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Relaxational Dynamics in the Glassy, Supercooled Liquid, and Orientationally Disordered Crystal Phases of a Polymorphic Molecular Material

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Relaxational Dynamics in the Glassy, Supercooled Liquid, and Orientationally Disordered Crystal Phases of a Polymorphic Molecular Material PHYSICAL REVIEW B VOLUME 59, NUMBER 14 1 APRIL 1999-II Relaxational dynamics in the glassy, supercooled liquid, and orientationally disordered crystal phases of a polymorphic molecular material M. Jime´nez-Ruiz, M. A. Gonza´lez, and F. J. Bermejo Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientı´ficas, Serrano 123, E-28006 Madrid, Spain M. A. Miller and Norman O. Birge Department of Physics and Astronomy and Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824 I. Cendoya and A. Alegrı´a Departamento de Fı´sica de Materiales, Facultad de Quı´mica, Apartado 1072, 20080 San Sebastia´n, Spain ~Received 14 September 1998! The relaxational dynamics of the ambient pressure phases of ethyl alcohol are studied by means of mea- surements of frequency dependent dielectric susceptibility. A comparison of the a relaxation in the super- cooled liquid and in the rotator phase crystal indicates that the molecular rotational degrees of freedom are the dominant contribution to structural relaxation at temperatures near the glass transition, the flow processes having lesser importance. Below the glass transition a secondary b relaxation is resolved for the orientational and structural glasses. Computer molecular-dynamics results suggest that localized molecular librations, strongly coupled to the low-frequency internal molecular motions, are responsible for this secondary relax- ation. @S0163-1829~99!00914-5# I. INTRODUCTION Most studies to date have been conducted on materials that can be studied within the supercooled liquid ~SCL! state The dynamics of glass-forming materials in the highly for a reasonable long period of time. This excludes simple viscous regime characteristic of the supercooled liquid have binary alloys, which may be described in terms of soft been extensively studied during recent years from theoretical spheres. Instead, materials with a complexity ranging from and experimental points of view. Empirically, some regulari- ionics to high polymers have been scrutinized4 and the re- ties have been found in a vast number of glasses, which lead sults have sometimes been interpreted within the framework to their identification as fingerprints of glassy dynamics. of theories couched for simple hard-sphere fluids. This prob- Mechanical or dielectric spectroscopies are sensitive to lem is only recently being addressed with the introduction of mode coupling approximations which take explicit account motions with frequencies stretching from macroscopic to 5 mesoscopic scales. This makes them especially suited for the of the nonsphericity of the particles and orientation depen- exploration of the dynamics of glass-forming liquids within dent forces. While the rotational dynamics in low viscosity molecular their highly viscous regimes ~i.e., shear viscosities about liquids is usually understood on hydrodynamic grounds 102 –1010 P). The strongest peak in the dielectric spectrum ~Stokes-Einstein-Debye!,6 such approximations usually known as the relaxation shows some features regarded as a break down within the SCL regime. This happens as a con- ‘‘universal.’’ These include a non-Debye behavior of its line sequence of the sluggish motions which take place when the shape and a temperature dependence of the relaxation time shear viscosity exceeds some tens of poises, which makes t(T) strongly departing from Arrhenius as the temperature is any description in terms of uncoupled rotations and center- decreased towards the glass transition temperature Tg , and of-mass diffusion of scarce value. In actual fact, on strict showing a divergence at temperatures below Tg . Such a de- hydrodynamic grounds, the same dependence on viscosity pendence is usually parameterized by means of the empirical ~and temperature! is expected for both mass diffusion and 1 Vogel-Tamman-Fulcher law. Secondary, b, or sub-Tg relax- rotational coefficients. Therefore a test of the hydrodynamic ations are detectable at higher frequencies, usually but not predictions could be carried out by studying a single system exclusively at temperatures well below Tg . They commonly where one of the two kinds of motions can be cleanly iso- show Arrhenius behavior and have been measured for many lated by experimental means. glassy systems, albeit reliable data are scarce since the sig- As described previously, ethyl alcohol can be prepared in nals are very weak and broad. Whether this relaxation arises two phases showing quantitatively close ‘‘glassy’’ behaviors from intramolecular motions which remain active below Tg , at the same temperatures and very close densities, one of a view defended by Wu2 and others, or it is an intrinsic these phases with topological disorder ~liquid and glassy! property of glasses due to local rearrangements taking place and the other showing orientational disorder ~rotator crystal within some minima of the potential energy,3 still needs to be and orientational glass phases!.7 By orientationally disor- clarified. dered crystals, we refer to those crystalline solids where the 0163-1829/99/59~14!/9155~12!/$15.00PRB 59 9155 ©1999 The American Physical Society 9156 M. JIME´ NEZ-RUIZ et al. PRB 59 molecular centers of mass sit at lattice positions of a long- ethyl alcohol ~ethanol! obtained from Quantum Chemical range periodic array, while molecular orientations remain Company of Tuscola, Illinois. To avoid contamination with disordered. Such disorder may be of dynamic nature as hap- water vapor, filling of the capacitor was carried out at room pens in the rotator phase crystal ~RP! where the molecules temperature in a dry nitrogen atmosphere. For the low- execute whole body rotations,8,9 or static as in the orienta- temperature measurements (T,110 K) a sealed coaxial ca- 16 tional glass ~OG! state which is attained by freezing the ro- pacitor identical in design to that used in our previous work tations into random orientations. Both phases thus provide a was used. After filling the capacitor, the sample cell was way to separate the contributions of the rotational and center- frozen, evacuated of remaining nitrogen gas, and sealed. The sample cell was then mounted inside a nested cryocan ar- of-mass motions to the dynamics. 16 The RP OG rotational freezing transition exhibits most rangement described elsewhere. Finally, the entire cryocan of the characteristics→ of a purely dynamical phenomenon. It assembly was immersed in a double-walled glass dewar. This design provided a temperature control within 0.05 K shows a jump in the specific heat across the transition10 con- over the range 4–300 K, and minimized the occurrence of comitant with a jump in the thermal expansivity,11 without temperature gradients across the sample. For the measure- any other change attributable to a structural transition. Also ments at T.150 K, in the liquid state, and high frequencies, and in parallel with the canonical liquid glass, the transi- → 12 1 MHz–1 GHz, a parallel plate sample capacitor was tion can be induced by application of moderate pressures. mounted as part of the inner conductor of the coaxial mea- To investigate in more detail the close similarity between surement line. The temperature control in this case was the relaxation processes of ethanol in the amorphous and achieved by employing a nitrogen-jet heating/cooling system orientational glass form, low-temperature specific-heat mea- with temperature stability better than 0.1 K. surements and neutron time-of-flight ~TOF! spectroscopy The sample preparation regarding the normal glass phase have been carried out, and the results reveal that at low tem- and the rotator phase ~RP! crystal is straightforward since it perature the vibrational density of states of these two phases only involves a quench below Tg597 K at a rate greater is very similar.14 Such dynamic proximity has also been ex- than 6 K/min ~glass! and a subsequent annealing at 110 K tended to macroscopic scales by means of dielectric spectros- during half an hour ~RP!. The orientational glass ~OG! is 15 OG copy measurements, and the results lead to the consider- formed from the RP by cooling down below Tg 597 K. ation of this material as an ideal candidate for studies of the The measurements of the a relaxation span the frequency glass transition. range from 1 mHz–10 MHz, of the liquid state from 1 Here, our purpose is to provide as detailed a comparison MHz–1 GHz, and the b relaxation from 10 Hz–10 MHz. as possible of the relaxational processes taking place at both A number of computer simulations of the glass, super- sides of the glass transitions in glassy and orientationally cooled liquid, and crystalline modifications of the material disordered crystalline ethanol by means of dielectric spec- have been carried out.17 The results relevant for this work are troscopy. Our aim goes well beyond that of our previous those obtained from a simulation using the optimized poten- report15 since a far wider spectral band, comprising both a tial model for liquid simulations ~OPLS!,18 which describes and b relaxations is considered. Since preliminary data15 in- reasonably well the static and some dynamic properties of dicated a close analogy between the main relaxation appear- the normal-liquid phase.19 The system consisted of 216 mol- ing in both SCL and RP phases, here we pursue a study of ecules at densities of 0.941 g cm23 and a temperature of 80 the relaxation detectable below freezing in both solids as K which corresponds to the glass phase, and 0.888 g cm23 well as of the high-frequency wing of the spectra of both and 180 K, which, within the computer model corresponds to SCL and RP. The motivation behind this is the possibility the SCL. The initial configuration was obtained from a offered by such a chemically simple material to unravel quench of the liquid at 298 K at a cooling rate of dT/dt some of the details of the dynamics at microscopic scales 50.1 K ps21.
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