Investigating disordered phases of C2Cl6 using an information theory approach

Many materials of interest and processes relevant to life are based in disordered phases. This disorder can be either positional, orientational or both as in the case of liquids. Unfortunately, the study of disordered phases is inherently difficult given the lack of periodicity as in ordered crystals. In this work we use neutron and X-ray diffraction experiments together with molecular dynamics simulations to study the local order and molecular movements in the disordered phases of hexachloroethane both in the liquid phase and in its plastic phase. The latter is a phase in which the molecular centres of mass form a long-range ordered crystalline lattice but molecules can rotate more or less freely. The concurrent use of diffraction experiments (Neutron and X-ray) and molecular dynamics simulations show that liquid structure mimics that of the disordered crystal at short distances. In order to extend the analysis to long distances, we have borrowed magnitudes from information theory that allow us to measure disorder and correlation. We also use the Kullback-Leibler divergence, an indicator of how similar two structures are to study the differences between plastic and liquid phases, as well as the structural difference at varying temperatures. We thus also offer in this work a common framework to characterize the structure of any disordered phase firmly based on probability and information theory. The advantage of our proposed methodology is that it can be used both to characterize the disorder and to perform comparisons of disordered materials with different degrees of freedom such as liquids and disordered crystals.Peer ReviewedPostprint (published version

Reversible and irreversible colossal barocaloric effects in plastic crystals

The extremely large latent heat exchanged in phase transitions involving strong molecular orientational disordering has recently led to the proposal of plastic crystals as a feasible solution for solid-state barocaloric eco-friendly cooling technologies. Here we determine the reversible barocaloric response of four plastic crystals derived from neopentane [C(CH3)4]: (NH2)C(CH2OH)3 (TRIS for short), (NH2)(CH3)C(CH2OH)2 (AMP), (CH3)C(CH2OH)3 (PG) and (CH3)3C(CH2OH) (NPA). All of them display colossal entropy changes at their ordered-plastic phase transition, which is a primal requirement for competitive barocaloric materials. However, we show that it is also important to verify that the large barocaloric effects can be achieved using pressures that, while being moderate, are large enough to overcome the pressure-dependent hysteresis. From this quantity and using the quasi-direct method, we determine the minimum pressure needed to achieve reversible barocaloric effects, prev, for each compound. Specifically, we find a small and moderate prev for PG and NPA, respectively, which therefore display colossal reversible barocaloric effects comparable to harmful fluids used in current refrigerators and thus confirm the potential of plastic crystals as excellent alternatives. Instead, in TRIS and AMP, the obtained prev is excessive to yield reversible barocaloric effects useful for cyclic applications.This work was supported by the MINECO projects MAT2016-75823-R and FIS2017-82625-P, the DGU project 2017SGR-42, the UK EPSRC grant EP/M003752/1, and the ERC Starting grant no. 680032. X. M. is grateful for support from the Royal Society

Crystalline tetrazepam as a case study on the volume change on melting of molecular organic compounds

The volume change on melting is a rarely studied quantity and it is not well understood even if it must reflect the changes in interaction between the solid and the liquid state. It is part of the solid-state information for materials and pharmaceuticals and it is important for the reliability of polymorph stability study results. Using the crystal structure of monoclinic tetrazepam at 150 K and at room temperature, in addition to powder X-ray diffraction as a function of the temperature, the specific volume of tetrazepam has been determined over a large temperature domain. In combination with a pressure-temperature curve for the melting of tetrazepam, its volume change on melting could be determined. With this information and previous data from the literature, the assumption that the volume of the solid increases on average with 11% on melting has been investigated. It can be concluded that this value is not constant; however so far, no simple relationship has been found to relate the solid state to its volume change on melting and using 11% remains best practice. A comparison of the tetrazepam crystal structure with diazepam and nordiazepam has been provided too.Peer ReviewedPostprint (author's final draft

Colossal barocaloric effects in adamantane derivatives for thermal management

Plastic crystals are currently attracting interest because their solid-state caloric functionality could be used to tackle climate change in two critical areas: (i) more environmentally friendly cooling and heating driven by pressure and (ii) passive waste heat management. Here, we suggest that plastic crystals could also be used for active pressure-assisted (i.e., barocaloric) waste heat management. In contrast to the barocaloric cooling/heating cycle, for active barocaloric waste heat management, the hysteresis may not be a constraint and transition temperatures above ambient are usually desired. In contrast to passive waste heat management, the application of pressure can be an advantage to actively control the absorption and delivery of heat by the plastic crystal. Here, we have investigated the pressure-induced caloric response at the first-order phase transitions occurring above room temperature of three plastic crystals derived from adamantane: 1-adamantanol, 2-adamantanol, and 2-methyl-2-adamantanol. Colossal barocaloric effects have been found for two of them under small pressure changes of 50 MPa. This behavior occurs thanks to a colossal transition entropy change and a large transition sensitivity to pressure, which can simultaneously take place due to enormous transition volume changes. The balance between configurational and volumic entropy changes at the transition has also been discussed. For 2-adamantanol, in addition to the transition to the plastic phase, the less energetic triclinic-to-monoclinic transition at lower temperatures has also been analyzed. The transition temperatures above ambient make these compounds suitable for waste heat management and, thanks to a small hysteresis, also for industrial cooling and heat pumping.Peer ReviewedPostprint (published version

Structure and reorientational dynamics of 1-F-adamantane

The polymorphism and the dynamics of a simple rigid molecule (1-fluoro-adamantane) have been studied by means of X-ray powder diffraction and broadband dielectric spectroscopy. At temperatures below the melting point, the molecule forms an orientationally disordered Phase I with a cubic-centered structure (Phase I, Fm (3) over barm, Z = 4). This phase possesses eight equilibrium positions for the fluorine atom, with equal occupancy factors of 1/8. A solid-solid phase transition to a low-temperature tetragonal phase (Phase II, P (4) over bar2(1)c, Z = 2) reduces the statistical disorder to only four possible equivalent sites for the fluorine atom, with fractional occupancies of 1/4. The dynamics has been rationalized under the constraints imposed by the space group of the crystal structure determined by powder X-ray diffraction. The dielectric spectroscopy study reveals that the statistical disorder in Phase II is dynamic in character and is associated with reorientational jumps along the two-and three-fold axes. In the dielectric loss spectra, the cooperative (alpha) relaxation exhibits a shoulder on the high-frequency side. This remarkable finding clearly reveals the existence of two intrinsic reorientational processes associated with the exchange of the F atom along the four sites. In addition to suchPeer ReviewedPostprint (published version

Reversible and irreversible colossal barocaloric effects in plastic crystals

The extremely large latent heat exchanged in phase transitions involving strong molecular orientational disordering has recently led to the proposal of plastic crystals as a feasible solution for solid-state barocaloric eco-friendly cooling technologies. Here we determine the reversible barocaloric response of four plastic crystals derived from neopentane [C(CH3)4]: (NH2)C(CH2OH)3 (TRIS for short), (NH2)(CH3)C(CH2OH)2 (AMP), (CH3)C(CH2OH)3 (PG) and (CH3)3C(CH2OH) (NPA). All of them display colossal entropy changes at their ordered-plastic phase transition, which is a primal requirement for competitive barocaloric materials. However, we show that it is also important to verify that the large barocaloric effects can be achieved using pressures that, while being moderate, are large enough to overcome the pressure-dependent hysteresis. From this quantity and using the quasi-direct method, we determine the minimum pressure needed to achieve reversible barocaloric effects, prev, for each compound. Specifically, we find a small and moderate prev for PG and NPA, respectively, which therefore display colossal reversible barocaloric effects comparable to harmful fluids used in current refrigerators and thus confirm the potential of plastic crystals as excellent alternatives. Instead, in TRIS and AMP, the obtained prev is excessive to yield reversible barocaloric effects useful for cyclic applications

Barocaloric response of plastic crystal 2-methyl-2-nitro-1-propanol across and far from the solid-solid phase transition

Plastic crystals have emerged as benchmark barocaloric (BC) materials for potential solid-state cooling and heating applications due to huge isothermal entropy changes and adiabatic temperature changes driven by pressure. In this work we investigate the BC response of the neopentane derivative 2-methyl-2-nitro-1-propanol (NO2C(CH3)2CH2OH) in a wide temperature range using x-ray diffraction, dilatometry and pressure-dependent differential thermal analysis. Near the ordered-to-plastic transition, we find colossal BC effects of $\simeq$400¿J¿K-1¿kg-1 and $\simeq$5¿K upon pressure changes of 100¿MPa. Although reversible effects at the transition are obtained only from higher pressure changes due to hysteretic effects, we do obtain fully reversible BC effects from any pressure change in individual phases, that become giant at moderate pressures due to very large thermal expansion, especially in the plastic phase. From our measurements, we also determine the crystal structure of the low-temperature phase and estimate the contribution of the configurational disorder and the volume change to the total transition entropy change.Peer ReviewedPostprint (author's final draft

Relationship between the two-component system 1-Br-adamantane + 1-Cl-adamantane and the high-pressure properties of the pure components

The temperature-composition phase diagram of the two-component system 1-Br-adamantane and 1-Cl-adamantane has been determined by means of thermal analysis techniques and X-ray powder diffraction from the low-temperature phase to the liquid state. The crossed isopolymorphism formalism has been applied to the two-component system to infer the normal pressure properties of the orthorhombic metastable phase of 1-Cl-adamantane at normal pressure. The experimental pressure-temperature phase diagrams for the involved compounds are related to the two-phase equilibria determined at normal pressure and inferences about the monotropic behavior of the aforementioned orthorhombic phase are discussed.Peer ReviewedPostprint (author's final draft

Transitions de phase et desordres structuraux dans le compose bidimentionnel a structure perovskite : NH3(CH2)5NH3CdCl4

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc