Thermal properties of halogen-ethane glassy crystals: Effects of orientational disorder and the role of internal molecular degrees of freedom

Dynamics in the ordered and disordered phases of barocaloric adamantane

High-entropy order-disorder phase transitions can be used for efficient and eco-friendly barocaloric solid-state cooling. Here the barocaloric effect is reported in an archetypal plastic crystal, adamantane. Adamantane has a colossal isothermally reversible entropy change of 106 J K^-1 kg^-1. Extremely low hysteresis means that this can be accessed at pressure differences less than 200 bar. Configurational entropy can only account for about 40% of the total entropy change; the remainder is due to vibrational effects. Using neutron spectroscopy and supercell lattice dynamics calculations, it is found that this vibrational entropy change is mainly caused by softening in the high-entropy phase of acoustic modes that correspond to molecular rotations. We attribute this difference in the dynamics to the contrast between an 'interlocked' state in the low-entropy phase and sphere-like behaviour in the high-entropy phase. Although adamantane is a simple van der Waals solid with near-spherical molecules, this approach can be leveraged for the design of more complex barocaloric molecular crystals. Moreover, this study shows that supercell lattice dynamics calculations can accurately map the effect of orientational disorder on the phonon spectrum, paving the way for studying the vibrational entropy, thermal conductivity, and other thermodynamic effects in more complex materials.

Disordered Crystals Reveal Soft Quasilocalized Glassy Excitations

Structural glasses formed by quenching a melt are known to host a population of low-energy quasilocalized (nonphononic) excitations whose frequencies ω follow a universal ∼ω^{4} distribution as ω→0, independently of the glass formation history, the interparticle interaction potential, or spatial dimension. Here, we show that the universal quartic law of nonphononic excitations also holds in disordered crystals featuring finite long-range order, which is absent in their glassy counterparts. We thus establish that the degree of universality of the quartic law extends beyond structural glasses quenched from a melt. We further find that disordered crystals, whose level of disorder can be continuously controlled, host many more quasilocalized excitations than expected based on their degree of mechanical disorder-quantified by the relative fluctuations of the shear modulus-as compared to structural glasses featuring a similar degree of mechanical disorder. Our results are related to glasslike anomalies experimentally observed in disordered crystals. More broadly, they constitute an important step toward tracing the essential ingredients necessary for the emergence of universal nonphononic excitations in disordered solids.

Low-Temperature Heat Capacity Anomalies in Ordered and Disordered Phases of Normal and Deuterated Thiophene

We measured the specific heat C_p of normal (C₄H₄S) and deuterated (C₄D₄S) thiophene in the temperature interval of 1 ≤ T, K ≤ 25. C₄H₄S exhibits a metastable phase II₂ and a stable phase V, both with frozen orientational disorder (OD), whereas C₄D₄S exhibits a metastable phase II₂, which is analogous to the OD phase II₂ of C₄H₄S and a fully ordered stable phase V. Our measurements demonstrate the existence of a large bump in the heat capacity of both stable and metastable C₄D₄S and C₄H₄S phases at temperatures of ∼10 K, which significantly departs from the expected Debye temperature behavior of C_p ≈ T³. This case study demonstrates that the identified low-temperature C_p anomaly, typically referred to as a "Boson-peak" in the context of glassy crystals, is not exclusive of disordered materials.

Multiple glass transitions in vapor-deposited orientational glasses of the most fragile plastic crystal Freon 113

Stable glass formation for both structural glass and as-deposited glassy crystal at deposition temperatures below T_g.

Disentangling α and β relaxation in orientationally disordered crystals with theory and experiments

We use a microscopically motivated generalized Langevin equation (GLE) approach to link the vibrational density of states (VDOS) to the dielectric response of orientational glasses (OGs). The dielectric function calculated based on the GLE is compared with experimental data for the paradigmatic case of two OGs: freon-112 and freon-113, around and just above T_{g}. The memory function is related to the integral of the VDOS times a spectral coupling function γ(ω_{p}), which tells the degree of dynamical coupling between molecular degrees of freedom at different eigenfrequencies. The comparative analysis of the two freons reveals that the appearance of a secondary β relaxation in freon-112 is due to cooperative dynamical coupling in the regime of mesoscopic motions caused by stronger anharmonicity (absent in freon-113) and is associated with the comparatively lower boson peak in the VDOS. The proposed framework brings together all the key aspects of glassy physics (VDOS with the boson peak, dynamical heterogeneity, dissipation, and anharmonicity) into a single model.

Glassy Anomalies in the Low-Temperature Thermal Properties of a Minimally Disordered Crystalline Solid

The low-temperature thermal and transport properties of an unusual kind of crystal exhibiting minimal molecular positional and tilting disorder have been measured. The material, namely, low-dimensional, highly anisotropic pentachloronitrobenzene has a layered structure of rhombohedral parallel planes in which the molecules execute large-amplitude in-plane as well as concurrent out-of-plane librational motions. Our study reveals that low-temperature glassy anomalies can be found in a system with minimal disorder due to the freezing of (mostly in-plane) reorientational jumps of molecules between equivalent crystallographic positions with partial site occupation. Our findings will pave the way to a deeper understanding of the origin of the above-mentioned universal glassy properties at low temperature.

Thermodynamic and Kinetic Fragility of Freon 113: The Most Fragile Plastic Crystal

We present a dynamic and thermodynamic study of the orientational glass former Freon 113 (1,1,2-trichloro-1,2,2-trifluoroethane, CCl_{2}F-CClF_{2}) in order to analyze its kinetic and thermodynamic fragilities. Freon 113 displays internal molecular degrees of freedom that promote a complex energy landscape. Experimental specific heat and its microscopic origin, the vibrational density of states from inelastic neutron scattering, together with the orientational dynamics obtained by means of dielectric spectroscopy have revealed the highest fragility value, both thermodynamic and kinetic, found for this orientational glass former. The excess in both Debye-reduced specific heat and density of states (boson peak) evidences the existence of glassy low-energy excitations. We demonstrate that early proposed correlations between the boson peak and the Debye specific heat value are elusive as revealed by the clear counterexample of the studied case.

Orientational relaxations in solid (1,1,2,2)tetrachloroethane

We employ dielectric spectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassy transition at 156 ± 1 K, corresponds to a cooperative motion by which each molecule rotates by 180° around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectric spectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids.