Mapping the structural trends in zinc aluminosilicate glasses.

The structure of zinc aluminosilicate glasses with the composition (ZnO)x(Al2O3)y(SiO2)1-x-y, where 0 ≤ x < 1, 0 ≤ y < 1, and x + y < 1, was investigated over a wide composition range by combining neutron and high-energy x-ray diffraction with 27Al magic angle spinning nuclear magnetic resonance spectroscopy. The results were interpreted using an analytical model for the composition-dependent structure in which the zinc ions do not act as network formers. Four-coordinated aluminum atoms were found to be in the majority for all the investigated glasses, with five-coordinated aluminum atoms as the main minority species. Mean Al-O bond distances of 1.764(5) and 1.855(5) Å were obtained for the four- and five-coordinated aluminum atoms, respectively. The coordination environment of zinc was not observed to be invariant. Instead, it is dependent on whether zinc plays a predominantly network-modifying or charge-compensating role and, therefore, varies systematically with the glass composition. The Zn-O coordination number and bond distance were found to be 4.36(9) and 2.00(1) Å, respectively, for the network-modifying role vs 5.96(10) and 2.08(1) Å, respectively, for the charge-compensating role. The more open coordination environment of the charge-compensator is related to an enhanced probability of zinc finding bridging oxygen atoms as nearest-neighbors, reflecting a change in the connectivity of the glass network comprising four-coordinated silicon and aluminum atoms as the alumina content is increased.

Rheology of supercooled P-Se glass-forming liquids: From networks to molecules and the emergence of power-law relaxation behavior.

The effect of the network-to-molecular structural transformation with increasing phosphorus content in PxSe100-x (30 ≤ x ≤ 67) supercooled liquids on their shear-mechanical response is investigated using oscillatory shear rheometry. While network liquids with 30 ≤ x ≤ 40 are characterized by shear relaxation via a network bond scission/renewal process, a Maxwell scaling of the storage (G') and loss (G″) shear moduli, and a frequency-independent viscosity at low frequencies, a new relaxation process emerges in liquids with intermediate compositions (45 ≤ x ≤ 50). This process is attributed to an interconversion between network and molecular structural moieties. Predominantly molecular liquids with x ≥ 63, on the other hand, are characterized by a departure from Maxwell behavior as the storage modulus shows a linear frequency scaling G'(ω) ∼ ω over nearly the entire frequency range below the G'-G″ crossover and a nearly constant ratio of G″/G' in the terminal region. Moreover, the dynamic viscosity of these rather fragile molecular liquids shows significant enhancement over that of network liquids at frequencies below the dynamical onset and does not reach a frequency-independent regime even at frequencies that are four orders of magnitude lower than that of the onset. Such power-law relaxation behavior of the molecular liquids is ascribed to an extremely broad distribution of relaxation timescales with the coexistence of rapid rotational motion of individual molecules and cooperative dynamics of transient molecular clusters, with the latter being significantly slower than the shear relaxation timescale.

Observation of a reentrant structural transition in an arsenic sulfide liquid.

A fundamental and much-debated issue in glass science is the existence and nature of liquid-liquid transitions in glass-forming liquids. Here, we report the existence of a novel reentrant structural transition in a S-rich arsenic sulfide liquid of composition As2.5S97.5. The nature of this transition and its effect on viscosity are investigated in situ using a combination of differential scanning calorimetry and simultaneous Raman spectroscopic and rheometric measurements. The results indicate that, upon heating significantly above its glass transition temperature (261 K), the constituent Sn sulfur chains in the structure of the supercooled liquid first undergo a Sn⇌S8 chain-to-ring conversion near ∼383 K, which is exothermic in nature. Further heating above 393 K alters the equilibrium to shift in the opposite direction toward an endothermic ring-to-chain conversion characteristic of the well-known λ-transition in pure sulfur liquid. This behavior is attributed to the competing effects of enthalpy of mixing and conformational entropy of ring and chain elements in the liquid. The existence of reentrant structural transitions in glass-forming liquids could provide important insights into the thermodynamics of liquid-liquid transitions and may have important consequences for harnessing novel functionalities of derived glasses.

Structural model for amorphous aluminosilicates.

An analytical model is developed for the composition-dependent structure of the amorphous aluminosilicate materials (M2O)x(Al2O3)y(SiO2)1-x-y and (MO)x(Al2O3)y(SiO2)1-x-y, where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1. The model is based on a simple set of reactions and contains a single adjustable parameter p (0 ≤ p ≤ 1). The latter is found from 27Al solid-state nuclear magnetic resonance (NMR) experiments in the regime where R = x/y ≥ 1, aided by new experiments on the magnesium and zinc aluminosilicate systems. The parameter p decreases linearly as the cation field strength of M+ or M2+ increases, as per the observation previously made for the degree of aluminum avoidance [Lee et al., J. Phys. Chem. C 120, 737 (2016)]. The results indicate that as the cation field strength increases, there are less fourfold coordinated aluminum atoms to contribute toward the glass network, and Al-O-Al bonds become more prevalent in a progressive breakdown of Loewenstein's aluminum avoidance rule. The model gives a good account of the composition-dependent fraction of non-bridging oxygen (NBO) atoms for R ≥ 1, as assessed from the results obtained from solid-state NMR experiments. An extension of the model to (M2O3)x(Al2O3)y(SiO2)1-x-y glasses leads, however, to an excess of NBO atoms, the proportion of which can be reduced by invoking network-forming fivefold coordinated Al atoms and/or oxygen triclusters. The model provides a benchmark for predicting the structure-related properties of aluminosilicate materials and a starting point for predicting the evolution in the structure of these materials under the extreme conditions encountered in the Earth's interior or in processes such as sharp-contact loading.

Structure of diopside, enstatite, and magnesium aluminosilicate glasses: A joint approach using neutron and x-ray diffraction and solid-state NMR.

Neutron diffraction with magnesium isotope substitution, high energy x-ray diffraction, and 29Si, 27Al, and 25Mg solid-state nuclear magnetic resonance (NMR) spectroscopy were used to measure the structure of glassy diopside (CaMgSi2O6), enstatite (MgSiO3), and four (MgO)x(Al2O3)y(SiO2)1-x-y glasses, with x = 0.375 or 0.25 along the 50 mol. % silica tie-line (1 - x - y = 0.5) or with x = 0.3 or 0.2 along the 60 mol. % silica tie-line (1 - x - y = 0.6). The bound coherent neutron scattering length of the isotope 25Mg was remeasured, and the value of 3.720(12) fm was obtained from a Rietveld refinement of the powder diffraction patterns measured for crystalline 25MgO. The diffraction results for the glasses show a broad asymmetric distribution of Mg-O nearest-neighbors with a coordination number of 4.40(4) and 4.46(4) for the diopside and enstatite glasses, respectively. As magnesia is replaced by alumina along a tie-line with 50 or 60 mol. % silica, the Mg-O coordination number increases with the weighted bond distance as less Mg2+ ions adopt a network-modifying role and more of these ions adopt a predominantly charge-compensating role. 25Mg magic angle spinning (MAS) NMR results could not resolve the different coordination environments of Mg2+ under the employed field strength (14.1 T) and spinning rate (20 kHz). The results emphasize the power of neutron diffraction with isotope substitution to provide unambiguous site-specific information on the coordination environment of magnesium in disordered materials.

Nucleation and early stage crystallization in barium disilicate glass.

Barium disilicate is one of the glass-ceramic systems where internal nucleation and crystallization can occur from quenched glass upon heat treatment without requiring nucleating agents. The structural origin of the nano-clusters formed during low temperature heat treatment is of great interest in gaining a fundamental understanding of nucleation kinetics in silicate glasses. Here, we present experimental investigations on the low temperature heat treatment of barium disilicate (BaO·2SiO2) glass. Several experimental techniques were used to characterize the structural nature of barium disilicate glasses that were heat treated between the glass transition temperature, Tg, and the peak temperature of crystal growth, Tcr. The data show that small amounts of crystallites including BaSi2O5 as well as other higher Ba/Si ratio phases are formed. Moreover, unlike that reported for lower BaO content (BaO<33mol%) barium silicate glass or the analogous Li2O-SiO2 glasses, no clear evidence is observed for liquid/liquid phase separation in barium disilicate glass.

Observation of a dynamical crossover in the shear relaxation processes in supercooled selenium near the glass transition.

Shear relaxation in supercooled selenium liquid near its glass transition over the viscosity range of 106 Pa s-1012 Pa s is studied using oscillatory parallel plate rheometry. The results demonstrate the presence of a slow, Debye-like relaxation process and a fast, cooperative relaxation process that are attributed, respectively, to the Se-Se bond scission/renewal dynamics and the segmental motion of selenium chains. The slow process displays a nearly-Arrhenius temperature dependence, while the fast process is strongly non-Arrhenius, and their combined contribution to viscosity is estimated using the Maxwell relation. The slow process is found to be coupled to viscous flow over the entire viscosity range. In contrast, the fast process becomes a major contributor to viscosity, and hence, to fragility only near Tg. This dynamical crossover is likely a fundamental characteristic of fragile liquids that represents a temperature dependent evolution of their free energy landscape. The fragility of supercooled selenium liquid appears to be remarkably closely linked to the temperature dependence of the shear modulus of the slow process, thus validating the prediction of the elastic shoving model.

Rheology of the λ transition in liquid sulfur: Insights from arsenic sulfide liquids.

The frequency dependence of the storage and loss shear moduli and viscosity of AsxS100-x glass-forming liquids (x = 5, 10, 15, 40) are obtained over a frequency range covering nearly 15 orders of magnitude using parallel plate rheometry. The S-rich (x ≤ 15) liquids are characterized by a ring-to-chain structural transition near Tλ ∼ 120 °C, and their rheological behavior below Tλ strongly resembles that of long-chain and entangled polymers as well as that recently speculated for liquid sulfur above its λ transition. These AsxS100-x liquids are characterized by the coexistence of a slow and a fast relaxation process with similar activation energy. Both processes are coupled to viscosity, but differ in time scale by several orders in magnitude and are assigned, respectively, to the reptation and the Rouse dynamics of the Sn chains in these liquids. Such complex polymer-like rheological behavior disappears in the As40S60 liquid, characterized by corner-shared AsS3/2 pyramids, where a single average shear relaxation time typical of simple liquids instead emerges.

Isotropic rotation vs. shear relaxation in supercooled liquids with globular cage molecules.

The temperature dependence of the rotational dynamics of P4Se3 molecules in the glass-forming molecular liquid P5Se3 is studied using two-dimensional (31)P nuclear magnetic resonance spectroscopy. Unlike typical molecular glass-forming liquids, the constituent molecules in the P5Se3 liquid perform rapid isotropic rotation without significant translational diffusion in the supercooled regime and this rotational process shows a decoupling in time scale from shear relaxation by nearly six orders of magnitude at the glass transition. This dynamical behavior of liquid-like rotation and localized translation appears to be universal to glass-forming liquids with high-symmetry globular molecules that are characterized by an underlying thermodynamically stable plastic crystal phase.

Unravelling the mechanism of pressure induced polyamorphic transition in an inorganic molecular glass.

The atomic structure of a germanium doped phosphorous selenide glass of composition Ge2.8P57.7Se39.5 is determined as a function of pressure from ambient to 24 GPa using Monte-Carlo simulations constrained by high energy x-ray scattering data. The ambient pressure structure consists primarily of P4Se3 molecules and planar edge shared phosphorus rings, reminiscent of those found in red phosphorous as well as a small fraction of locally clustered corner-sharing GeSe4 tetrahedra. This low-density amorphous phase transforms into a high-density amorphous phase at ~6.3 GPa. The high-pressure phase is characterized by an extended network structure. The polyamorphic transformation between these two phases involves opening of the P3 ring at the base of the P4Se3 molecules and subsequent reaction with red phosphorus type moieties to produce a cross linked structure. The compression mechanism of the low-density phase involves increased molecular packing, whereas that of the high pressure phase involves an increase in the nearest-neighbor coordination number while the bond angle distributions broaden and shift to smaller angles. The entropy and volume changes associated with this polyamorphic transformation are positive and negative, respectively, and consequently the corresponding Clapeyron slope for this transition would be negative. This result has far reaching implications in our current understanding of the thermodynamics of polyamorphic transitions in glasses and glass-forming liquids.

Third order cascaded Raman wavelength shifting in chalcogenide fibers and determination of Raman gain coefficient.

Cascaded Raman wavelength shifting up to three orders from 1553 nm to 1867 nm is demonstrated in As(2)S(3)-chalcogenide fibers. Due to a long zero dispersion wavelength for the sulfide fiber (>4.5 mum), pumping the fiber at 1553 nm results in generation of cascaded Stokes orders based on stimulated Raman scattering. Using the threshold power for the Raman orders, we estimate the Raman gain coefficient for the As(2)S(3) fibers to be ~5.7x10(-12) m/W at 1550 nm. Observation of higher Raman orders is limited by damage to the fiber at input intensities >1 GW/cm(2).