Two-dimensional structural phase transitions in the stage-1 OsF6-graphite intercalated compound

Neutron scattering has been used to study the temperature dependence of the in-plane structure of the stage-1 graphite intercalation compound C9OsF6. The low-temperature structure has a two-dimensional character and can be described as a nearly hexagonal domain-wall lattice in which sharp domain walls separate regions of (2×2)R0°structure on a 60-A scale. The domain-wall lattice is imperfect with a correlation length that only extends over a few such regions. The in-plane intercalant structure is disordered above 280 K and resistivity measurements show a clear hysteresis in the transition region above 250 K, indicating a first-order transition. The structural data are consistent with coexistence of a solidlike and fluidlike phase in the transition region, as expected for constant volume melting. The results suggest that this class of intercalation compounds is a good realization of a four-state Potts model with effective dimensionalities less than 3.