When a glassy material or its liquid state is subjected to sufficiently high pressure, significant changes can take place in the short- and medium-range structure, vibrational density of states, and physical properties. It is crucial to determine and understand the structure-property relations under high pressure from both scientific and technological perspectives, since the glass structures frozen-in under elevated pressure may give rise to properties unattainable under ambient pressure. However, the structural and topological origins of the pressure-induced changes in macroscopic properties are not yet well understood. Here, we address this problem by subjecting various isostatically compressed glasses to isothermal annealing at ambient pressure and monitor the relaxation of glass structure and properties as a function of time and temperature. For different glass systems, density is found to relax in a stretched exponential manner with an exponent close to the Phillips value of 3/5 for relaxation in three dimensions when both short- and long-range interactions are activated [1]. For a compressed soda lime borate glass, we find that upon annealing at 0.9T_g, the pressure-induced increase in boron coordination remains unchanged, while the pressurized values of macroscopic properties such as density, refractive index, and hardness are relaxing [2]. Hence, the pressure-induced changes in macroscopic properties are not necessarily attributed to changes in the short-range order in the glass, but rather to changes in overall atomic packing density and medium-range structures. Moreover, we show that the relaxation mechanism depends on the annealing temperature and different physical properties (e.g., density and hardness) are found to relax on different timescales. [1] M. M. Smedskjaer, S. J. Rzoska, M. Bockowski, J. C. Mauro, Journal of Chemical Physics140, 054511 (2014). [2] M. M. Smedskjaer, R. E. Yougnman, S. Striepe, M. Potuzak, U. Bauer, J. Deubener, H. Behrens, J. C. Mauro, Y. Z. Yue, Scientific Reports4, 3770 (2014).

This talk reports on two interesting new phenomena recently discovered in glasses.
The first part of the talk will discuss the elastic properties of vitreous silica submitted to high pressures in a diamond anvil cell. We show that the compressibility of a silica sample immersed in helium or neon fluid is much smaller than expected from its elastic properties measured by Brillouin light scattering. It results from gas atom penetration into the interstitial free volume of the glass network. This adsorption-induced expansion can be described by a generalized poromechanical model. The second part of the talk will address the long-standing question of the nature of the glassy state. It is commonly believed that far below the glass transition temperature, dynamics in glasses is frozen, i.e. relaxations occur on time scales too large to be observed. X-rays photon correlation spectroscopy is used to reveal the existence of surprisingly fast atomic rearrangements in a sodium silicate glass, even at room temperature. These results challenge our conception of the glassy state and call for a new microscopic theory.
References:
(1) C. Weigel, A. Polian, M. Kint, B. Rufflé, M. Foret, and R. Vacher, Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities, Phys. Rev. Lett. 109, 245504 (2012).
(2) B. Coasne, C. Weigel, A. Polian, M. Kint, J. Rouquette, J. Haines, M. Foret, R. Vacher, and B. Rufflé, submitted.
(3) B. Ruta, G. Baldi, Y. Chushkin, B. Rufflé, L. Cristofolini, A. Fontana, M. Zanatta, and F. Nazzani, Revealing the fast atomic motion of network glasses, Nat. Commun. 5, 3939 (2014).

The constituents of any network glass can be broadly classified as either network formers or network modifiers. Network formers, such as SiO₂, Al₂O₃, B₂O₃, P₂O₅, etc., provide the backbone of the glass network and are the primary source of its rigid constraints. Network modifiers play a supporting role, such as charge stabilization of the network formers or alteration of the network topology through rupture of bridging bonds and introduction of floppy modes. The specific role of the modifiers depends on which network formers are present in the glass and the relative free energies of modifier interactions with each type of network former site. This variation of free energy with modifier speciation is responsible for the so-called mixed network former effect, i.e., the nonlinear scaling of property values in glasses having fixed modifier concentration but a varying ratio of network formers. In this talk, a general theoretical framework is presented describing the statistical mechanics of modifier speciation in mixed network glasses. The model provides a natural explanation for the mixed network former effect and also accounts for the impact of thermal history and relaxation on glass network topology.

The development of effective new tools for structural characterization of disordered materials and systems is becoming increasingly important as such tools provide the key to understanding, and ultimately controlling, their properties. The relatively novel technique of correlograph analysis (i.e. the approach calculating angular autocorrelations within diffraction patterns) promises unique advantages for probing the local symmetries of disordered structures. Because correlograph analysis examines a component of the high-order four-body correlation function, it is more sensitive to medium-range orders than conventional diffraction methods. Generally, electron correlograph analysis would be implemented in a transmission electron microscope (TEM) operating in the STEM mode where a nanoscale probe can be controllably created and translated to regions of interest on the specimen. It can be primarily used for characterizing local diffraction symmetry of disordered solid materials, such as amorphous semiconductors, oxides, and metallic glasses. Like fluctuation electron microscopy, electron correlograph analysis is a statistical method, replying on a sufficiently large sampling to distinguish between random and truly persistent structural correlations. The Friedel correlation in the mean electron correlograph unveils the crystallinity of samples with medium-range orders, an important structural parameter that cannot be measured by other techniques. In the presentation, I will describe the practical experimental method and common systematic errors of electron correlograph analysis. First, I will discuss the effects of beamstop, diffraction center locating, and distortions on the electron correlograph analysis. Second, by using both experimental data and numerical simulations, I will elucidate why those seemingly real correlations showing up in one or a few correlographs do not actually reflect the local symmetries of disordered structures, and emphasize that reliable structural information about the sample can only be obtained from the mean correlograph, which is averaged over results from an adequate number of individual diffraction patterns (typically > 50).

I will review our previous work in B₂O₃ glass [1,2] as well as its recent extensions in the liquid phase and in other systems. Using ab-initio simulations, we predicted new low-density crystalline polymorphs [1] which provide a framework to understand the glass properties [2]. Thanks to NMR, the new crystals are shown to be structurally much more similar to the glass than is any of the known polymorphs. Some possible routes to synthetise these crystals will be mentionned. I will also discuss the implications of these findings for the liquid behavior. In particular, the possible existence of a liquid-liquid transition, much similar to that hypothetized in water will be raised.
[1] G. Ferlat et al., Nature Materials, 11, 925 (2012)
[2] G. Ferlat et al., Phys. Rev. Lett., 101, 065504 (2008)

Amorphous transparent conducting oxides (TCO) are of considerable technological interest due to their unusually high electron mobilities, excellent optical properties, wide thermal stability and ease of processing. Significant progress has been made regarding the synthesis and optimization of these thin films (notably Zn and Sn-doped In₂O₃) for a wide range of applications. However, the structural changes upon doping as well as the exact role played by the short and intermediate-range order in defining their valuable optical and electronic properties is still lacking. This knowledge is desirable as it can be leveraged to direct research efforts towards other materials more abundant than Indium, or even point towards compounds hitherto unexplored. Short-range order (typically < 5Å) can be obtained through analysis of extended x-ray absorption fine structure (EXAFS) data which yields a partial structure function centered on the absorber atom. A complementary technique yielding structural information on longer length-scales, i.e. medium-range order, is to analyze the complete pair distribution function (PDF) obtained from total X-ray scattering data. Essentially, total scattering data is the properly corrected and normalized elastic scattering i.e. I(Q) vs. Q, where Q is the scattering vector. The PDF is a real-space distribution of atoms obtained through an appropriately weighted Fourier transform of this function. Taken together, EXAFS and PDF data contain important structural information such as bond distances, bond angles, coordination numbers and structural coherence lengths which are otherwise challenging to access in poorly ordered materials. In this study, we analyze EXAFS and grazing-incidence PDF data on amorphous In-Zn-O films with varying In:Zn ratios. Samples of a given composition but possessing significantly different electronic conductivities displayed only subtle changes in both XAS and PDF data. Further, the PDF data show clear ordering up to almost 3 nm indicating that the films are not truly amorphous. These observations have implications for both theoretical and experimental studies and will be discussed along with other structural insights on the In-Zn-O thin films with the broader aim of arriving at a better understanding of the short and medium-range order in amorphous TCO materials.

Drawing inspiration from nature, where some organisms can control the short-range order of amorphous minerals, we successfully manipulated the short-range order of amorphous alumina by surface and size effects. By utilizing the Atomic Layer Deposition (ALD) method to grow amorphous nanometrically thin films, combined with state-of-the-art electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), we show experimentally that the short-range order in such films is strongly influenced by size. This phenomenon is equivalent to the well-known size effect on lattice parameters and on the relative stability of different polymorphs in crystalline materials. Additionally, direct measurements of crystallization temperature have shown a dramatic increase in amorphous to crystalline phase transition temperature for thinner amorphous Al₂O₃ films. Finally, we show that the short-range order changes while still in the amorphous phase, before the amorphous to crystalline transformation takes place.
¹ Leonid Bloch, Yaron Kauffmann, and Boaz Pokroy. Crystal Growth & Design. In press 2014.

Originally, rigidity in glasses was formulated by J.C. Phillips in order to understand the relationship between cooling speed, which measures glass formation ability, and chemical composition. Although in the last years much of this picture has been confirmed by many experiments and simulations, still it is not clear how minimal cooling speed depends on the composition. Here a simple model is provided in which two key ingredients are considered in the glass, metastable states and their multiplicity. Metastable states are considered as in two level system models. However, their multiplicity and topology allows a phase transition in the thermodynamic limit, while a transition to the glass is obtained for fast cooling. By solving the corresponding master equation, the minimal speed of cooling required to produce the glass is obtained as a function of the distribution of metastable and stable states. This allows to understand cooling trends due to rigidity considerations in chalcogenide glasses as in metallic glasses doped with group IV impurities, like the Pd-Ni-P-Si compound.

We report that the properties of metallic glasses can be dramatically tailored by slow deposition at high temperatures without altering chemical composition. The glass transition and crystallization temperatures of the metallic glass can be increased as high as 83 K and 269 K, respectively, compared to the room-temperature deposited one and melt-spun ribbons. The high-temperature deposited glass also shows ultrahigh hardness, over 60 % higher than room-temperature deposited glass. Atomic structure characterization reveals that the exceptional properties of the high-temperature glasses are associated with abundance of medium range order contains mainly distorted icosahedral order with partially cubic symmetry. The finding of the high-temperature deposited glass gives insight on atomic mechanisms of metallic glass formation and has important impact on the technological applications of metallic glasses.

That shear causes dilatation in densely packed granular systems has been known and studied since the days of Osborne Reynolds (1885). A similar effect occurs in dense packings of hard spheres, which are a useful analogues of metallic glasses. It is now possible to follow this process directly in colloidal hard-sphere glasses, by tracking the invidual particles by confocal microscopy. In dense hard-sphere systems, the moduli are strong functions of the density (they diverge at close packing). These moduli can be measured from the thermal distribution of the local strain energy densities. These measurements show a decrease in the shear modulus (and hence density) upon plastic shear deformation, and a relaxation back to the initial state after the deformation stops. Microscopic mechanisms for these density changes as well as their implications for strain softening and shear instabilities in metallic glasses will be discussed.

Owing to the high cooling rates necessary for glass formation in metallic glasses, parts with a complex geometry have been difficult to cast directly. The metallic glasses' remarkable softening behaviour above the glass transition temperature T_g has led many to suggest, then develop, thermoplastic forming as an alternative manufacturing method for complex parts. Cerium-based BMGs, exhibiting very low T_g as well as a large supercooled liquid region (SCLR), provide an excellent platform on which to study such thermoplastic forming. In order to investigate and develop new thermoplastic forming processes, the SCLR of these alloys must first be appropriately characterised. Heating rate, processing temperature, and strain rate have been examined; their effects on viscosity, flow, and devitrification explored; and the results and conclusions are hereby reported.

Many materials have a high propensity to crystallize as this is the energetically most favorable state; it is thus difficult to make these materials amorphous. However, for certain applications, it would be highly beneficial to make these materials amorphous as many of their physicochemical properties differ from those of their crystalline couterparts. For example, amorphous materials have a much higher solubility than crystalline ones. I will present a microfluidic spray drier, we call it a microfluidic nebulator, that produces very small drops through the use of supersonic air. These small drops serve as containers to form small amorphous nanoparticles; the size of these nanoparticles is determined by the number of solute molecules contained in the drop. The nanoparticle structure is determined by the probability for a crystalline nucleus to form as the drop evaporates; this probability is typically very high in supersaturated solutions as crystalline solids readily form under these conditions. However, the formation of a crystalline nucleus itself entails some time delay. We demonstrate that the nebulator can kinetically suppress the formation of crystalline nuclei; thereby, it produces amorphous nanoparticles from many different materials, even from materials that have a very high propensity to crystallize.

One of the most intriguing issues in supercooled liquids and glassy materials is their dynamic relaxation behaviors, which underlies their physical and mechanical properties. Recently, Yu et al. found that the b-relaxations in metallic glass-forming liquids is sensitive to the chemical interactions among all the constituent atoms. In other words, b-relaxations in metallic glasses are linked to a large mixing enthalpy for the atoms.
We use mechanical spectroscopy to investigate the mechanical relaxation (a and b relaxations) in several typical bulk metallic glasses in isochronal route as well as isothermal mode: Pd-Ni-Cu-P, La-Ni-Cu-Al and Cu-Zr-Al-Dy systems. We investigated the dependence of the mechanical relaxation on the physical aging to study the evolution of the localized atomic mobility with the physical aging. The pronounced JG b relaxations in metallic glasses are associated to large negative values of the enthalpy of mixing among the constituting atoms, these results are in good agreement with the recent report by Yu et al.

Resistive switches offer the prospect of improved performance, efficiency and scalability over current data storage methods. Many device architectures have been proposed, reliant upon a wide variety of materials whose conductance switches in a non-volatile manner with the application of an applied field. Silicon-based switching materials are of particular interest in these devices as they offer the added potential for integration into existing CMOS infrastructures. It is of great importance that the underlying physics of switching is well-understood, such that device optimisation and integration into commercial hardware may be realised. Our device layers are sputter-deposited to create a 37nm thick, amorphous, non-stoichiometric silicon-rich layer of silica sandwiched between conductive electrodes. We report on the material changes leading to reversible resistive switching in silicon suboxide using secondary ion mass spectroscopy (SIMS), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and conductive atomic force microscopy (cAFM). Analysis with a range of techniques serves to highlight the broad dynamics of device behaviour, and supports the model of an ionic and defect-dependent switching mechanism which relies upon the presence of nanoscale grain boundaries within the amorphous silica switching layer.

Our recent research accomplishments led us believe that Ga substitution on Al sites need not permit it to remain in trivalent state only. The monovalent Ga atom, has atomic radius is greater than that of trivalent one. In contrast, pentavalent state of Ga has atomic radius smaller than that of trivalent one. The change of valency of Ga in metallic glass composition will lead to change in geometry of main cluster that promotes glass forming ability of the melt. Further, the mixed states of Ga may facilitate formation of clusters of different types leading to phase separation in the melt and thereby in metallic glasses. Keeping these in mind we have recently investigated two such compositions. They refer to Zr_69.5Al_7.5minus;xGa_xCu₁₂Ni₁₁ and Ce₇₅Al_25-xGa_x metallic glass compositions. The purpose of this presentation will be to bring out aforesaid features after Ga substitution on the nature of glass forming ability based on our studies on these alloy systems.
The first part of the presentation deals with the behavior of Zr_69.5Al_7.5minus;xGa_xCu₁₂Ni₁₁ (x = 0, 1.5, 7.5 at.%) alloys. A comparison between their nanohardness and reduced elastic modulus values of the as-synthesized glassy phase with their nanocomposites has been made. The indentation characteristics of a novel Ga substituted glass composition corresponding to x = 7.5 have shown significant improvement in regard to hardness and elastic modulus. The evidence of pile up has been observed in case of as-synthesized glassy ribbons. The load (P) versus depth (h) curves for as-synthesized melt-spun ribbons displayed the presence of displacement burst, which are known as pop-ins. The amount of energy per unit volume required for the shear band formation in glassy state has been estimated based on the pop-ins observed in P-h curve. This seems to decrease with Ga addition. Based on transmission electron microscopic observations of indented glassy specimen, the possibility of nanocrystallization has been ruled out.
The glass forming ability and indentation characteristics of melt-spun Ce₇₅Al_25-xGa_x (x = 0, 2, 4 and 6 at.%) metallic glasses will also be presented. The substitution of Ga decreases the glass transition temperature, while increases the supercooled liquid region. The small amount of Ga substitution has led to appearance of second diffuse halos in the X-ray diffraction (XRD) pattern. The observation of phase separation is thus asserted. The transmission electron microscopic images led us to conclude the formation of nano-amorphous domains after Ga substitution in a glassy matrix. The load dependent hardness behaviour of metallic glasses with and without Ga substitution has been studied. The substitution of Ga improves the microhardness property of Ce-Al alloy. The formation of shear bands around the indentation periphery has been observed. The value of yield strength and Meyer exponent of these alloys has been compared.

We investigate the crystallization of an amorphous eutectic Ni-P alloy, fabricated by electroless deposition as a 10 um thick continuous layer. The purpose of this work is to obtain a detailed understanding of the microscopic mechanisms of crystallization and phase formation upon heating. To this end, we combine a variety of complimentary characterization techniques. DSC (differential scanning calorimetry) during isothermal and isochronal heating reveals the crystallization kinetics. XRD (X-ray diffractometry) provides quantitative information on phase composition as a function of tempering time. Conventional-, high-resolution-, and analytical TEM (transmission electron microscopy) and TEM-based electron diffraction provide high-spatial-resolution information on phase nucleation and spatial distribution of atom species, particularly the crystallography of the nucleating crystalline phases (Ni3P and Ni) and the spatial distribution of phosphorus in the partially and completely crystallized alloy. Our results confirm earlier observations, according to which crystallization proceeds by formation of barrel-shaped grains. Internally, these exhibit a radial microstructure of of slightly misoriented subgrains of Ni3P containing Ni needles with unique ORs (orientation relationships) to Ni3P. However, the preferred Ni-Ni3P ORs we observe differ from those reported in the literature. Combining our observations on the structure and microstructure of partially and completely crystallized Ni-P with the observed crystallization kinetics leads to a new model for the microscopic mechanism of crystallization. This may be useful for understanding the effect of ternary alloying elements on the crystallization behavior and temperature.

V-Zr amorphous thin films potentially have a number of applications as coatings for components subject to aggressive chemical and physical conditions. In this work V-Zr thin films were produced by magnetron sputtering at room temperature and were subsequently characterized and modelled; using atomic scale techniques, to understand the alloys&’ stability and the relationship the amorphous material has to the crystalline Zr, V and V₂Zr phases.
Atomic scale modelling using density functional theory was used to investigate three compositions in the V-Zr system: equimolar VZr, the V rich V₂Zr and Zr rich VZr₂ compositions. Amorphous supercells were created and minimised under constant pressure with a variety of starting densities and supercell sizes. We find that the simulation conditions are pivotal in reproducing the experimentally observed structure. Bulk X-ray diffraction techniques were used as a first check to compare experiment and theory with encouraging results.
Ensembles of electron nanodifraction patterns were collected from the three formulations of amorphous V-Zr films. The electron beam was tuned to the diameter of the nearest-neighbour clusters. In this geometry, and collecting diffraction patterns from sub-4 nm thick regions, angular correlations in the diffraction patterns reflect the symmetries of nearest-neighbour polyhedra. [1] An angular autocorrelation technique was used to quantify the statistical incidence of symmetries in the diffraction patterns of each specimen as a function of the scattering angle. It was found that the symmetries in the diffracted intensities from all the films could not be explained by BCC local ordering alone, but required the presence of more complex intermetallic local ordering, such as the C15 Laves phase (V₂Zr). The experimental data matched simulations from the amorphous supercells.
[1] A. C. Y. Liu, M. J. Neish, G. Stokol, G. A. Buckley, L. A. Smillie, M. D. de Jonge, R. T. Ott, M. J. Kramer, and L. Bourgeois, Phys. Rev. Lett., 110, 205505 (2013)

The stability of undercooled liquids against crystallization is affected by initial viscosity of the stable liquids, viscosity change of the melt due to temperature drop, free energy difference between undercooled liquids and crystalline phases, liquid/solid interface energy, bulk density, non-uniform nucleation, cooling rate of particle and so many other factors. If the cooling rate is fast enough that diffusion among the atoms is limited, the amorphous phase is uniformly solidified. This behavior is called glass transition behavior, and several metallic alloys with such behavior have been discovered, which can be vitrified with only radiative cooling. For example, containerless high-temperature/high-vacuum electrostatic levitation (ESL) technique, which could minimize heterogeneous sites, is useful for investigation of the undercooled liquid behavior. In particular, several high glass