A. Criado

Collective excitations in liquid methanol: A comparison of molecular, lattice‐dynamics, and neutron‐scattering results

The collective dynamics of liquid methanol‐d4 is studied by means of molecular‐dynamics simulation. The model potential is validated by means of lattice energy calculations and shows a very good agreement with the experimentally obtained crystal structure. Center‐of‐mass density and momentum fluctuations are investigated in the (Q,ω) region which is also accessible to inelastic neutron‐scattering (INS) techniques. A simple viscoelastic model previously used for the analysis of INS data is tested against the dynamic structure factor computed from the simulation. A direct comparison with the INS results themselves is also made and qualitative agreement is found. Also, a tentative assignment of the peaks appearing in the current–current correlations is made on the basis of lattice‐dynamics calculations for the polycrystalline low‐temperature α phase.

Crystal growth and characterization of a new nonlinear optical material: Urea L-Malic acid

A new nonlinear optical organic crystal, viz. Urea L-Malic acid, has been grown from aqueous solution employing the techniques of solvent evaporation and slow cooling. The grown crystals were found to be transparent in the visible region. The complete morphology of the crystal has been worked out and its hardness was evaluated using Vickers indentation method. The grown crystals were characterized employing several techniques such as micro-carbon-hydrogen-nitrogen analysis, X-ray diffraction, thermogravimetric analysis, differential thermal analysis, UV-VIS-IR spectroscopy and refractive index measurement. Preliminary measurements indicate that the second harmonic generation efficiency at a fundamental wavelength of 1064 nm is roughly three times that of KDP.

Thermodynamic and structural properties of the two isomers of solid propanol

Abstract We have measured the specific heat, Cp, of the two chemical isomers of propanol, both at low temperatures (1.5–30 K) and around the glass-transition region (≈100 K ) . Although the two substances present only the stereoisomeric difference, their glass-transition temperatures differ ( T g ≃98 K for 1-propanol and T g ≃115 K for 2-propanol), though with comparable changes in specific heat. At temperatures below 10 K, both isomers in the glassy state have the typical maximum in Cp/T3. Nevertheless, the magnitude of the low-temperature specific heat is much larger in 2-propanol than in 1-propanol, a difference which is also found in the Debye contributions in the corresponding crystalline states. To investigate the structural differences between the two isomers of propanol, and their possible influence on the thermodynamic properties, X-ray scattering experiments have been performed on samples of the glassy and crystalline states. In the glassy state, both isomers present the expected amorphous pattern, with characteristic distances of 0.40±0.06 and 0.42±0.07 nm for 1- and 2-propanol, respectively. Preliminary structural analysis of the crystalline states would assign a triclinic structure for 1-propanol and a monoclinic structure for 2-propanol.

On the microscopic origin of the “boson” peak in glassy materials

Abstract The origin of the low-frequency (“boson”) peak characteristic of the vibrational spectra of most glassy materials is discussed. A comparison between the generalized frequency distributions of several archetypal glasses with those of their ground states (harmonic crystals) is presented, and a correlation between the frequencies corresponding to the maxima of the lowest frequency peaks in the dynamic structure factors of the crystals and those of the glass phase, is inferred. The microscopic motions leading to the appearance of the “boson” peak as well as the relevance of the present findings regarding current discussions on glassy dynamics are finally commented on.

Microscopic dynamics of ortho-terphenyl in its polycrystalline and glassy forms

The microscopic dynamics of polycrystalline ortho-terphenyl are investigated by means of the concurrent use of harmonic lattice dynamics calculations and Raman spectroscopy. In order to reproduce the spectra and thermodynamics of the crystal up to about 70 K, the motions of several internal modes are considered in detail. An empirical force field, able to account for the observed crystal structure, thermodynamics and Raman spectra up to such temperature, is then developed. Estimates of the anharmonic effects are derived from the temperature dependence of vibrational frequencies of those modes of collective or hybrid character (below 6 THz). The Raman spectra of the glass are finally discussed by comparison with the polycrystalline case. The implications of the present findings regarding studies on the glass transition of complex materials are discussed.

Lattice Dynamics and Thermal Crystallographic Parameters in Phenothiazine

A computer program has been developed to study the lattice dynamics of molecular crystals in the harmonic approximation with the external Born-von Karman formalism and an atom-atom potential function. Dispersion curves are obtained for monoclinic phenothiazine together with frequency distribution functions and external mode contribution to thermodynamic functions. Lattice dynamical T, L and S rigid-body tensors are obtained and individual thermal tensors are compared with experiment. The disagreement with respect to experimental results is of the same order as the disagreement with a Schomaker-Trueblood fit of experimental data.

Collective low-frequency excitations in a molecular glass

The collective dynamics of an organic molecular glass former has been investigated by means of coherent inelastic scattering of neutrons at several temperatures below the glass transition. The excitations observed by means of the inelastic neutron scattering response are characterized by the wavevector dependences of their average frequencies. The inelastic spectra are analysed on the basis of a lattice dynamics calculation for the polycrystalline reference state as well as a molecular dynamics simulation of a computer-generated glass. The excitation frequencies are discussed in terms of average omega m quantities, defined as the peak maxima of the longitudinal current-current autocorrelation function. An assessment of the reliability of the simulation results is finally made by comparison with the experimentally accessible magnitudes.

Calorimetric and X-ray characterization of the non-isothermal crystallization of the metallic glass Ni89P11C (wt.%)

Abstract The crystallization of the Ni81P19C (at.%) quenched alloy is studied by calorimetric (DSC) and X-ray diffraction techniques. The transformation occurs in one stage (at 641 K) with ΔH=3.7±0.1 kJ/mol. The crystallization phases formed are Ni(fcc) and Ni3P(bct) and the phase proportions in the crystalline mixture, derived from the X-ray data, agree with those corresponding to the nominal composition of the amorphous alloy.

Crystallization kinetics of an Fe-Co based metallic glass

Crystallization kinetics of the Fe67Co18Si1B14 metallic glass has been studied from electrical conductivity measurements. Results are interpreted on the basis of a modification of the Germainet al. model (J. Non-Cryst. Solids23 (1977) 93). The characteristics of kinetics for different annealing temperatures are analysed and the activation energy is evaluated

Rotational freezing in plastic crystals: a model system for investigating the dynamics of the glass transition

The dynamics of the rotational freezing transition of the rotator phase (RP) crystal of ethanol into its orientational glass (OG) phase is monitored by measurements of molecular rotational components in the quasielastic neutron scattering spectrum. Such a transition is known to share several features in common with the canonical liquid glass transition (both taking place within the same temperature interval), and therefore some of the results from such a study should be of relevance for our understanding of the latter. We show that the basic features of the RP OG transition can be understood in terms of a hard-needle model by means of a mapping of the aspect ratio onto a temperature scale.