In contrast to standard thermodynamic models, we observe phase coexistence
over an extended temperature range at a first-order surface phase transitio
n. We have measured the domain evolution of the Si(111)-(7 X 7) to (1 X 1)p
hase transition with temperature, using low-energy electron microscopy. Com
parison with detailed, quantitative theoretical predictions shows that coex
istence is due to long-range elastic and electrostatic domain interactions.
Phase coexistence is predicted to be a universal feature of surface phase
transitions.