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

Abstract

Experimental studies of the phase transitions in the adsorption system Li/Mo͑112͒ 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 ϭ0.6 form chains normal to the troughs of the substrate. The commensurate p(4ϫ1) phase, which is completed at a coverage, , of 0.25 monolayers ͑ML͒, seems to be truly long range ordered, whereas the p(2ϫ1) phase at ϭ0.5 still contains domain boundaries even at the lowest temperature of 100 K. Both undergo temperaturedriven order-disorder phase transitions. In contrast, the incommensurate phases existing in the coverage range ϭ0.66-0.90 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°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.

First-principles calculation of Li adatom structures on the Mo(112) surface

“…First, faint spots of the p(4ϫ1) phase appear already at a coverage ⌰ ϭ0.1. 7 Our calculation shows a stable p(8ϫ1) structure at ⌰ϭ0.125, which is characterized by the same binding as the p(4ϫ1) structure for ⌰ϭ1/4. The (8ϫ1) pattern is not observed experimentally.…”

“…8͒ is found. 7 Our calculations show that this structure is relatively stable ͑Table III͒ although the magnitude of the adsorbate-substrate potential modifies slightly the equidistant spacing between adatoms. A finite temperature (TϾ100 K) maintained during experiment may slightly activate Li adatoms, thus allowing them to keep uniform spacing in a row.…”

“…The latter is comparable to the size of fluctuations in activation energy for self-diffusion at close packed ͑111͒ surface of Al and Cu. For coverages ⌰Ͼ0.5, experiment 7 suggests the appearance of stable incommensurate Li structures. Close to ⌰ ϭ2/3 a p(1.5ϫ1) structure ͑Fig.…”

Lithium incorporation into a silica thin film: Scanning tunneling microscopy and density functional theory

“…1,4,7 Even on the Mo͑112͒ surface without oxide film, the development of ͑4 ϫ 1͒ and ͑2 ϫ 1͒ superstructures has been found in LEED upon alkali deposition. 4,5 In contrast, the Li forms small assemblies at the silica-Mo interface already at relatively low exposure, although the calculated Bader charge of +0.8͉e͉ clearly indicates the ionic character of the Li species. The difference to bare metal surfaces can be related to the presence of the dielectric silica film on the Mo support, which contributes to the screening of the Li + -Li + interaction.…”

“…1,2,4 At intermediate coverage, the alkali atoms often self-assemble into regular arrays on the metal surface in order to maximize their mutual distance. [5][6][7] By this means, the repulsive dipole-dipole interaction and the subsequent layer depolarization are restrained. Already this simple picture, first suggested by Langmuir and Gurney, 8 provides a good description for the general adsorption behavior of alkali atoms on metal surfaces.…”