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 B1g1 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 C33 for HOPG was found to be 3.40 x 1011 dyn/cm2 at 1 bar with a pressure coefficient of (1/C33)(dC33/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 SbCl5-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 (SbCl5). Elastic-neutron-scattering studies up to 20 kbar were undertaken to search for stage transformations, but no evidence of any phase transition was observed.
B. Alzyam et al., "High-Pressure Neutron-Scattering Studies of Graphite and Stage-Two Graphite-SbCl5," Physical Review B (Condensed Matter), vol. 38, no. 2, pp. 1544-1547, American Physical Society, Jul 1988.
The definitive version is available at https://doi.org/10.1103/PhysRevB.38.1544
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