Evidence for Orientational Glass States in Two Dimensions: Monolayers of $C_2{\mathrm{Cl}}_2{F}_4$ and $C_2{\mathrm{ClF}}_5$ Physisorbed on Graphite

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.