2D ISING CRITICAL-BEHAVIOR AT AN ISOMORPHOUS STRUCTURAL PHASE-TRANSITION IN C2F6 MONOLAYERS PHYSISORBED ON GRAPHITE

C2F6 monolayers physisorbed on graphite have been investigated by heat capacity and x-ray diffraction measurements. With increasing temperat ure, the adsorbate shows a sequence of three 2D solid phases, all of t hem having a triangular lattice, followed by the 2D fluid stale. One o f the transitions belongs to the universality class of the 2D Ising mo del and is of a new orientational order-disorder disorder type. [S0031 -9007(98)05379-4].