Crystallographic tilting during heteroepitaxial growth of rubidium iodide on mica

Heteroepitaxial deposits of RbI on mica were prepared by the evaporation of aqueous solutions under controlled conditions. Using four-circle x-ray dif fractometer measurements to determine the orientation of the alkali halide lattice relative to the mica substrate, we find that the RbI [111] axis is often tilted relative to the substrate normal, along specific in-plane azim uths. Crystallographic tilting is a well-known phenomenon during heteroepit axial growth of semiconductors and metals, but our results are somewhat unu sual, since they involve the growth of an ionic crystal on mica. It is gene rally acknowledged that tilting is driven by epitaxial misfit accommodation , and for a net tilt to occur the interfacial system requires a symmetry-br eaking component. In the vast majority of previous studies the broken symme try is due to substrate miscut, but miscut is entirely absent in the case o f cleaved mica substrates. In order to explain our results, we show that th e tilting of RbI on mica is made possible by the inherent threefold symmetr y of the [111] axis in fcc crystals, and we also show that the observed til t angles are consistent with the lattice parameters of Rbl and mica.