Structure Analysis of Water Ice Crystallites on NaCl(001), KCl(001), and CaF2(111) by Reflection High-Energy Electron Diffraction

Interactions of water with ionic-crystal surfaces, such as NaCl(001), KCl(001), and CaF2(111), were investigated in terms of epitaxial ice growth and hydration based on diffraction patterns of transmitted high-energy electrons. On the NaCl(001) and KCl(001) surfaces, domains of hexagonally arranged I-h(0001) and/or I-c(111) crystallites grow commensurately with the substrates. The bulk lattice constant of ice I-c is close to that of KCl (mismatch of 1.3%), so grains of ice I-c(001) can also be formed epitaxially on KCl(001) depending on its surface condition. On the CaF2(111) surface, ice is formed with the epitaxial relationship of, for example, [1 (1) over bar 00](Ih)// [1 (1) over barO](CaF2) because of good surface lattice-constant matching (1.2%). Results indicate that the structure of ice crystallites and their growth are controlled not only by the lateral lattice parameter but also by the microscopic surface structure of the ionic crystals. In fact, the crystallization pathways of amorphous solid water can be modified or quenched when NaCl is hydrated at defect sites. Results are compared to those of transition-metal and silicon substrates reported previously to gain insight into what controls nucleation and growth processes of water ice on solid surfaces.