Crystal/glass phase change in K1−xRbxSb5S8 (x = 0.25, 0.50, 0.75) studied through thermal analysis techniques

K_1−xRb_xSb₅S₈ (x = 0.25, 0.5, 0.75) is a well-defined single-phase system that undergoes a reversible phase-change. We determined the activation energy of glass transition and crystallization, respectively, for the three compositions using the Kissinger and Ozawa-Flynn-Wall equations. The results have shown that for K_0.25Rb_0.75Sb₅S₈ the crystallization mechanism could be interpreted in terms of a single-step reaction. For the other two compositions the glass-to-crystal transformation is a process of increasing mechanistic complexity with time and it involves simultaneously several different nucleation and growth events. The slope of the lines in the Avrami plots was observed to be independent of heating rate for K_0.25Rb_0.75Sb₅S₈ and the mean value of the activation energy was found to be 262 ± 6 kJ/mol. For the other two compositions, the slope varies with the heating rate. In the K_0.25Rb_0.75Sb₅S₈ glasses, bulk nucleation with three-dimensional crystal growth appears to dominate the phase-change process.     

Light scattering by longitudinal acoustic modes in molecular supercooled liquids I: phenomenological approach

We derive expressions for the intensity of the Brillouin polarized spectrum of a molecular liquid formed of axially symmetric molecules. These expressions take into account both the molecular dielectric anisotropy and the modulation of the local polarisability by density fluctuations. They also incorporate all the retardation effects which occur in such liquids. We show that the spectrum splits into a q-independent rotational contribution and q-dependent term, which reflects the propagation of longitudinal acoustic modes. In the latter, the two light scattering mechanisms enter on an equal footing and generate three scattering channels. We study the influence of the two new channels and show that they may substantially modify the Brillouin line-shape when the relaxation time of the supercooled liquid and the period of the acoustic excitation are of the same order of magnitude. Received 14 August 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: franosch@hmi.de     

Free energy landscape from path-sampling: application to the structural transition in LJ38

We introduce a path-sampling scheme that allows equilibrium state-ensemble averages to be computed by means of a biased distribution of non-equilibrium paths. This non-equilibrium method is applied to the case of the 38-atom Lennard-Jones atomic cluster, which has a double-funnel energy landscape. We calculate the free energy profile along the Q₄ bond orientational order parameter. At high or moderate temperature the results obtained using the non-equilibrium approach are consistent with those obtained using conventional equilibrium methods, including parallel tempering and Wang-Landau Monte Carlo simulations. At lower temperatures, the non-equilibrium approach becomes more efficient in exploring the relevant inherent structures. In particular, the free energy agrees with the predictions of the harmonic superposition approximation.     

Frequency-dependent specific heat near the glass transition measured with the photoacoustic effect

For the supercooled salt melt 0.4 Ca(NO₃)₂–0.6 KNO₃ the photoacoustic amplitude and phase shift are measured in the glass transition region. The transition temperature is defined by the maximum in the phase angle. It is shifted to lower temperatures for lower frequencies. A frequency-dependent specific heat can be calculated from the data in the dispersion region. The frequency dependence of the transition temperature can be fitted by a power law with exponent =8.8±1.This work is part of a Ph. D. thesis at the Technische Universität München     

Variational calculation of some S-states of Coulomb three-body systems

A generalized Hylleraas-type basis set with three nonlinear parameters is proposed to study three-body systems interacting via coulomb forces within the framework of non-relativistic quantum mechanics. This basis set improves the rate of convergence with respect to previous ones, specially for non-symmetric systems and excited states of two electron atoms. Accurate binding energies and other properties for S-states of helium-like ions, muonic molecules and the positronium negative ion are reported. Received 21 July 2000 and Received in final form 4 October 2000     

Growth exponent in the Domany-Kinzel cellular automaton

Dynamic heterogeneities, i.e. the presence of molecules with different mobilities, have been established as one of the key features of the physics of supercooled liquids. Here we study in detail how the mobility of an individual molecule fluctuates with time. Our analysis is based on a time series of molecular dynamics simulations for a low molecular weight glass-former, propylene carbonate. We find that the variation of mobility with time of initially slow molecules significantly differs from that of initially fast molecules. We explicitly show the relation to the rate memory parameter which recently has been introduced to quantify the mobility fluctuations as observed via multidimensional NMR experiments. In this way qualitative agreement between simulation and experiment can be established although the time scales of simulation and NMR experiment differ by many orders of magnitude. Received 10 April 2000 and Received in final form 21 September 2000     

Very fast relaxation in polycarbonate glass

Relaxations in amorphous bis-phenol A polycarbonate are studied by neutron scattering, as a function of temperature below the glass transition. Two different processes are observed. One is very fast, with a characteristic time (∼ 0.3 ps), that is independent of temperature and momentum transfer. Conversely the other is slower, with a time which is dependent on temperature and momentum transfer. The very fast localized anharmonic motion is interpreted by the overdamping of low-frequency vibrational modes, by nearby dynamic holes. The slower relaxation is thermally activated and momentum transfer dependent. It corresponds to molecular group motions and possibly to the short-time regime of the segmental relaxation. Received 29 February 2000 and Received in final form 13 June 2000     

Simulation study of size distributions and magic number sequences of clusters during the solidification process in liquid metal Na

To investigate the size distributions of various clusters formed during solidification processes, a molecular dynamics (MD) simulation study has been performed for a system consisting of 10⁶ liquid metal Na atoms. With the cluster-type index method (CTIM), it is demonstrated that the basic clusters of (13 3 6 4), (13 1 10 2), (14 2 8 4), (14 4 4 6) and (12 0 12 0) and their combinations play a critical role in the microstructure transitions. Also, using a new method to classify all the clusters in the system, the size distributions of various clusters clearly reveal magic number characteristics. The total magic number sequence can be regarded as the superposition of all partial magic numbers corresponding to the related group levels of clusters. The first 10 magic numbers are present in order 14, 22, 28, 34, 41(43), 46(48), 52(54), 57(59), 61(66), 70(74),… (the numbers in the parentheses are the second magic numbers corresponding to the same group level of clusters). This magic number sequence is compared with the experimental and computational results of other authors.     

Thermal properties and crystallization of iron phosphate glasses containing up to 25 wt% additions of Si-, Al-, Na- and U-oxides

The thermal properties (expansion, T_g and T_SOFT.) of glasses, having 56-66% P₂O₅, 14.8-34.2% Fe₂O₃ and 2-25 wt% additions of SiO₂, Al₂O₃, Na₂O and UO₂, were comparatively estimated from dilatometric measurements in similar conditions. The T_g reversibility was clearly verified by varying the heating rates between 1 and 5 °C min⁻¹. From linear equations fits of the various glass properties as functions of the six components it is suggested the iron, sodium and uranium oxides decrease the thermal expansion (for 50 < T ? 300 °C), T_g and T_SOFT. From DTA/XRD analysis of three glasses it was confirmed the crystallization tendency decreased with increasing the UO₂ level in the glasses. Leaching test data for two compositions containing Na₂O suggest addition of UO₂ increases the chemical durability of the related glass. The roles of UO₂, Na₂O and Fe-oxide species as structural components of the glass network are discussed.     

NMR self-diffusion study of organic glasses: COANP,MBANP, PNP,NPP

A pulsed field gradient proton spin-echo NMR self-diffusion study of organic glasses COANP, MBANP, PNP and NPP in their liquid and weakly supercooled states was performed. The NMR phase diagrams, based on the proton NMR transverse relaxation time (T ₂) temperature hysteresis data of these materials, clearly give evidence of the onset of a glass phase on cooling the isotropic liquids below their respective melting temperatures. The self-diffusion data exhibit in the supercooled glassy state a non-Arrhenius behaviour and can be described in terms of the Vogel-Fulcher modification of the Arrhenius law,D=D exp{–E _a /[k _B (T–T _VF )]}. The activation energiesE _a and Vogel-Fulcher temperaturesT _VF are 83.2 meV and 239 K for COANP, 66 meV and 249 K for MBANP, and 85 meV and 245 K for PNP, respectively. The flow viscosity data obtained for COANP in the same temperature region as well conform to the Vogel-Fulcher behaviour, exp{E _a ⁽⁾ /[k _B (T–T _VF )]}, withE _a ⁽⁾ =80.4 meV andT _VF =239 K. In case of COANPD was found to increase with decreasing diffusion time in the supercooled (glassy) melt just belowT _M whereas no such behaviour was found aboveT _M .