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Order-disorder in higher-stage ( n = 3 , 4 ) potassium graphites

1983; American Physical Society; Volume: 27; Issue: 10 Linguagem: Inglês

10.1103/physrevb.27.6385

1095-3795

Mitsuhiro Mori, S. C. Moss, Y. M. Jan,

Surface and Thin Film Phenomena

The disordered and ordered states in stages-3 and -4 potassium graphite (\ensuremath{\sim}${\mathrm{C}}_{36}$K and ${\mathrm{C}}_{48}$K) have been studied with x-ray diffraction on highly oriented pyrolytic graphite samples. The disordered state of both is similar and is confirmed to consist of independent (uncorrelated) layers of intercalant with a liquidlike in-plane structure, incommensurate with the graphite-net planes. The in-plane ordered structures of the two are essentially identical to each other and quite similar to, but somewhat expanded over, the stage-2 structure. The transition to some form of two-dimensional (2D) modulated or ordered state in ${\mathrm{C}}_{36}$K occurs smoothly on cooling with an apparent onset at \ensuremath{\sim}190 K of ordering reflections similar to those observed in stage 2. This is characterized as well by a continuous shift and sharpening of the 2D pattern below the ordering onset and a concomitant development of weak three-dimensional (3D) order at $T<131$ K. This behavior is to be contrasted with the sharp, apparently second-order, transition in stage 2 (\ensuremath{\sim} ${\mathrm{C}}_{24}$K) in which the modulation peaks and 3D order set in at the same temperature.