Electron-Beam-Assisted Formation of Simple-Cubic Silver

Accessing crystal phases under nonequilibrium conditions is a central challenge for expanding the landscape of materials. Simple-cubic (sc) structures are considered energetically unfavorable in elemental solids and have previously been confirmed only in polonium. Here, we report the direct observation of a previously unrecognized sc-Ag formed under far-from-equilibrium conditions. Using electron beam irradiation inside a high-resolution transmission electron microscope as a trigger, we induced the radiolytic decomposition of silver iodide (AgI) and tracked the in situ formation of silver crystals in real time. Continuous imaging revealed that sc-Ag formation strongly depends on the polymorph of the precursor and occurs selectively from metastable γ-AgI. The sc-Ag exhibited unusually high chemical reactivity, undergoing gradual oxidation into silver oxide under high-vacuum conditions where conventional fcc-Ag remains stable. These findings demonstrate that irreversible crystal formation pathways can kinetically stabilize crystal structures inaccessible under near-equilibrium conditions and extend a previously inaccessible structural limit of metallic bonding.