STRUCTURE AND PHASE-DIAGRAM OF TETRAMETHYLGERMANIUM ADSORBED ON GRAPHITE (0001) PLANES

X-ray scattering experiments have been performed on tetramethylgermani um (TMG) physisorbed on (0001) planes of exfoliated graphite (papyex). At coverages below 0.9 monolayers (ML) and temperatures below T = 130 K, TMG adsorbs in a commensurate root 7 x root 7 structure. A first-o rder phase transition near T = 110 K leads to a uniaxially compressed phase of the adsorbate with a dipod down structure. In the region of 1 ML and below T = 150 K, TMG forms an incommensurate hexagonal structu re and undergoes a transition to the incommensurate dipod down structu re at T = 130 K. A continuous melting transition of the two-dimensiona l structure has been observed for all investigated coverages.