The melting of intercalant layers in C24Cs and C36Cs

Abstract We have investigated the detailed ordering of the cesium intercalant layers in single-crystal and pyrolytic forms of C 24 Cs and C 36 Cs using X-ray diffuse scattering techniques. A two-dimensional RDF analysis reveals that the Cs layer has a disordered triangular structure at 300 K with a 5.95A nearest-neighbor distance and is thus, on average, not registered with the graphite bounding layers. At 165 K the Cs atoms start to order into a uniform triangular lattice which is modulated by the graphite structure. Interlayer positional correlations start to appear at about this temperature but they are limited in range by stacking faults in the graphite host. A new transition to a mixed registered phase is observed at 50 K. The Cs layers in single crystals of C 24 Cs and C 36 Cs are found to exhibit three-dimensional long-range orientational order at all temperatures. Below 228 K a rotation of the Cs layers relative to the graphite 〈100〉 directions is observed with a mean-field-like temperature exponent. The rotation is interpreted in terms of the Novaco and McTague orientational epitaxy mechanism. Satellite reflections around each graphite reflection are attributed to static distortion wave effects arising from the incommensurability of the Cs layer. The orientational lock-in of the Cs layer at 228 K is probably related to a resistivity anomaly that has been observed recently in C 24 Cs at approximately this temperature.