We demonstrate that
the heat capacity Boson peak (BP)-like anomaly
appearing in fully ordered anharmonic molecular crystals emerges as
a result of the strong interactions between propagating (acoustic)
and low-energy quasi-localized (optical) phonons. In particular, we
experimentally determine the low-temperature (<30 K) specific heat
of the molecular crystal benzophenone and those of several of its
fully ordered bromine derivatives. Subsequently, by means of theoretical
first-principles methods based on density functional theory, we estimate
the corresponding phonon dispersions and vibrational density of states.
Our results reveal two possible mechanisms for the emergence of the
BP-like anomaly: (i) acoustic–optic phonon avoided crossing,
which gives rise to a pseudo-van Hove singularity in the acoustic
phonon branches, and (ii) piling up of low-frequency optical phonons,
which are quasi degenerate with longitudinal acoustic modes and lead
to a surge in the vibrational density of states at low energies.
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