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

Experimental studies of the phase transitions in the adsorption system Li/M o(112) are presented. This system is a model system for highly anisotropic interactions. From measurements of the half-widths of the low-energy electr on diffraction spot profiles a phase diagram is derived for the whole submo nolayer region of coverage in the temperature range 100-500 K. The commensu rate 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 co verage, theta, of 0.25 monolayers (ML), seems to he truly long range ordere d, whereas the p(2x1) phase at theta=0.5 still contains domain boundaries e ven at the lowest temperature of 100 K. Both undergo temperature driven ord er-disorder phase transitions. In contrast, the incommensurate phases exist ing in the coverage range theta=0.66-0.90 form chains along the troughs, wh ich are only weakly coupled normal to the troughs of the substrate. These p hases exhibit two coverage-driven phase transitions from rectangular to obl ique units cells and back at critical coverages of 0.66 and 0.85, respectiv ely, and represent floating solids. As a function of temperature, they unde rgo a two-dimensional melting transition. Close to the critical coverages, the melting temperatures show a sharp drop below the temperature range acce ssible in our experiments. Both functional dependences of the angular devia tion from 90 degrees and of the melting temperature on coverage are in good agreement with a phenomenological theoretical model, assuming an instabili ty of the shear modulus of the adsorbate unit cell at the critical coverage s.