STRUCTURES AND PHASE-DIAGRAMS OF TETRAMETHYLTIN AND TETRAMETHYLLEAD ADSORBED ON GRAPHITE

Layers of tetramethyltin and tetramethyllead on exfoliated graphite ha ve been investigated in the monolayer regime with X-ray and neutron di ffraction. At coverages Theta > 1 monolayer Sn(CH3)(4) on graphite has a rectangular unit cell (plane group pg) containing two molecules and with antiferroordering (tripod-down/onepod-down orientation). Slightl y distorted rectangular unit cells were found for the deuterated compo und Sn(CD3)(4) (gamma = 92.2 degrees), which was used for the neutron diffraction, and also for Pb(CH3)(4) on graphite (gamma = 89.1 degrees ). The lattice constants for the three systems are almost identical th ough the structures are incommensurate with the graphite lattice. With decreasing coverage, only the tetramethyltin-on-graphite system under goes a solid-solid phase transition. The two-dimensional (2D) melting transitions were found to be of first order. (C) 1997 Elsevier Science B.V.