Structural transitions in alkali graphite intercalation compounds

Structural transitions induced by pressure, temperature and chemical potential changes were studied in alkali graphite intercalation compounds using x-ray diffraction. A unique first-order commensurate-commensurate transition, from the ambient 2 x 2 to a {dollar}surd3{dollar} x {dollar}surd3{dollar} superstructure, was observed at high pressure in KC{dollar}sb8{dollar}; this in-plane densification is consistent with the observation of a fractional stage which occurs during the accompanying sequence of staging transitions. At low temperatures, stage 2 and stages 4 to 10 potassium graphite compounds were found to undergo both in-plane and stage ordering transitions. The room temperature in-plane fluid structure expanded upon freezing to a commensurate {dollar}surd7{dollar} x {dollar}surd7{dollar} solid in stages n {dollar}geq{dollar} 5 and to a weakly incommensurate superlattice in stage 4 + 5; no out-of-plane correlations were found in these cases suggesting an upper limit of n = 4 for the appearance of 3D effects. The nature of the freezing transition changed with stage according to the symmetry of the low temperature phase. Using the Hendricks-Teller model for one-dimensional disorder, the average stage and stage disorder in these compounds were shown to decrease with low temperature; this is consistent with the conservation of sample mass as well as an entropy-driven stage ordering transition. More complex low temperature behaviour was found in stage 2; depending on the initial sample stoichiometry, the ordered in-plane phase was either a single phase modulated solid or a complex multiphase structure, while the stage structure tended towards greater phase separation or higher average stage. Overall, the stage disorder in these metastable constant concentration samples was shown to decrease with stage as predicted by the domain model of stage disorder. This effect was found to persist under equilibrium conditions for cesium graphite. The cesium in-situ diffraction data revealed that both stage purity and phase separation increase with decreasing stage. Stage 5 and higher were observed to be highly miscible while stages 3 and 2 completely phase separate; the intermediate stages were found to be partly miscible. The miscibility was also found to modulate during a stage n to (n {dollar}-{dollar} 1) transition, suggesting that the kinetics are island-growth-dominated towards the middle of the transition.