Phase transitions in the adsorption system Li/Mo(112)

Experimental studies of the phase transitions in the adsorption system Li/Mo(112) are presented. This system is a model system for highly anisotropic interactions. From measurements of the half-widths of the low-energy electron diffraction spot profiles a phase diagram is derived for the whole submonolayer region of coverage in the temperature range 100-500 K. The commensurate low-coverage phases below theta=0.6 form chains normal to the troughs of the substrate. The commensurate p(4X1) phase, which is completed at a coverage, theta, of 0.25 monolayers (ML), seems to he truly long range ordered, whereas the p(2x1) phase at theta=0.5 still contains domain boundaries even at the lowest temperature of 100 K. Both undergo temperature driven order-disorder phase transitions. In contrast, the incommensurate phases existing in the coverage range theta=0.66-0.90 form chains along the troughs, which are only weakly coupled normal to the troughs of the substrate. These phases exhibit two coverage-driven phase transitions from rectangular to oblique units cells and back at critical coverages of 0.66 and 0.85, respectively, and represent floating solids. As a function of temperature, they undergo a two-dimensional melting transition. Close to the critical coverages, the melting temperatures show a sharp drop below the temperature range accessible in our experiments. Both functional dependences of the angular deviation from 90 degrees and of the melting temperature on coverage are in good agreement with a phenomenological theoretical model, assuming an instability of the shear modulus of the adsorbate unit cell at the critical coverages.