Raman light scattering and c-axis resistivity evidence for a pressure-induced stage transformation in PdAl2Cl8 intercalated graphite

We have examined several samples of first- to third-stage PdAl2Cl8-intercal ated graphite under hydrostatic pressures up to 1 GPa. In stage-1 highly or iented pyrolytic graphite-(HOPG) and single-crystal-graphite-based material s, the c-axis resistivity decreases sharply above a few kilobars; pressure release induces a reversible return to the initial value only in the case o f the latter sample. Raman spectra taken in situ under pressure on a HOPG-b ased material show similarly irreversible effects. Analysis of the spectra taken on higher-stage samples leads to the conclusion that hydrostatic pres sure beyond a few kilobars increases the density of the intercalate within the graphitic galleries, transforming the initial sample to a higher-stage material. Since there is no loss of intercalate, the overall intercalate-to -host charge transfer remains constant so that the Raman frequency is appro ximately the same for both first- and second-stage products. This is an unu sual situation in which there is thus an apparent lack of Raman signature i n spite of the stage change.