First-order orientational-disordering transition on the (111) surface of ${\mathrm{C}}_{60}$

The temperature dependences of the triplet exciton, of the vibrational modes, and of the low-energy diffraction pattern of the (111) surface of well-ordered ${\mathrm{C}}_{60}$ films indicate that this surface undergoes a first-order orientational order-disorder phase transition at 225±3 K, well below the temperature of the corresponding phase transition in the bulk of the same samples, which we find to be 260 K. The bulk phase transition is therefore initiated by the surface. To our knowledge, this is the first example of a surface orientational phase transition, a phenomenon that should occur on several other molecular crystal surfaces. © 1996 The American Physical Society.