Higher-Order Commensurate Phase of CF3Cl Physisorbed on Graphite

X-ray diffraction experiments have been carried out on monolayers and submonolayers of the polar molecule C${\mathrm{F}}_3$Cl physisorbed on the (001) planes of exfoliated graphite. The coverage-temperature phase diagram shows three crystalline phases: an incommensurate phase, a commensurate 2×2 phase, and a higher-order commensurate phase. For the latter phase an arrangement of ferroelectric chains with a stacking sequence $p1\mathrm{}$ is proposed which belongs to the oblique plane group $\mathrm{ABCDA}\dots$. Surprisingly the lattice parameters exhibit hexagonal relations ($\sqrt{73}\times \sqrt{73}R5.8\mathrm{}°$).