  The lattice dynamics of coesite has been studied by a combination of diffuse
x-ray scattering, inelastic x-ray scattering and an ab initio lattice dynamics
calculation. The combined technique gives access to the full lattice dynamics
in harmonic description and thus eventually provides detailed information on
the elastic properties, the stability and metastability of crystalline systems.
The experimentally validated calculation was used for the investigation of
eigenvectors, mode character and their influence on the density of vibrational
states. High symmetry sections of the reciprocal space distribution of diffuse
scattering and inelastic x-ray scattering spectra as well as the density of
vibrational states and the dispersion relation are reported and compared to the
calculation. A critical point at the zone boundary is found to contribute
strongly to the main peak of the low energy part in the density of vibrational
states. Comparison with the most abundant SiO2 polymorph - alpha-quartz -
reveals similarities and distinct differences in the low-energy vibrational
properties.
