Atomic resolution for non-equilibrium structures in the steady state and for structural transformations at the interface between NaCl(c) and water

Observations of both nanoscale and microscale images were carried out on a NaCl(001) surface in water using atomic force microscopy (AFM). The observed higher density of steps and the observed convex and concave steps in water result from the forward motion of monatomic steps and crystal growth of the so-called step-flow mode. The non-equilibrium structure having a unit cell of C(2 x 2) structure has been observed for the first time on terraces. Since every Naf on terraces is located at the same height as Cl-, each Na+ as well as each Cl- have been observed as protrusions by AFM. The metastable configuration at the interface between the NaCl(001) surface and water was transformed, at steps, into the equilibrium structure of NaCl(c), about 3 h after the NaCl(001) surface had been covered with its own saturated water. After about 30 min, however, Na+ still remains at the same height as Cl- at steps, or sodium ions leave the surface at steps. The reason why the forward motion of monatomic steps takes place at the interface between the NaCl(001) surface and water is the decrease of about 0.05 nm in the distance of like nearest neighbours in the non-equilibrium structure having a unit cell of C(2 x 2) structure on terraces after its structural transformation to the equilibrium structure of NaCl(c) with the aid of water molecules.