Theory of order-disorder transitions in the graphite intercalation compounds C8Cs, C8Rb, and C6Li

The order-disorder transitions in the stage-1 graphite intercalation compounds C8Cs, C8Rb, and C6Li are analyzed within a Landau-Ginzburg theory. The transitions are all found to be first order, but for different reasons: In C8Cs the Landau expansion includes third-order umklapp terms, whereas in C8Rb the ordered phase is stablized by sixth-order anisotropy terms. The transition in C6Li is described by the three-dimensional three-state Potts model, which also contains cubic invariants. Experiments are suggested to test the predictions. © 1979 The American Physical Society.