Pressure-induced staging transitions and stage disorder in dilute potassium-graphite intercalation compounds.

Abstract

We extend our previous study of high-pressure staging transitions and stage disorder to dilute potassium-graphite intercalation compounds, using the Hendricks-Teller analysis of elastic neutron scattering (00L) profiles measured at 300 K over a broad q range. The dilute regime begins at a concentration [K]/[C]\ensuremath{\lesssim}1/33, all samples in this regime behaving qualitatively the same at high pressure. A continuous transition starts immediately near P=0, leading to a well-ordered higher stage with no apparent connection to in-plane density or structure. This is stable only over a small range, transforming abruptly at slightly higher P to a still higher stage or stage mixture, the index of which is consistent with the specific in-plane density imposed by commensurate 2\ifmmode\times\else\texttimes\fi{}2 lockin. Disorder generally increases with average stage index (i.e., with increasing P or decreasing [K]/[C] ratio), consistent with the theory of Kirczenow. Disorder in the most dilute samples is better represented by a skewed distribution of many stage packages than by a random mixture of two or three different stage cells.