Susceptibility study of physisorbed oxygen layers on graphite

Susceptibility measurements on oxygen (O₂) physisorbed on exfoliated graphite are reported for coverages of up to {ρ}=3. For 0{≤}{ρ}{≤}1 solidification proceeds by one first-order transition at T'{}25 K associated with a nearly discontinuous decrease of the susceptibility (width about {Δ}T{}0.5 K) and a nearly {δ}-like anomaly in the specific heat. For the second and higher coverages solidification spreads over a temperature interval as broad as {Δ}T{}8 K. This more intricate solidification process is associated with a nearly linear variation of the susceptibility with temperature. We propose to identify this with a two-phase coexistence region corresponding to the {θ} phase previously proposed on the basis of low-energy electron diffraction measurements. Specific-heat measurements reveal only the low-temperature limit of the solidification process. On the basis of the present measurements we are able to describe some new structures in the O₂-graphite phase diagram which allow a more consistent view of all of the hitherto known details.