C L E A N E N E R G Y T R A N S I T I O N A N D S U S T A I N A B L E T E C H N O L O G I E S

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

1 1 4 H I G H L I G H T S 2 0 2 3 I

Chemistry under sheer force monitored by X-ray diffraction

High-pressure X-ray diffraction experiments have demonstrated that extreme pressure can destroy the ionic bonds in silver iodide, transforming it to elemental silver and iodine. The ability to change the chemistry of materials with mechanical force is an ideal approach for the synthesis of new materials.

The use of mechanical force to prompt chemical reactions is well known. In 1820, when Michael Faraday used trituration (grinding) in a mortar to induce the mechanical reduction of silver chloride with zinc, tin, iron and copper, he gave us probably the first experimental example of mechanochemistry. Mechanochemistry directly converts mechanical energy to chemical energy, or chemical potential. Mechanical milling is the most common way of performing mechanochemistry, but the force applied is limited, so many materials are still chemically stable under such relatively gentle pressures. But what happens if the applied pressure is high enough? How can we better understand ionic bonding at the critical point of chemical reaction?

In this work, the mechanochemistry of silver iodide (AgI) was studied at extremely high pressures. At relatively low pressures, AgI exists in various solid phases. At pressures above about 0.5 GPa, AgI has an ionic structure like rock salt and, above about 10 GPa, it transitions to a superionic solid. In its superionic phase, the iodine atoms behave like a liquid, flowing through the silver atoms, which remain solid. This exotic phase makes it a promising candidate for making battery electrolytes.

Fig. 92: X-ray diffraction (XRD) patterns of compressed AgI. a) Evolution of XRD patterns from 6.1 to 41.5 GPa. b) FCC AgI-III at 10.9 GPa. c) Mixture of AgI, Ag, and I at 11.9 GPa. AgI sample partially decomposes and the peaks of FCC-Ag and Immm-type

I appear. Inset figures in (b) and (c) are caked two-dimensional diffraction patterns, whose y-axes are the azimuth angle.