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PRINCIPAL PUBLICATION AND AUTHORS

Mechanochemistry and the evolution of ionic bonds in dense silver iodide, J. Li (a), Y. Geng (a), Z. Xu (a), P. Zhang (b), G. Garbarino (c), M. Miao (d), Q. Hu (e), X. Wang (a), J. Am. Chem. Soc. Au. 3, 2, 402-408 (2023); https:/doi.org/10.1021/jacsau.2c00550 (a) School of Physics and Electronic Information, Yantai University, Yantai (China) (b) School of Physics and Electronic Engineering, Linyi University, Linyi (China) (c) ESRF (d) Department of Chemistry and Biochemistry, California State University, Northridge, California (USA) (e) Center for High Pressure Science and Technology Advanced Research, Beijing (China)

In conclusion, the results reveal the sophisticated chemistry of simple inorganic compounds under extreme conditions, and the combined computational and experimental approach may aid in the design of new paths for chemical reactions.

Fig. 93: Phase diagram of the Ag−I compound. Inset figures are the trajectories of the supercell structure taken from molecular dynamics (MD) simulation, with ordering solid in AgI-III, superionic solid in AgI-V and decomposed AgI at the end of MD simulation.

A high-pressure experiment was performed at beamline ID15B, where a pair of diamond anvils was used to compress a tiny chip of silver iodide (AgI) to extremely high pressures, equivalent to 420 000 atmospheres, and high-energy, angular-dispersive X-ray diffraction (XRD) experiments were carried out to measure the structure of the AgI sample under such pressurised conditions. Along the pressurisation trajectory, the disappearance of solid AgI and the emergence of elemental Ag and I was clearly observed in the diffraction patterns (Figure 92). The data show that the external mechanical stress weakens the Ag−I ionic bonds, making AgI chemically unstable. Once the chemical limit of AgI is reached and overcome by increasing pressure, the sample decomposes into elementary silver and iodine due to the collapse of ionicity. Each bond is seen to have its own chemical limit.

This experiment was pioneered by state-of-the-art computational modelling in which the evolution of Ag-I bonding and its properties were calculated at high-pressures. Using a structural searching algorithm implemented by the CALYPSO (crystal structure analysis by particle-swarm optimisation) software program, it was possible to derive the ground state of compounds at relevant pressures. Calculating the free energy of Ag, I and the stable AgI phases gave a predicted decomposition pressure, which was consistent with experimental data. The phase diagram of AgI was thus extended and updated, as shown in Figure 93. Computer modelling further predicts that such pressure-induced chemistry should also occur in other ionic solids, such as silver chloride and silver bromide, but at even higher pressures.