Impurity-induced changes of overlayer symmetry and of phase transitions

We show in a Low Energy Electron Diffraction (LEED) experiment that small c oncentrations of oxygen atoms (2 to 12% of a monolayer) added to half a mon olayer of Sr on Mo(112), ordered in a c(2 x 2) structure, do not destroy or der, but introduce new correlations with different translational symmetry. Thus incommensurate structures with domain walls are formed and the c(2 x 2 ) is finally transformed into a p(2 x 1) structure. Temperature-driven tran sitions at constant average densities from incommensurate to short-range or der and finally to long-range order at high temperature can be described by simultaneous melting and domain wall evaporation.