S C

IE N

T IF

IC H

IG H

LI G

H T

S M

A T

T E

R A

T E

X T

R E

M E

S

2 5 I H I G H L I G H T S 2 0 2 1

Unconventional routes to synthesis and unexpected new cubic symmetry in group IV alloys A new cubic group IVA alloy, Ge-Sn, a material actively investigated for optoelectronic applications to overcome conventional cubic Si s deficiencies, was synthesised. Using high-pressure, high-temperature experiments, reactivity barriers between Ge and Sn were removed. For this, conventional routes to synthesis were neither followed nor required.

The group IVA column of elements (C, Si, Ge, Sn, Pb) occupies a prominent position in the periodic table. The cubic diamond symmetry (Fd-3m) is a principal contributor to this position, as exemplified by super-hard cubic C and through cubic Si, the material driver behind the electronics industry. Using high-pressure, high-temperature multi- anvil experiments coupled with in-situ X-ray diffraction measurements at beamline ID06-LVP, an unexpected new cubic group IVA alloy, body-centred cubic (Im-3m) Ge-Sn was prepared.

Ge-Sn is one of the most actively investigated systems for direct band gap formation as opposed to Si s indirect band-gap. However, Ge and Sn do not react in the bulk at ambient pressure; hence, investigations have been largely confined to thin films. A Ge-Sn solid solution was synthesised upon heating Ge and Sn at pressures from 13 to 28 GPa using multi-anvil methods at beamline ID06-LVP and double-sided diamond anvil laser-heating at DESY, both characterised in situ with X-ray diffraction measurements, making it possible to access regions

in the new phase diagram where barriers to reactivity were removed. The new material substantially enriches the seminal group IVA alloy materials landscape by introducing an eightfold coordinated cubic symmetry (Im-3m) (Figure 12a), which markedly expands on the conventional tetrahedrally coordinated cubic one (Fd-3m).

Several unusual aspects are associated with the synthesis and structure of this new cubic group IVA solid solution. Firstly, Ge and Sn have unlike crystal structures and significantly different atomic radii in the 13-28 GPa range, so shouldn t form a solid solution. Secondly, Ge does not adopt the Im-3m symmetry under any conditions, and thus should hinder alloy formation with this symmetry. However, upon alloying with Sn, solid solution formation is enhanced, with the formation pressure of pure Im-3m lowered by 500,000 atmospheres from that at which elemental Sn can adopt this symmetry as a single phase. Thirdly, melting, normally an integral part of synthesis to promote homogenisation, reactivity and crystal formation, is detrimental here, in contrast to pure solid-state reaction, which leads to high-quality crystal formation.

The explanation for the first unusual aspect is that the atomic radii within the coordination polyhedra of the reactant elements are not the appropriate compatibility manometer. Instead, it is the radii that the elements will adopt in the coordination polyhedra of the new solid solution (Figure 12b). For the second, stabilisation of the new solid solution is favoured by the entropic contribution of Ge. Temperature also contributes favourably to product formation with respect to relative specific volumes of product to reactants. Thirdly, the local Sn liquid structure

Fig. 12: a) Evolution of X-ray diffraction patterns at ID06-LVP upon heating of a b-Ge (I41/amd) and t-Sn (I4/mmm) mixture at 15.9 GPa and formation of a bcc structure with Im-3m symmetry. b) Ge-Sn atomic radii ratio (%) pressure dependence {(Sn radius-Ge radius)/Sn radius} x 100. The red line is the ratio above which solid solutions are not favoured.