X-ray study of the order—disorder transition in high-stage alkali metal intercalated graphite single crystals

Abstract

Abstract Graphite intercalation compounds display a variety of structural properties because of their composite nature (graphite + intercalate) and their layered arrangement. Alkali metal intercalated graphite compounds undergo order-disorder phase transitions when the temperature is varied in the range 300–10 K. The disordered state shows true two-dimensional character, whereas three-dimensional coupling takes place on ordering. Results of single-crystal X-ray diffractometric and photographic studies of stage-2 KC24 single crystals are presented. The positional and orientational correlations of the modulated liquid phase have been studied from 300 K down to the temperature transition Tu = 123.5° K. At the transition, the hexagonal incommensurate solid structure of the alkali metal is modulated by the graphite potential. This transition is discussed in terms of the relaxed-close packed structure model (Dicenzo, 1982). At low temperature a second transition takes place at TL ≈ 95 K. It is found to correspo...