Phase transitions in the graphite intercalation compound C₂₄Rb

The phase transitions in C₂₄Rb have been studied by means of resistivity, a.c. calorimetry and neutron scattering measurements. Four anomalies in resistivity are observed at temperatures 60, 110, 165 and ca. 210 K, while corresponding anomalies in specific heat are observed. The neutron scattering study reveals two structural changes: (1) Above 165 K the Rb atoms have neither long-range intra- or interlayer correlation, while below 165 K they form a non-registered triangular lattice which is modulated by the graphite potential and has a preferred orientation relative to the graphite lattice. The modulation is interpreted in terms of the static distortion wave. The Rb layers have a stacking order with a period of three CRbC sandwiches. (2) Below about 110 K the stacking of Rb layers changes to that of a much longer period than the phase above 110 K.