2-DIMENSIONAL PHASE-SEPARATION FOR CO ADSORBED ON SI(111)

We have used low energy electron microscopy to monitor the high-temper ature behavior of surface phases in the Co/Si(111) system at submonola yer coverages. We observe a reversible phase separation from a disorde red ''(1 x 1)'' phase to well-defined coexisting regions of ordered (7 x 7)-reconstructed and disordered phases. The transition temperature is depressed by nearly 200 degrees C by the addition of 0.1 monolayer of cobalt. The shape of the coverage-temperature phase boundary allows us to estimate the latent heat of the (7 x 7) order-disorder transiti on.