Evidence for orientational glass states in two dimensions: Monolayers of C2Cl2F4 and C2ClF5 physisorbed on graphite.

Abstract

Monolayers of ${\mathrm{C}}_{2}{\mathrm{Cl}}_{2}{\mathrm{F}}_{4}$ and ${\mathrm{C}}_{2}{\mathrm{ClF}}_{5}$ physisorbed on exfoliated graphite have been investigated by x-ray diffraction and heat capacity measurements. Both systems crystallize in a triangular lattice with dynamic orientational disorder at higher temperatures and freeze into an orientational glass state at low temperature. For ${\mathrm{C}}_{2}{\mathrm{ClF}}_{5}$ the orientational glass state is observed only in the extended monolayer region, whereas the monolayer shows long-range orientational ordering in a three-sublattice structure.