Structures and phase diagrams of tetramethyltin and tetramethyllead adsorbed on graphite

Abstract

Layers of tetramethyltin and tetramethyllead on exfoliated graphite have been investigated in the monolayer regime with X-ray and neutron diffraction. At coverages Θ > 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). Slightly distorted rectangular unit cells were found for the deuterated compound Sn(CD3)4 (γ = 92.2°), which was used for the neutron diffraction, and also for Pb(CH3)4 on graphite (γ = 89.1°). The lattice constants for the three systems are almost identical though the structures are incommensurate with the graphite lattice. With decreasing coverage, only the tetramethyltin-on-graphite system undergoes a solid-solid phase transition. The two-dimensional (2D) melting transitions were found to be of first order.