Session L27: Glassy Systems

Pressure Dependence of the Glass-Transition Temperature for Intermediate and Fragile Glass-Forming Systems

The glass transition temperature, T$_{g}$, is determined over the pressure range 0 to 80 kbar using a diamond anvil cell (DAC). Two methods are used: i) the onset or disappearance of pressure gradients indicated by ruby fluorescence, and ii) a new method in which T$_{g}$(P) is determined by significant changes in slope in the P-T curve during pseudo-isobaric temperature ramps. This slope change accompanies the significant change in volume expansion coefficient between the highly viscous, metastable, supercooled liquid state and the solid glassy state. Good agreement is found in the T$_{g}$(P) curve using the two methods. While the second method does not allow for quantitative determinations of the volume expansion coefficients of these systems, qualitative results can be obtained. It is found, e.g., that differences in the volume expansion coefficient upon crossing the glass transition are much greater for low-pressure fluids than for the much denser fluids at high pressure. Data will be presented for glycerol, an intermediate strength glass-forming system, as well as the fragile glass former salol.   

Pressure Raman experiments on Ge$_{x}$As$_{x}$Se$_{1-2x}$ glasses$^{\ast }$

It is known$^{1}$ that variations in the non-reversing enthalpy associated with glass transitions, $\Delta $H$_{nr}$(x), display a global minimum ($\sim $0) in the 0.09 $<$ x $<$ 0.16 range and the term increases at x $>$ 0.16 and at x $<$ 0.09 in the titled glasses. In this reversibility window, glasses are thought to in the Intermediate phase and form stress-free networks. Since the size of As, Ge and Se are nearly the same, Raman pressure experiments using a DAC provide a useful way to check the stress-free nature of glasses in the window$^{2}$. Preliminary results at x = 0.11, and 0.14, compositions in the reversibility window, reveal Raman frequency of the symmetric stretch of Ge(Se$_{1/2})_{4}$ tetrahedra to blue-shift linearly with external pressure (P) once P$>$0. At x = 0.18, a composition in the stressed-rigid phase, a blue-shift of the mode is also observed but only once P exceeds a threshold (P$_{c})$ value of 14 kbar. The present finding of a finite value of P$_{c}$ at x = 0.18, but its vanishing at x = 0.11 and 0.14, is quite similar to a previous one in binary Ge$_{x}$Se$_{1-x}$ glasses$^{2}$. We are now examining other glass compositions in the present ternary. $^{\ast }$ Supported by NSF grant DMR 04-56472 $^{1}$ T. Qu et al. Mater. Res. Soc. Symp. Proc. \textbf{754}, 111 (2003). $^{2}$ F. Wang et al. Phys. Rev. B, \textbf{71}, 174201 (2005).   

Two Simple Models of Monoatomic Glass Formers

Glass formation in one component systems remains a challenge for computer simulations, and therefore most studies to date have been done on binary mixtures. Here we explore the origin of resistance to crystallization of single component systems for two examples: modified silicon potential (Stillinger-Weber model) and Lenard-Jones ellipsoids (Gay-Berne model of liquid crystals). To produce glass formers these potentials were tuned by optimization of the parameter of three-body interaction (for the former) and aspect ratio (for the later). The kinetic properties and the potential energy landscapes of both models are studied.   

Tunable structures and properties in vitreous silica

We studied the structures and properties of vitreous silica samples prepared by specific high pressure processing routes, using molecular dynamics simulations based on a charge-transfer three-body potential. Our study shows that the ability of the glass to undergo irreversible densification is inherently connected to its anomalous thermo-mechanical properties, such as the minimum in the bulk modulus at $\sim $2-3 GPa and the negative thermal expansion while under pressure. These behaviors can be tuned by controlling the pressure under which the initial glass was quenched. By preparing silica glass in ways that eliminates anomalous thermo-mechanical behaviors, e.g., by quenching a melt under pressure, the propensity of the glass to undergo irreversible densification can be eradicated. Such ``pressure-treated'' silica glass is less susceptible to radiation damage and can potentially increase the lifetime of many optical components.   

Molecular origin of the giant conductivity enhancement in (Ag$_{2}$S)$_{x}$(As$_{2}$S$_{3})_{1-x}$ glasses

The solid electrolyte additive Ag$_{2}$S is found to homogeneously alloy with base As$_{2}$S$_{3}$ glass at low concentrations ( x $<$ 6 {\%}, single: T$_{g}$ = T$_{g}^{high} \quad \sim $ 210C$_{ })$, but it rapidly segregates as a Ag-rich glass phase at medium concentrations ( 6{\%} $<$ x $<$ 20{\%}, bimodal : T$_{g}^{high}$ and T$_{g} \quad ^{low} \quad \sim $ 170C ), and becomes the principal glass phase populated at higher x $>$ 35 {\%} (single: T$_{g}^{low})$ as revealed by modulated calorimetric measurements. The stoichiometry of the Ag-rich (T$_{g}^{low}$ phase) is suggested to be near AgAs$_{3}$S$_{7}$ at x $\sim $ 25{\%} but becomes closer to that of Smithite (AgAsS$_{2})$ at x $>$ 40{\%}, as revealed by Raman scattering. In the 9{\%} $<$ x $<$ 14{\%} composition range, one observes, in calorimetric experiments, the opening of a reversibility window, and a pronounced increase in the fractional population, R(x) of the Ag-rich glass phase, both of which correlate well with the 5-orders of magnitude increase in electrical conductivity$^{1,2}$ across this compositional interval. In the same interval molar volumes on our samples show a local plateau. These observations suggest a new interpretation of the giant electrical conductivity enhancement observed at x $>$ 15{\%} in the present electrolyte glass system. $^{1}$ E.A. Kazakova and Z.U.Borisova, Fiz. Khim.Stekla \textbf{6}, 424(1980).   

Enhanced Icosahedral Order in Supercooled Liquid Iron

As part of a study of metallic glass-forming ability, we perform first-principles molecular dynamics simulations of supercooled liquid Iron. Analyzing the results according to the icosahedral order parameter $\hat {W}_6 $, we find that Iron exhibits enhanced icosahedral order compared with supercooled Copper and compared with dense random packing. Voronoi analysis confirms the enhanced order is in the form of 13-atom icosahedral clusters as well as characteristic Frank-Kasper type disclinated icosahedra. Upon further cooling the sample crystallizes to a BCC lattice, with the icosahedra clustering to form a novel point defect.   

Full Recovery of Electron Damage in Glass at Ambient Temperatures

An unusually complete recovery of the electron beam induced damage in a CaO-Al$_{2}$O$_{3}$-SiO$_{2}$ glass was discovered. Nanoscale measurements carried out in a scanning transmission electron microscope show that the Ca ions migrate about 10 nm away during irradiation and return during recovery. Oxygen atoms are trapped largely as molecular oxygen and do not migrate. Electron energy loss measurements demonstrate that for glass to return completely to the original compositional and structural state the following processes must take place: First, no mass loss should occur. Thus the irradiation time should be less than the time necessary for significant mass-loss to occur. Second, diffusion must be sufficient at the ambient temperature for atoms to migrate back to suitable bonding sites. Third, the role of oxygen is critical: unless oxygen is available for recombination with the displaced atoms then recovery is incomplete. Finally, the observation that the system recovers so completely (structurally, as well as compositionally) after such a substantial perturbation is evidence that the initial state of the glass must be a very stable thermodynamic minimum [1]. [1] K.A. Mkhoyan et al., Phys. Rev. Lett. \textbf{96}, 205506 (2006).   

Glass Transition and Structure in the Phase Field Crystal Model

The dynamics of the glass transition and structure of the disordered phase are studied using the Phase Field Crystal (PFC) model in two and three dimensions. It is shown that a kinetically driven glass transition is produced in 3D for sufficiently large cooling rates. Analysis of free energy barriers indicates that the glass phase is more accessible from the liquid than the crystalline phase, but will not be stable for long times unless a critical cooling rate is exceeded. Below the critical cooling rate an equilibrium BCC structure is established. A Vogel-Fulcher type divergence in the density autocorrelation function is produced as the glass transition temperature is approached, signifying fragile glass forming behavior. As well the structure factor of the glass phase shows the characteristic split second peak. Notable differences between results in 2D and 3D will be discussed, as well as results for pure and binary systems.   

Ion-conduction and rigidity/flexibility of glasses

The (AgI)$_{x}$(AgPO$_{3})_{1-x }$ solid electrolyte glass system has been examined extensively although a consensus on the increase of electrical conductivity with x data has been elusive. Here we show that the variability of the data is likely due to water contamination. Our work is on specifically prepared \textit{dry} samples which display glass transition temperatures T$_{g}$(x) that are at least 50\r{ }to 100\r{ }C higher than those reported hitherto. In Raman scattering the frequency of the P-O$_{t}$ bonds in PO$_{4}$ tetrahedra of long chains is found to systematically red-shift with increasing x, and to display thresholds near x= x$_{c}$(1) =0.095(3)(stress-transition) and x =x$_{c}$(2) = 0.379(5)(rigidity transition). Calorimetric measurements show a reversibility window in the 0.09 $<$ x $<$ 0.38 range. Room temperature electrical conductivity, $\sigma $(x), increases with x to display thresholds near x$_{c}$(1) and x$_{c}$(2), and a logarithmic increase at x$>$ x$_{c}$(2) with a power-law $\mu $ = 1.78(10) that is in good agreement with theoretical predictions$^{1}$. Properties of flexibility and rigidity of backbones commonplace in covalent systems$^{2}$ is a concept that extends to solid electrolyte glasses as well. \newline $^{1}$Richard Zallen, Physics of Amorphous Solids \newline $^{2}$ P. Boolchand et al. Phil. Mag 85, 3823 (2005)   

Locations of metal ions in the new glasses in the alumina-calcia-monazite (LaPO$_{4})$ system

The new group of glasses synthesized from calcium aluminate (CaAl$_{2}$O$_{4})$ or C12A7 (CaO)$_{12}$(Al$_{2}$O$_{3})_{7}$ with varying fractions of La-monazite (LaPO$_{4})$ has been characterized by electron microscopy, $^{31}$P and $^{27}$Al NMR, Raman scattering and chemical methods. These techniques have yielded information concerning the environments of the metal ions Al and La in the glasses. A substantial negative shift of the principal $^{31}$P NMR line at all monazite fractions, along with Raman spectra showing that PO$_{4}$ groups do not share bridging oxygens, places La within the second coordination shell surrounding P. P-Al TRAPDOR double NMR experiments show that aluminum and phosphorus are also closely coordinated, accounting for a second, more negatively shifted line in the $^{31}$P single resonance spectra. Models for the structures of these glasses have been constructed for a range of monazite contents, and will be presented.   

X-ray and Neutron Scattering Studies of Methyl Iodide Confined in GelTech$_{\copyright}$ Glass

X-ray diffraction and neutron scattering studies of methyl iodide confined in 200 {\AA} GelTech$_{\copyright }$ glass have revealed a never before observed intermediate solid phase of methyl iodide. Bulk methyl iodide has one phase transition below room temperature, at 207 K where it transitions from a liquid to an orthorhombic solid. Neutron scattering studies of the diffusion of methyl iodide confined in the 200 {\AA} pores show a transition from a liquid to a solid at 203 K. X-ray diffraction measurements support this finding and identify the transition as one from a liquid to the never before observed amorphous solid. The amorphous solid remains down to 168 K upon cooling at which point a second transition appears from the amorphous solid to an orthorhombic solid, which upon indexing is identical to the bulk.   

Nonlinear susceptibility of the dipolar glass

Glassy behavior is seen in various systems, among which are structural glasses, electron glasses, spin glasses and electric dipolar glasses. The latter two seem to share the same effective Hamiltonian. However, experimentally some fundmental differences are seen between these two systems, most notably the divergence of the nonlinear susceptibility in the spin glass system and its absence in the electric dipolar glass. Here we discuss the similarities and differences between these two systems leading to the above mentioned phenomenon.   

Conductivity thresholds and glass structure in (K$_{2}$O)$_{x}$(GeO$_{2}$)$_{1-x}$ glasses

There are reports of conductivity thresholds with glass composition in solid electrolyte glasses. In the titled glass system, a seven order of magnitude increase in conductivity\footnote{Jain et al JNCS 222, 361 (1997).} occurs at x $>$ 0.10. The origin of the observation remains an open question. In titled glasses, we show that glass structure probed by the elastic behavior of its backbone shows two thresholds, a stress transition near x = 0.04 and a rigidity transition near x = 0.09. These elastic thresholds emerge from the reversibility window\footnote{S. Chakravarty et al. J.C.M.P 17,L1-7 (2005).} observed in calorimetric measurements, and in Raman scattering experiments that show scattering strength of the 520 cm$^{-1}$ mode of 3-member rings to show a global maximum in the reversibility window. The pronounced increase of conductivity apparently occurs when backbones become flexible at x $>$ 0.09, permitting K$^{+}$ ions to freely diffuse. The correlation between the electrical, thermal and optical properties of the present solid electrolyte glasses may well be a generic feature of these materials.