High-pressure neutron-scattering studies of graphite and stage-two graphite-SbCl5

Abstract

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 ν(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×${10}^{\mathrm{-}2}$ ${\mathrm{kbar}}^{\mathrm{-}1}$ (independent of q); the elastic constant $C_{33}$ for HOPG was found to be 3.40×${10}^{11}$ dyn/${\mathrm{cm}}^2$ at 1 bar with a pressure coefficient of (1/${\mathit{C}}_{33}$)(${\mathit{dC}}_{33}$/dP)=2.91×${10}^{\mathrm{-}2}$ ${\mathrm{kbar}}^{\mathrm{-}1}$. The measured c-axis spacings were 6.71 and 12.72 Å for HOPG and stage-2 ${\mathrm{SbCl}}_5$-intercalated graphite, respectively, at atmospheric pressure; the corresponding compressibilities (1/c)(dc/dP) were -2.24×${10}^{\mathrm{-}3}$ ${\mathrm{kbar}}^{\mathrm{-}1}$ (HOPG) and -2.28×${10}^{\mathrm{-}3}$ ${\mathrm{kbar}}^{\mathrm{-}1}$ (${\mathrm{SbCl}}_5$). Elastic-neutron-scattering studies up to 20 kbar were undertaken to search for stage transformations, but no evidence of any phase transition was observed.