Konôpková (e), Angew. Chem. Int. Ed. 60, 16, 9009-9014 (2021); https:/doi.org/10.1002/anie.202016179, Unconventional Route to High-Pressure and -Temperature Synthesis of GeSn Solid Solutions, G. Serghiou (a), N. Odling (c), H.J. Reichmann (b), K. Spektor (d), W.A. Crichton (d), G. Garbarino (d), M. Mezouar (d), A. Pakhomova (e), J. Am. Chem. Soc. 143, 21, 7920-7924 (2021); https:/doi.org/10.1021/jacs.1c03765 (a) School of Engineering, University of Edinburgh (UK) (b) Deutsches GeoforschungsZentrum, GFZ, Potsdam (Germany) (c) School of Geosciences, University of Edinburgh (UK) (d) ESRF (e) Deutsches Elektronen-Synchrotron, DESY, Hamburg (Germany)

Fig. 13: a) Evolution of X-ray diffraction patterns at ID06-LVP upon heating of a c-Ge (Fd-3m) and t-Sn (I4/

mmm) mixture at 9.9 GPa and formation of a tetragonal b-Ge0.44Sn0.56 structure with I41/amd symmetry.

b) Cubic diamond-structured Ge0.68Sn0.32 (Fd-3m) recovered at 1 atm and 298 K characterised on ID27.

This phase has the same symmetry as c-Si.