Orientational order and phase transitions in a two-dimensional triangular octopolar array

As a prototypical model for a system of methane molecules adsorbed on a surface of graphite, we have carried out a classical molecular field theoretical investigation of a two-dimensional triangular octopolar array. Two successive phase transitions are found. The upper one is of second order, while the lower one is of first order. The changes in site symmetries are D6h→D3h→Cs in the order of decreasing temperature.