Orientational phase transitions in a lithium overlayer on Mo(112)

We study the unusual behavior of the incommensurate lattice formed Ly Li on the anisotropic trough-like surface of Mo(112). Starting at a coverage of 0.66, we found a continuous transformation of a rectangular into an oblique unit cell. This oblique cell transforms back into a rectangular cell at co verages higher than 0.85. Both transitions are explained by an instability in the shear modulus of the layers at the respective critical coverages. It leads to the development of smectic-like fluctuations and finally to a mel ting transition. For the oblique phase below melting, long-range correlatio ns along the troughs, but correlations over less than 10 lattice constants normal to the troughs are observed in experiment. The experimental results are in reasonable agreement with the theoretical model proposed.