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11 págs.; 9 figs. ; The temperature dependence of the parameters characterizing the first peak in the S(Q) and S(Q,E=0) diffraction patterns of vitreous boron trioxide is considered in some detail. The analysis of the experimental spectra is aided by results derived from molecular-dynamics simulations, which enable us to isolate the most relevant features driving the variation with temperature of both structure factors. The relevance of the present results regarding some recent phenomenological approaches developed towards the understanding of the dynamics of glasses at intermediate temperatures is finally discussed. ©1996 American Physical Society ; Work was performed in part under D.G.I.C.Y.T (Spain) Grant No. PB92-0114-C03-01. Financial support from the Large Scale Facilities Programme of the European Union for the measurements carried at Risoe National Laboratory (Denmark) is acknowledged. J.D. wishes to thank CONICET (Argentina) and CSIC (Spain) for financial support. ; Peer Reviewed

7 pags., 7 figs., 1 tab. ; We applied the recently developed Generalized Langevin equation (GLE) approach for dielectric response of liquids and glasses to link the vibrational density of states (VDOS) to the dielectric response of a model orientational glass. The dielectric functions calculated based on the GLE, with VDOS obtained in experiments and simulations as input, are compared with experimental data for the paradigmatic case of 2-Adamantanone at various temperatures. The memory function is related to the integral of the VDOS times a spectral coupling function γ(ωp), which tells the degree of dynamical coupling between molecular degrees of freedom at different eigenfrequencies. With respect to previous empirical fittings, the GLE-based fitting reveals a broader temperature range over which the secondary relaxation is active. Furthermore, the theoretical analysis provides clear evidence of secondary relaxation being localized within the THz (0.5-1 THz) range of eigenfrequencies, and thus not too far from the low-energy modes involved in α relaxation. In the same THz region, the same material displays a crowding of low-energy optical modes that may be related to the secondary relaxation. ; B.C. acknowledges the financial support of CSCCambridge Scholarship. J.L.T. acknowledges MINECO (No. FIS2017-82625-P) and the Catalan government (No. SGR2017-042) for financial support.

7 pags., 10 figs., 2 tabs. ; Trans-1,2-dichloroethene (HClC=CClH) has several structural and dynamic anomalies between its low- and high-density liquid, previously found through neutron scattering experiments. To explain the microscopic origin of the differences found in those experiments, a series of molecular dynamics simulations were performed. The analysis of molecular short-range order shows that the number of molecules in the first neighbor shell is 12 for the high-density liquid and 11 for the low-density one. It also shows that the angular position of the center of mass of the first neighbor is roughly the same although the molecular orientation is not. In both liquids the first neighbor and its reference molecule arrange mainly in two configurations, each being the most probable in one of the liquids. First neighbors in the configuration that predominates in the high-density liquid tend to locate themselves closer to the reference molecule, an evidence that they are more strongly bonded. This arrangement facilitates a better packing of the rest of molecules in the first neighbor shell so that on average an additional molecule can be included, and is proposed to be the key in the explanation of all the observed anomalies in the characteristics of both liquids. © 2012 American Institute of Physics. ; This work was supported by the Spanish Ministry of Science and Innovation (FIS2008-00837) and by the Government of Catalonia (2009SGR-1251).

We report on the interplay between the short-range order of molecules in the liquid phase of 1,1,2,2-tetrachloro-1,2-difluoroethane and the possible molecular conformations, trans and gauche. Two complementary approaches have been used to get a comprehensive picture: analysis of neutron-diffraction data by a Bayesian fit algorithm and a molecular dynamics simulation. The results of both show that the population of trans and gauche conformers in the liquid state can only correspond to the gauche conformer being more stable than the trans conformer. Distinct conformer geometries induce distinct molecular short-range orders around them, suggesting that a deep intra- and intermolecular interaction coupling is energetically favoring one of the conformers by reducing the total molecular free energy. © 2011 American Physical Society. ; This work was supported by the Spanish Ministry of Science and Innovation (Grant No. FIS2008-00837) and by the Government of Catalonia (Grant No. 2009SGR-1251). ; Peer Reviewed

10 págs.; 6 figs.; 4 tabs. ; © 2015 AIP Publishing LLC. The thermal conductivity, specific heat, and specific volume of the orientational glass former 1,1,2-trichloro-1,2,2-trifluoroethane (CCl2F–CClF2, F-113) have been measured under equilibrium pressure within the low-temperature range, showing thermodynamic anomalies at ca. 120, 72, and 20 K. The results are discussed together with those pertaining to the structurally related 1,1,2,2-tetrachloro-1,2-difluoroethane (CCl2F–CCl2F, F-112), which also shows anomalies at 130, 90, and 60 K. The rich phase behavior of these compounds can be accounted for by the interplay between several of their degrees of freedom. The arrest of the degrees of freedom corresponding to the internal molecular rotation, responsible for the existence of two energetically distinct isomers, and the overall molecular orientation, source of the characteristic orientational disorder of plastic phases, can explain the anomalies at higher and intermediate temperatures, respectively. The soft-potential model has been used as the framework to describe the thermal properties at low temperatures. We show that the low-temperature anomaly of the compounds corresponds to a secondary relaxation, which can be associated with the appearance of Umklapp processes, i.e., anharmonic phonon-phonon scattering, that dominate thermal transport in that temperature range. ; This work was financially supported in part by the Spanish Ministry of Science and Innovation (Grant Nos. FIS2014-54734-P, FIS2011-23488, and MAT2014-57866- REDT), by the Catalan Government (Grant No. 2014SGR- 0581) and by the Comunidad de Madrid through program NANOFRONTMAG-CM (No. S2013/MIT-2850), as well as by the joint NAS Ukraine and Russian Foundation for Basic Research project "Metastable states of simple condensed systems" (Agreement No. N 7/-2013). ; Peer Reviewed

6 págs.; 3 figs.; 2 tabs. ; PACS number s : 65.60. a, 66.70. f, 61.43. j, 63.50. x ; The thermal conductivity k(T) of the orientational glass state of 1,2-difluoro-1,1,2,2-tetrachloroethane (CFCl2-CFCl2, Freon 112) and cyanocyclohexane (C6H11CN) has been measured under equilibrium pressure within the temperature range 2-100 K. The results show that the soft-potential model is able to account for low-temperature data and, indeed, quantitative agreement of all data considered is found within this realm. The details beyond such temperatures are heavily dependent on chemical details and a large plateau is observed for Freon 112 which is rationalized in terms of resonant scattering of phonons by simple oscillators. Such a view is given additional support by the presence of a strong low-frequency feature in the generalized frequency spectrum of the former material as proven by inelastic neutron-scattering spectroscopy. ©2010 The American Physical Society ; This work was financially supported in part by the Spanish Ministry of Science and Innovation Grants No. FIS2008-00837 and No. MAT2007- 65711-C-4-01 and the Catalan Government Grant No. 2009SGR-1251 and joint projects of NAS of Ukraine and the Russian Foundation for Fundamental Research Grant No. N 9-2009. ; Peer Reviewed

The thermal conductivity κ(T) of the fully ordered stable phase II, the metastable phase III, the orientationally disordered (plastic) phase I, as well as the nonergodic orientational glass (OG) phase, of the glass former cyclohexanol (C 6H 11OH) has been measured under equilibrium vapor pressure within the 2-200 K temperature range. The main emphasis is here focused on the influence of the conformational disorder upon the thermal properties of this material. Comparison of results with those regarding cyanoclyclohexane (C 6H 11CN), a chemically related compound, serves to quantify the role played by the terminal groups -OH and -CN on the phonon scattering processes. The picture that emerges shows that motions of such groups do play a minor role as scattering centers, both within the low-temperature orientationally ordered phases as well as in the OG states. The results are analyzed within the Debye-Peierls relaxation time model for isotropic solids comprising mechanisms for long-wave phonon scattering within the OG and orientational ordered low-temperature phases, as well as others arising from localized short-wavelength vibrational modes as pictured by the Cahill-Pohl model. By means of complementary neutron and Raman scattering we show that in the OG state the energy landscapes for both compounds are very similar. © 2012 American Physical Society. ; This work was financially supported in part by the Spanish Ministry of Science and Innovation (Grant No. FIS2008-00837) and the Catalan Government (Grant No. 2009SGR-1251) ; Peer Reviewed

4 págs.; 3 figs.; PACS number s : 64.70.Ja, 61.05.fm, 61.20. p, 61.25.Em ; We present a study of the dynamics and structural changes for trans-1,2-dichloroethylene between high- and low-density liquids using neutron-scattering techniques diffraction, small-angle neutron scattering, and time of flight spectroscopy and molecular-dynamics simulations. We show that changes in the short-range ordering of molecules goes along with a change in the molecular dynamics: both structure and dynamics of the highdensity liquid are more cooperative than those of the low-density liquid. The microscopic mechanism underlying the cooperative motions in the high-density liquid has been found to be related to the backscattering of molecules due to a strong correlation of molecular ordering. ©2010 The American Physical Society ; This work was supported by the Spanish Ministry of Science and Technology Grants No. FIS2008-00837 and No. BES-2007-17418, by the Government of Catalonia Grant No. 2009SGR-1251 and by the European Commission MI3/FP7. ; Peer Reviewed