Emergence of glassy features in halomethane crystals

Abstract

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 have studied the halomethane family CBrnCl4−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 )/T 3 and in the reduced density of states (g(ω)/ω 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 have 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.

Low-energy optical phonons induce glassy-like vibrational and thermal anomalies in ordered crystals

“…In particular, [21] has reported experimental observations of a strong boson peak in perfectly ordered crystals of halomethanes due to low-lying optical modes, whereas [22,23] experimentally observed the upturn of the VDOS predicted here in organic molecular systems with high degree of crystallinity (and presumably very soft optical modes) such as starch and glucose. Similar behaviors have been measured also in thermoelectric crystals [24,25] where a boson peak in the VDOS is also observed, and where, interestingly, low-energy vibrations (so-called rattling) of caged compounds give rise to an upturn in the VDOS at vibrational energies below the boson peak [18].…”

“…The latter effect shifts the boson peak to lower frequencies, as shown in figure 4, and in turn the red-shifted boson peak produces an upturn in the specific heat as  T 0. This upturn is highly reminiscent of the TLS tunneling mechanism which is usually invoked to explain this upturn of the specific heat at low T. However, the TLS mechanism assumes a random distribution of TLS, and hence it is unlikely to explain this upturn which has been observed experimentally in ordered crystals in [21]. Instead, the mechanism proposed here for the upturn does not rely on any disorder assumption, and is uniquely provided by the red-shift of the boson peak caused by the strongly damped optical phonon.…”

Theory of the phonon spectrum in host-guest crystalline solids with avoided crossing

“…Our work provides further evidence of the universality of the BP anomalies beyond the realm of glasses and disordered systems towards complex crystalline and ordered solids. Moreover, together with recent experimental and theoretical results [7,9,11,18], it opens up the way of realizing technologically relevant materials with crystal-like electronic behaviour and glass-like phononic behaviour. The presence of these features seems to be more universal and general than thought before and presumably tightly connected with anharmonic damping mechanisms and softly-gapped modes.…”

Detection of Boson Peak and Fractal Dynamics of Protein by Terahertz Time-Domain Spectroscopy

“…Interestingly, such universal dynamics of disordered materials also appears in a few single crystals. As discovered in recent studies, BPs or amorphous-like thermal behaviours appear in single crystals, such as thermoelectric materials [6][7][8][9] , pure relaxor materials 10 , and atypical organic materials 11,12 , and theoretical studies for elucidating anomalous phenomena are underway 8,[13][14][15] .…”