Phase diagram and phase transitions of monolayer and bilayer CF4 on graphite.

Abstract

The phase diagram of ${\mathrm{CF}}_{4}$ adsorbed on graphite in the monolayer and bilayer region is mapped out in an ac heat-capacity study. In the monolayer region, the resultant phase diagram shows the existence of five different solid phases. The melting transition can proceed from the (2\ifmmode\times\else\texttimes\fi{}2) commensurate, I(S) (an incommensurate phase), and the hexagonal incommensurate (HI) phases. Strongly-first-order melting is found from the (2\ifmmode\times\else\texttimes\fi{}2) and the I(S) phases. In contrast, a sharp and small heat-capacity peak followed by a broad anomaly, similar to the melting of Ar on graphite in the submonolayer region, is observed at the melting of the HI phase. In the bilayer region, four different regions of coexisting solid phases are found. The melting of the bilayer solid occurs at a triple point at 89.05 K. Our data also suggest a bilayer critical point at 93 K.