High-Pressure Neutron-Scattering Studies of Graphite and Stage-Two Graphite-SbCl₅

The longitudinal-acoustic (LA) phonons propagating along the c axis in highly oriented pyrolytic graphite (HOPG) were investigated as a function of pressure up to 20 kbar using inelastic-neutron-scattering techniques. The phonon frequencies varied as (P,q)=A(P)sin[c(P)q/2], where A(P) and c(P) indicate the pressure dependences of the zone-center LO frequency of B _{1g 1} symmetry and of the c-axis lattice spacing, respectively. From the measurements, the mode Grüneisen parameter for the LA branch was estimated to be 1.5 x 10 ^-2 kbar ^-1 (independent of q); the elastic constant C ₃₃ for HOPG was found to be 3.40 x 10 ¹¹ dyn/cm ² at 1 bar with a pressure coefficient of (1/C ₃₃ )(dC ₃₃ /dP)=2.91 x 10 ^-2 kbar -1 . The measured c-axis spacings were 6.71 and 12.72 Å for HOPG and stage-2 SbCl ₅ -intercalated graphite, respectively, at atmospheric pressure; the corresponding compressibilities (1/c)(dc/dP) were -2.24 x 10 ^-3 kbar ^-1 (HOPG) and -2.28 x 10 ^-3 kbar ^-1 (SbCl ₅ ). Elastic-neutron-scattering studies up to 20 kbar were undertaken to search for stage transformations, but no evidence of any phase transition was observed.