Abstract
The Sn specific densities of phonon states in the SnSe subunits of [(SnSe)1.04]m[MoSe2]n ferecrystals with
(m,n) = (1,1), (4,1) and in bulk SnSe were derived from nuclear inelastic scattering by the 119Sn Mössbauer
resonance. Using different measurement configurations, phonons with polarization parallel and perpendicular
to the ferecrystal plane were specifically probed. Vibrational properties and phonon spectral
weight are found to strongly depend on the phonon polarization and layer count m. A highly peculiar
feature of these ferecrystal densities of phonon states is the emergence of rather sharp high energy
vibrational modes polarized perpendicular to the ferecrystal plane, which contrasts with usual findings in
thin layered structures and nanostructured materials in general, and a depletion of modes with a gap
appearing between acoustic and high energy modes. The spectral weight of these phonons depends on
the overall SnSe content, m, but cannot be unambiguously attributed to SnSe–MoSe2 interfaces.
Considering the low energy part of lattice dynamics, ferecrystals exhibit rather low average phonon group
velocities as compared to the speed of sound in the long wavelength limit. For the (1,1) ferecrystal, this
effect is most pronounced for vibrations polarized in the ferecrystal plane. Thus, an experimental microscopic
origin for the vibrational and bonding anisotropy in subunits of ferecrystals is provided.
