Propiedades vitreas emergentes en cristales moleculares

One of the most complex and unsolved problems in physics is the glass transition problem, in which the resulting state, the glass, exhibits unique and anomalous properties. During the transition, a kinetic freezing phenomenon occurs, in which a state of equilibrium of a many-body disordered system transforms into a non-equilibrium state, with similar mechanical properties to a crystalline salid. The case of the structural glass represents the paradigmatic of these systems. This state is achieved when a molecular or atomic liquid is supercooled below the crystallization temperature and to temperatures so low that, the viscosity and characteristic relaxation times diverge (a similar divergence can be reached also by over-pressurizing the liquid). However, liquids are not the only systems to produce glassy states. In fact, any disordered system that exhibits sorne interna! dynamics can in principie be supercooled to a non-ergodic state with "frozen" disorder. The resulting phase is considered as a glassy-like state, by its very non-ergodic nature. Ali these systems exhibit a set of anomalous features compared to those of crystalline solids. These anomalies, up to now universal to for glasses, include the wellcknown boson peak, visible in the low-temperature specific heat (and also in low-energy vibrational density of states), a linear temperature dependence below 1-2 K in the specific heat, and a drastic drop of the thermal conductivity below 100 K with respect to their crystalline counterpart. In spite of the attempts of the last 50 years to find a universal explanation for these anomalous glassy features, their fundamental origin is still a matter of debate. In this thesis we have focused on the study of these anomalies in molecular glasses obtained from translationally ordered and orientationally disordered phases, in which the dynamics of molecular reorientations can be frozen under certain conditions, for example, by fast cooling. The resulting glassy state is, then, an orientational glass. The orientationaly disordered systems presented in this work are actually crystalline phases of rigid molecules. The corresponding lattice symmetries impose restrictions to molecular orientations wich determine a statistical and controlled disorder in the system. These cases have been chosen due to the high reduction of com plexity of the resulting glass com pared to structural glas ses, due to the existence of translational arder. We propase that the study of these peculiar systems with restricted disorder can shed light onto the origin of low-temperature specific heat anomalies (and low-energy density of states anomalies) and on the relevance of the disorder on these physical magnitudes. The materials chosen for this study are mainly insulating molecular systems such as: the halomethane family CBr$_n$Cl$_{4-n}$ with n=0,1,2, two adamantane derivatives, 2-adamantanone and 1-fluoro-adamantane, and three sollds formed by planar moleculés, thiophene (pristine and deuterated), parachloronitrobenzene, and pentachloronitrobenzene. A common conclusion could be drawn for in all these systems: the presence of low-energy optical excitations in which rigid molecules exhibit rotational-translational motions coupled to propagating acoustic waves. This phenomenon induces an excess of vibrational states that gives rise to the boson peak, regardless of the ordered or disordered nature of th¿¿ phase studied Furthermore, we discuss the anarmonicity of these vibrational modes and their influence on the boson peak and the anomaly visible below 1-2 K in the specific heat.no de los problemas de la física más complejos y no resueltos es el de la transición vítrea, en el que el estado resultante, el vidrio, manifiesta propiedades únicas y anómalas. En la transición se produce un fenómeno de congelación cinética donde un sistema de muchas partículas en un estado de equilibrio desordenado, pero ergódico, transforma a un estado de no equilibrio, no ergódico, con propiedades mecánicas similares a las de un sólido cristalino. El caso de los vidrios estructurales representa el paradigma por excelencia de estos sistemas. Este estado se consigue cuando un líquido molecular o atómico es subenfriado por debajo de su temperatura de cristalización, donde el enfriamiento (o aumento de presión) produce una divergencia de la viscosidad y de los tiempos característicos de relajación. Sin embargo, éstos no son los únicos sistemas que pueden dar lugar a uri estado vítreo. De hecho, cualquier sistema desordenado que presente una dinámica interna puede ser, en principio, subenfriado hasta alcanzar un estado no ergódico con desorden "congelado". La fase resultante puede ser considerada un tipo de vidrio, dado su carácter no ergódico. Estos sistemas manifiestan una serie de características anómalas en comparación con las de un sólido cristalino. Estas anomalías, consideradas hasta el presente como huellas características, y, por lo tanto, universales de los vidrios incluyen el famoso pico bosónico presente en el calor específico a bajas temperaturas (y también en la densidad de estados vibracional a bajas energías), una dependencia lineal con la temperatura por debajo de 1-2 K en el calor específico, y una reducción drástica en la conductividad térmica respecto al estado homólogo cristalino por debajo de 100 K. A pesar de los intentos de los últimos 50 años para encontrar una explicación universal de estas características anómalas de los vidrios, sus orígenes fundamentales son aún un tema de debate. En esta tesis nos hemos centrado en el estudio de estas anomalías de los vidrios moleculares formados a partir de fases traslacionalmente ordenadas, pero orientacionalmente desordenadas, donde la dinámica de reorientación molecular se ve "congelada" bajo ciertas condiciones, por ejemplo, un enfriamiento rápido. El estado vítreo resultante es, por tanto, un vidrio orientacional. Los sistemas orientacionalmente desordenados presentados en este trabajo son, de hecho, fases cristalinas de moléculas rígidas, cuyas simetrías de red imponen restricciones en las orientaciones moleculares, produciendo un desorden estadístico y controlado en el sistema, Estos casos se han escogido por la fuerte reducción de la complejidad del vidrio resultante en comparación con los vidrios estructurales, debido a la existenciá de un orden traslacional. De esta manera, se pretende ahondar en el origen de las anomalías de baja temperatura en el calor específico (y de baja energía en la densidad de estados vibracional) y la relevancia del desorden sobre estas magnitudes. Los materiales escogidos para este estudio son principalmente sistemas moleculares aislantes de naturaleza diversa como: la familia de halometanos CBr$_n$C1$_{4-n}$ con n=0, 1,2, dos derivados del adamantano, los compuestos 2-adamantanona y 1- fluoro-adamantano, y tres sólidos de moléculas planares, el tiofeno (normal y deuterado), el paracloronitrobenceno y el pentacloronitrobenceno. En todos estos sistemas se ha encontrado un denominador común: la presencia de excitaciones ópticas de baja energía donde las moléculas rígidas exhiben movimientos oscilatorios de tipo roto-traslacional acoplados a ondas de tipo acústico. Este fenómeno induce un exceso de estados vibracionales que dan lugar a la aparición del pico bosónico, independientemente del carácter ordenado o desordenado de la fase estudiada. Además, se discute la anarmonicidad de estos modos de vibración y su influencia en el pico bosónico y en la la anomalía visible por debajo de 1-2 K en el calor específico.Postprint (published version

Disentangling a and ß 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 Tg. The memory function is related to the integral of the VDOS times a spectral coupling function Y(¿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 ß 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 comparatively lower boson peak in the VDOS. The proposed framework brings together all the key aspects of glassy physics (VDOS with boson peak, dynamical heterogeneity, dissipation, anharmonicity) into a single model.Peer ReviewedPostprint (published version

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

We measured the specific heat Cp 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 Cp ˜ T3. This case study demonstrates that the identified low-temperature Cp anomaly, typically referred to as a “Boson-peak” in the context of glassy crystals, is not exclusive of disordered materials.Peer ReviewedPostprint (published version

Multiferroic and related hysteretic behavior in ferromagnetic shape memory alloys

We combine a Ginzburg–Landau model for a ferroelastic transition with the theory of micromagnetism to study the magnetostructural behavior leading to multicaloric effects in ferromagnetic shape memory alloys. We analyze the ferroelastic transition under different conditions of temperature, stress and magnetic field and establish the corresponding phase diagram. On the one hand, our results show that the proper combination of both fields may be used to reduce the transition hysteresis and thus improve the reversibility of the related elastocaloric effects, superelasticity and stress-mediated magnetocaloric effects. On the other hand, the stress-free magnetic field-driven and thermally driven magnetostructural evolution provides physical insight into the low-temperature field-induced domain reorientation, from which we derive strategies to modify the operational temperature ranges and thus the corresponding (magnetic) shape-memory effect.Peer ReviewedPostprint (published version

Heat capacity anomalies of the molecular crystal 1-fluoro-adamantane at low temperatures

Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T¿=¿180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.Peer ReviewedPostprint (published version

Emergence of glassy features in halomethane crystals

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 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(¿)/¿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 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.Peer ReviewedPostprint (published version

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.Peer ReviewedPostprint (published version

Fire protection for liquefied gas storage tanks

20.00; Translated from German; Paper at BAM meeting, Berlin Oct 1987Available from British Library Document Supply Centre- DSC:9022.381(HSE-Trans--12905)T / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Modelling shape-memory effects in ferromagnetic alloys

We develop a combined Ginzburg–Landau/micromagnetic model dealing with conventional and magnetic shape-memory properties in ferromagnetic shape-memory materials. The free energy of the system is written as the sum of structural, magnetic and magnetostructural contributions. We first analyse a mean field linearized version of the model that does not take into account long-range terms arising from elastic compatibility and demagnetization effects. This model can be solved analytically and in spite of its simplicity allows us to understand the role of the magnetostructural term in driving magnetic shape-memory effects. Numerical simulations of the full model have also been performed. They show that the model is able to reproduce magnetostructural microstructures reported in magnetic shape-memory materials such as Ni2MnGa as well as conventional and magnetic shape-memory behaviour

Multiferroic and related hysteretic behavior in ferromagnetic shape memory alloys

We combine a Ginzburg–Landau model for a ferroelastic transition with the theory of micromagnetism to study the magnetostructural behavior leading to multicaloric effects in ferromagnetic shape memory alloys. We analyze the ferroelastic transition under different conditions of temperature, stress and magnetic field and establish the corresponding phase diagram. On the one hand, our results show that the proper combination of both fields may be used to reduce the transition hysteresis and thus improve the reversibility of the related elastocaloric effects, superelasticity and stress-mediated magnetocaloric effects. On the other hand, the stress-free magnetic field-driven and thermally driven magnetostructural evolution provides physical insight into the low-temperature field-induced domain reorientation, from which we derive strategies to modify the operational temperature ranges and thus the corresponding (magnetic) shape-memory effect.Peer Reviewe