Electrically driven first-order phase transition of a 2D ionic crystal at the electrode/electrolyte interface

Liquid electrolytes adsorbed at the surface of metallic electrodes display a multitude of structures that can largely differ from the parent bulk system, both in terms of composition and local organization. In particular, the existence of disorder-order or order-order transitions has been increasingly reported in experimental and simulation studies, and the electrode potential identified as the corresponding driving force. The microscopic mechanisms and the stages of the process are, however, poorly understood, and the free energy variation during the transition remains insufficiently characterized. To fill this gap, we investigate the crystallization of the adsorbed layer in a prototypical molten salt-metal interface. We demonstrate that the transition from the disordered to ordered structures proceeds in two stages. Preordering effects are observed across a wide range of potentials, resulting in the formation of a polycrystalline structure on the electrode surface, before an abrupt ordering transition finally occurs. The preordering displays signature of a continuous transition. On the other hand, finite-size effects analysis proves the first-order character of the transition toward the monocrystalline state. Upon increasing the system size, a shift in the onset applied voltage is observed, accompanied by a dramatic increase in the free energy barrier. The latter reflects in the interfacial capacitance, which displays a peak that sharpens with increasing system size.