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The dispersion relation for the longitudinal acoustic lattice waves propagating in the direction of the hexad axis in (pyrolitic) graphite, at room temperature, has been determined by neutron spectrometry. The dispersion curve has very closely the form of a sine curve, with a maximum frequency of (3. 840. 06) 10^12 cps, for the zone-boundary phonon of wavelength c=6. 70. The transverse acoustic lattice waves were less well determined, but show roughly similar behavior with a maximum frequency at the zone boundary of (1. 30. 3) 10^12 cps. Values for certain elastic constants have been deduced from the measurements: in units of 10^11 dyn/cm^2, C₃₃=3. 90. 4, and C₄₄=0. 420. 2.
Dolling et al. (1962) studied this question.