P. A. Heiney, M. E. Huster, V. B. Cajipe, and J. E. Fischer, Synthetic Metals 12, 21-26 (1985).
We have used x-ray diffraction to study the structure of stage 4 through stage 11 potassium-intercalated graphite. Out of plane, the diffraction patterns are analyzed in terms of the Hendricks Teller model of a disordered 1D crystal. Stage 4-5 samples are best described by stage separation of two different package types, while higher-stage samples are better desribed by a broad distribution of many packages. In-plane, all samples show a high temperature 2D fluid structure of the K atoms. In the stage 4-6 samples, the fluid peak gradually sharpens on cooling and increases in intensity to form a 2D incommensurate (IC) superlattice. This evolution is consistent with a second order transition. By contrast, the higher-stage samples show a strongly first-order, hysteretic transition to a commensurate sqrt(7)Xsqrt(u) superlattice at 235K. In all cases, the expansion of the 2D lattice upon freezing agrees within experimental error with the 2-4% increase in out-of-plane density of K layers.
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