Ultrasonic studies of phase transitions in stage-4 antimony pentachloride intercalated graphite

The c-axis shear elastic constant C₄₄ and transverse sound attenuation α_T for stage-4 SbCl₅ intercalated graphite from 4 to 300 K are studied with ultrasonic pulse-echo techniques. No transverse sound echo is observable above 230 K. At constant temperatures between 230 and 200 K, both C₄₄ and α_T drift spontaneously toward lower bounds over a period of hours. A decrease in C₄₄ by a factor of ～ 2 is observed. Below 195 K, the sample becomes metastable. From 195 to 4 K, C₄₄ increases by 60% for the quenched sample, and by 30% for the annealed sample. This abnormal behavior of the elastic property can be interpreted by a disordered-liquid to incommensurate-solid phase transition of the intercalant layer at 230 K. The time-dependence of C₄₄ and α_T is attributed to the growth of islands with a slightly incommensurate solid structure.