Vibrational properties of amorphous and two crystalline modifications of arsenic as determined by inelastic neutron scattering

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Abstract
Inelastic neutron scattering experiments have been carried out on amorphous arsenic and on polycrystalline specimens of the rhombohedral (metallic) and orthorhombic modifications, all at 295 K. The measurements were made using three time-of-flight spectrometers with incident neutron wavelengths of 4·2, 4·8 and 1·86 Å thus providing a reasonably wide range of wave vector transfers (Q). Corrections have been made for multiphonon contributions to the scattering and values obtained for Z 1(ω)C(Q) where Z 1(ω) is the one-phonon density of states and C(Q) represents the coherence (interference) effects arising because arsenic is a predominantly coherent scatterer. It is shown that the data at not too low Q give a good approximation to Z 1(ω), particularly when averaged over a range of Q. The frequency distributions for the amorphous and orthorhombic modifications are very similar and differ significantly from that for the rhombohedral crystal. The coherence effects C(Q) are considered and it is shown that at higher Q they are dominated by nearest-neighbour correlations. At low Q and low ω, longer range correlations are important and the coherence effects observed in the amorphous sample are shown to be qualitatively explicable in terms of transverse acoustic modes and the normal wave vector conservation condition, with the continuous structure factor S(Q) acting as a reciprocal lattice vector.