M. Rovira-Esteva

Differences in first neighbor orientation behind the anomalies in the low and high density trans-1,2-dichloroethene liquid

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.

Comparison of the atomic level structure of the plastic crystalline and liquid phases of CBr2Cl2: neutron diffraction and reverse Monte Carlo modelling.

Neutron diffraction results obtained for plastic crystalline dichlorodibromomethane (CBr2Cl2) have been modelled by means of the reverse Monte Carlo method. Comparison with its liquid phase is provided at several levels of the atomic structure (total scattering structure factors, partial radial distribution functions, orientational and dipole-dipole correlations). The results reveal that the relative orientation of neighbouring molecules largely depends on the steric effect. The small dipole moment does not have as strong an influence as the steric effect on the short-range order. Our observations fit well with earlier findings presented for the series CBr(n)Cl(4-n) (n = 0, 1, 2, 4).

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

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.

Insights into the determination of molecular structure from diffraction data using a Bayesian algorithm

The determination of the molecular ordering in a liquid is still a controversial subject. There is no general consensus either on the methods to obtain reliable liquid structures or on the way to analyze them. Regardless of the method, it is very important to have a realistic molecular structure available that allows simulations to faithfully reproduce the sample features, and that minimizes the computing time in structure refinements. However, attention is not always paid to this point and molecular models coming from general force-fields are frequently used to undertake many of the analyses. We propose in this work to use a Bayesian scheme to fit the experimental data and produce reliable molecular models that can be used as the starting point of any simulation or refinement. The algorithm behind the proposed method is based on a Markov chain Monte Carlo procedure, as many other refinement programs such as reverse Monte Carlo or empirical potential structure refinement.

Fitting in a complex χ(2) landscape using an optimized hypersurface sampling.

Fitting a data set with a parametrized model can be seen geometrically as finding the global minimum of the χ(2) hypersurface, depending on a set of parameters {P(i)}. This is usually done using the Levenberg-Marquardt algorithm. The main drawback of this algorithm is that despite its fast convergence, it can get stuck if the parameters are not initialized close to the final solution. We propose a modification of the Metropolis algorithm introducing a parameter step tuning that optimizes the sampling of parameter space. The ability of the parameter tuning algorithm together with simulated annealing to find the global χ(2) hypersurface minimum, jumping across χ(2){P(i)} barriers when necessary, is demonstrated with synthetic functions and with real data.

Microscopic dynamics of glycerol: a QENS study

We report on a quasielastic incoherent neutron scattering (QENS) experiment on liquid glycerol. QENS data were collected at the temperature T=380 K and with a resolution (FWHM) R=55μeV. The analysis of the quasielastic signal enables us to draw a consistent picture of the diffusive mechanism on a picosecond time scale and to compare with most recent models for glycerol dynamics. Model selection, performed with the fitting algorithm FABADA, gives us a preliminary description about the motions of the glycerol molecules in its liquid state.

Erratum: Microscopic structures and dynamics of high- and low-density liquid trans-1,2-dichloroethylene [Phys. Rev. B 81, 092202 (2010)]

M. Rovira-Esteva, A. Murugan, L. C. Pardo, S. Busch, M. D. Ruiz-Mart́ın, M.-S. Appavou, J. Ll. Tamarit, C. Smuda, T. Unruh, F. J. Bermejo, G. J. Cuello, 6 and S. J. Rzoska Grup de Caracterització de Materials, Departament de F́ısica i Enginyeria Nuclear, ETSEIB, Universitat Politècnica de Catalunya, Diagonal 647, 08028 Barcelona, Catalonia, Spain Physik Department E13 and Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, Lichtenbergstr. 1, 85748 Garching, Germany Forschungszentrum Jülich GmbH, Institut für Festkörperforschung (IFF), Jülich Centre for Neutron Science (JCNS), FRM II outstation, Lichtenbergstr. 1, 85748 Garching, Germany ETH Zürich, Center for Radiopharmaceutical Science, Wolfgang-Pauli-Str. 10, CH-8093 Zürich, Switzerland Facultad de Ciencia y Tecnoloǵıa, Universidad del Páıs Vasco / EHU, P. Box 644, E-48080 Bilbao, Spain Institut Laue Langevin, 6 Rue Jules Horowitz, BP. 156, F-38042 Grenoble Cedex 9, France Institute of Physics, Silesian University, Uniwersytecka 4, 40-007 Katowice, Poland

FABADA: a Fitting Algorithm for Bayesian Analysis of DAta

The fit of data using a mathematical model is the standard way to know if the model describes data correctly and to obtain parameters that describe the physical processes hidden behind the experimental results. This is usually done by means of a χ2 minimization procedure. Although this procedure is fast and quite reliable for simple models, it has many drawbacks when dealing with complicated problems such as models with many or correlated parameters. We present here a Bayesian method to explore the parameter space guided only by the probability laws underlying the χ2 figure of merit. The presented method does not get stuck in local minima of the χ2 landscape as it usually happens with classical minimization procedures. Moreover correlations between parameters are taken into account in a natural way. Finally, parameters are obtained as probability distribution functions so that all the complexity of the parameter space is shown.

Neutron Diffraction as a Tool to Explore the Free Energy Landscape in Orientationally Disordered Phases

The temperature dependence of structural parameters of orientational glasses of the halogenomethane family, Freon 112 (FCl2C)-(CCl2F)) and Freon 112a (F2ClC)-(CCl3)) are studied at short- (molecular) intermediate- (orientational correlations) and long-range (lattice parameters) scales by means of neutron diffraction. The two materials which are chemical isomers display strikingly different properties in their ordering patterns resulting from a shift in balance between electrostatic and excluded-volume interaction. The relevance of these findings to our understanding of glassy phenomena is discussed.

A Procedure to Quantify the Short Range Order of Disordered Phases

Determination of the short- and intermediate-range structure of disordered materials is a necessary step to fully understand their properties. Despite of this, no generally accepted procedure exists to date to extract structural information from diffraction data. In this paper we describe a method which enables determination of the short-range structure of disordered molecular phases. This general method is applied to one of the first studied molecular liquids, carbon tetrachloride, and to its plastic phase being able to unravel the so called local density paradox: although molecules are closer in the liquid than in the plastic phase, the density of the former is lower than that of the later. The analysis of the short range order in both phases shows that although the minimal energy configuration allows a closer approach of molecules, it hinders the formation of the face centered cubic long range ordered lattice due to the difficulty of molecules to form stacked structures.