  Both structural glasses and disordered crystals are known to exhibit
anomalous thermal, vibrational, and acoustic properties at low temperatures or
low energies, what is still a matter of lively debate. To shed light on this
issue, we studied the halomethane family C-Br_n-Cl_4-n (n = 0,1,2) at low
temperature where, despite being perfectly translationally ordered stable
monoclinic crystals, glassy dynamical features had been reported from
experiments and molecular dynamics simulations. For n = 1,2 dynamic disorder
originates by the random occupancy of the same lattice sites by either Cl or Br
atoms, but not for the ideal reference case of CCl4. Measurements of the
low-temperature specific heat (Cp) for all these materials are here reported,
which provide evidence of the presence of a broad peak in Debye-reduced Cp/T^3
and in the reduced density of states g(w)/w^2 determined by means of neutron
spectroscopy, as well as a linear term in Cp usually ascribed in glasses to
two-level systems in addition to the cubic term expected for a fully ordered
crystal. Being CCl4 a fully ordered crystal, we also performed density
functional theory (DFT) calculations, which provide unprecedented detailed
information about the microscopic nature of vibrations responsible for that
broad peak, much alike the boson peak of glasses, finding it to essentially
arise from a piling up (at around 3 - 4 meV) of low-energy optical modes
together with acoustic modes near the Brillouin-zone limits.
