Phase transitions in argon films.

We present heat-capacity data detailing the evolution of the first six layers of argon adsorbed on graphite foam. The second and third layers have liquid-solid-gas triple points similar to the first layer. These layers exhibit a phase diagram consisting of two-dimensional solid, liquid, and gas phases on top of a solid film. Above the temperatures of the individual-layer triple points, the melting transition for each layer appears to be first order, and the first two layers show evidence of registry transitions prior to melting. For films of a total thickness of about four layers and up, the melting of each of the first three layers occurs at temperatures above the bulk triple point, as reported by Zhu and Dash [Phys. Rev. B 38, 11673 (1988)]. Our results confirm those of an ellipsometry study [H. S. Youn and G. B. Hess, Phys. Rev. Lett. 64, 918 (1990)) that found layering transitions above what were believed to be the layering critical-point temperatures. We observe heat-capacity peaks identified with these transitions and with melting transitions that join them with the low-temperature layering transitions. A phase diagram based on these data may represent the signature of a preroughening transition and a disordered flat phase in the bulk-crystal interface.