Journal
JOURNAL OF PHYSICAL CHEMISTRY LETTERS
Volume 13, Issue 22, Pages 5061-5067Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpclett.2c01224
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Funding
- National Academy of Sciences of Ukraine [117U002290]
- National Research Foundation of Ukraine [197/02.2020]
- Spanish MCIN under Ramon y Cajal Fellowship [RYC2018-024947-I]
- Spanish AEI/MCIN [PID2020-112975GBI00]
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This study demonstrates that the heat capacity Boson peak (BP)-like anomaly in fully ordered anharmonic molecular crystals is caused by the strong interactions between propagating (acoustic) and low-energy quasi-localized (optical) phonons. Through experimental measurements and theoretical calculations, the low-temperature specific heat of benzophenone and its fully ordered bromine derivatives is determined, revealing two mechanisms for the emergence of the BP-like anomaly.
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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