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.

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.

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.

Observation of Collective Molecular Dynamics in a Chalcogenide Glass: Results from X-ray Photon Correlation Spectroscopy.

The structural relaxation processes in a Ge3As52S45 molecular chalcogenide glass sample were directly studied by X-ray photon correlation spectroscopy (XPCS). XPCS was conducted at the first sharp diffraction peak at q = 1.16 Å-1 at temperatures ranging from 123 K to above the glass transition at 328 K, and the results showed two different dynamical regimes. At a low temperature, the observed glass dynamics are slow and dominated by X-ray-photon-induced effects, which are temperature independent. At a higher temperature, we observed a dramatic decrease in the fluctuation timescales, indicating that the dynamics were mainly due to the intermolecular correlation of the As4S3 molecule in the glass. The timescales in this high-temperature range agree well with those determined from measurements of the Newtonian viscosity. Our XPCS studies suggest an extended length scale of the relaxation process in glassy Ge3As52S45 from the single molecule to the intermolecular range across the glass transition, providing a unique direct probe of the dynamics beyond the length scales of the individual molecule.

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.

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.

Compositional Evolution of the Structure and Connectivity in Binary P-Se Glasses: Results from 2D Multinuclear NMR and Raman Spectroscopy.

The atomic structure of binary PxSe100-x glasses with 5 ≤ x ≤ 70 is investigated using Raman spectroscopy and two-dimensional 77Se and 31P isotropic/anisotropic correlation nuclear magnetic resonance (NMR) spectroscopy. These spectroscopic results, when taken together, demonstrate that the structure of PxSe100-x glasses with x ≤ 50 consists primarily of -Se-Se-Se- chain elements, pyramidal P(Se1/2)3 units, ethylene-like 2/2SeP-PSe2/2 units, and Se=P(Se1/2)3 tetrahedral units. The chain structure of Se becomes increasingly cross-linked by P-Se polyhedral units, and the degree of connectivity increases with a progressive increase in P content up to x ∼ 50, at which point the -Se-Se-Se- chain elements completely disappear, and the structure becomes highly rigid. The compositional variation of the Se-Se-Se environments as obtained from the 77Se isotropic NMR spectra reveals that the connectivity between the Se-Se and P-Se units in glasses with x ≤ 50 is intermediate to that of the random and the fully clustered scenarios. A further increase in P content results in the formation of P4Se3 molecules such that at x = 63, the structure becomes predominantly molecular, consisting of P4Se3 molecules likely held together via van der Waals forces. The structure of glasses with x > 63 is characterized by P4Se3 molecules and likely nonmolecular P4Se3-like species, along with amorphous red phosphorus-like regions. These P4Se3-like moieties and the amorphous red phosphorus-like units can connect to each other via P-P bonds, and their relative concentrations increase with increasing P content. This compositional evolution of structural connectivity of PxSe100-x glasses is shown to be consistent with the corresponding variation in the glass transition temperature.

Rheological characterization of complex dynamics in Na-Zn metaphosphate glass-forming liquids.

The viscoelastic behavior and shear relaxation in supercooled [NaPO3]x[Zn(PO3)2]1-x metaphosphate liquids with 0.2 ≤ x ≤ 1.0 are investigated using a combination of small amplitude oscillatory and steady shear parallel plate rheometry, resonant ultrasound spectroscopy, and differential scanning calorimetry. The results demonstrate that these liquids are thermorheologically complex with the coexistence of a fast and a slow relaxation process, which could be attributed to the segmental motion of the phosphate chains and the Zn-O bond scission/renewal dynamics, respectively. The segmental motion of the phosphate chains is found to be the dominant process associated with the shear relaxation for all metaphosphate liquids. The compositional evolution of the calorimetric fragility of these liquids is shown to be related to the conformational entropy of the constituent phosphate chains, which is manifested by the width of the relaxation time distribution for the segmental chain motion. This entropy decreases and the temporal coupling between the chain dynamics and Zn-O bond scission-renewal increases with the increasing Zn content as the higher field strength Zn modifier ions provide more effective cross-linking between the phosphate chains.

Kinetic and Calorimetric Fragility of Chalcogenide Glass-Forming Liquids: Role of Shear vs Enthalpy Relaxation.

The kinetic and calorimetric fragility indices m of binary As-Se and Se-Te chalcogenide liquids with a wide range of fragility are determined using a combination of parallel plate rheometry, beam bending viscometry, and conventional differential scanning calorimetry (DSC). It is shown that both sets of measurements lead to consistent m values only if the validity of the assumptions often implicit in the methodology for the estimation of m are considered. These assumptions are (i) the glass transition temperature Tg corresponds to a viscosity of ∼1012 Pa s and (ii) enthalpy and shear relaxation time scales τen and τshear are comparable near Tg. Both assumptions are shown to be untenable for highly fragile liquids, for which modulated DSC studies demonstrate that τen ≫ τshear near Tg. In these cases, the above-mentioned assumptions are shown to lead to consistently higher values for the kinetic fragility compared to its calorimetric counterpart.

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.

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.

Is the λ-transition in liquid sulfur a fragile-to-strong transition?

The abrupt and large increase in the viscosity of liquid sulfur above the λ-transition temperature Tλ corresponds to a reversible structural transformation in the form of a ring-to-chain polymerization reaction. The mechanistic connection between this structural transformation and viscosity is investigated by studying the compositional dependence of the shear relaxation behavior of supercooled SxSe100-x liquids as their structural evolution mimics that of liquid sulfur across the λ-transition. The results of steady and oscillatory shear parallel-plate rheometry indicate that the viscosity of these liquids is controlled by the S/Se-S/Se bond scission/renewal dynamics and the time scale of these dynamics rapidly increases as the relative concentrations of rings and chains in the structure become comparable. The coexistence of these two types of topological units in these liquids lowers the conformational entropy of the chain elements due to a steric hindrance from the ring elements, resulting in a rapid drop in the fragility as S is added to Se. The same topological effect resulting from the ring-to-chain transformation in liquid S renders the highly fragile molecular liquid below Tλ, a strong polymerized liquid above Tλ. Therefore, it is argued that the λ-transition of liquid S corresponds to a fragile-to-strong liquid-liquid transition.

Nature of the floppy-to-rigid transition in chalcogenide glass-forming liquids.

The viscoelastic properties of supercooled AsxSe100-x and GexSe100-x (0 ≤ x ≤ 30) liquids are studied using oscillatory parallel plate rheometry. The liquids with average selenium chain segment length L longer than ∼3 to 5 atoms or average coordination number ⟨r⟩ less than ∼2.2 are characterized by the coexistence of a low-frequency bond scission/renewal based relaxation process as well as high-frequency segmental chain dynamics. The latter process disappears for liquids with higher connectivity, thus implying a dynamical rigidity transition. The temporal decoupling of the high-frequency chain mode from that of the bond scission/renewal process and the shear modulus Gs associated with the low-frequency mode are shown to be unique functions of L or ⟨r⟩ and display strong similarity with the corresponding variation in the fragility m and the conformational entropy of the chain segments. When taken together, these results provide direct experimental support to the entropic rigidity argument originally proposed by Phillips but suggest a floppy-to-rigid transition of the structural network at ⟨r⟩ ∼ 2.2, instead of the conventional rigidity percolation threshold value of 2.4.

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.

Structure and Chemical Order in S-Se Binary Glasses.

The compositional evolution of the structure and chemical order in binary S xSe100- x glasses (0 ≤ x ≤ 90) is investigated using a combination of high-resolution 2D 77Se isotropic-anisotropic correlation NMR and Raman spectroscopy. The results indicate that the structure of S-Se glasses consists of two types of topological elements, namely polymeric [Se,S] n chains and eight-membered Se yS8 -y rings (0 ≤ y ≤ 8). The relative concentration of Se atoms monotonically decreases in the chain elements and concomitantly increases in the ring elements with increasing S concentration. Moreover, the Se speciation results are consistent with an average heterocyclic ring composition of Se1S7 at low S content (≤40 at. % S), while the composition shifts to Se1.5S6.5 at higher S content (≥60 at. % S), indicating increasing incorporation of multiple Se atoms in each ring element. The Raman spectra suggest that -Se-Se- association is favored, when more than one Se atom is incorporated in chains and rings. As in their elemental forms, the S and Se atoms retain their preference of forming rings and chains in binary S xSe100- x glasses, which predicts a linear compositional variation in the relative fractions of these topological elements. This structural evolution is consistent with the corresponding variation in the Tg and molar volume, both of which exhibit a linear decrease with increasing S concentration.

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.

Observation of Steady Shear-Induced Nematic Ordering of Selenium Chain Moieties in Arsenic Selenide Liquids.

Structural anisotropy induced by steady shear and its mechanistic relation with shear thinning are investigated in AsxSe100-x glasses (5 ≤ x ≤ 30) quenched from parent liquids subjected to shear rates ranging between 0 and 104 s-1 using polarized Raman spectroscopy and two-dimensional X-ray diffraction. When taken together, the results demonstrate significant shear-induced partial alignment of -Se-Se-Se- chain moieties in the flow direction of the extruded fibers. This alignment is reminiscent of nematic liquid crystals where orientational order exists without positional order. The degree of this structural alignment in quenched glasses appears to be practically independent of the shear rate, although the parent liquids undergo shear thinning at the highest shear rates. It is conjectured that any causal relationship between structural alignment and shear thinning in the liquid may be masked in the glassy state by the postextrusion structural relaxation of the parent liquid.

Fabrication of graded index single crystal in glass.

Lithium niobate crystals were grown in 3D through localized heating by femtosecond laser irradiation deep inside 35Li2O-35Nb2O5-30SiO2 glass. Laser scanning speed and power density were systematically varied to control the crystal growth process and determine the optimal conditions for the formation of single crystal lines. EBSD measurements showed that, in principle, single crystals can be grown to unlimited lengths using optimal parameters. We successfully tuned the parameters to a growth mode where nucleation and growth occur upon heating and ahead of the scanning laser focus. This growth mode eliminates the problem reported in previous works of non-uniform polycrystallinity because of a separate growth mode where crystallization occurs during cooling behind the scanning laser focus. To our knowledge, this is the first report of such a growth mode using a fs laser. The crystal cross-sections possessed a symmetric, smooth lattice misorientation with respect to the c-axis orientation in the center of the crystal. Calculations indicate the observed misorientation leads to a decrease in the refractive index of the crystal line from the center moving outwards, opening the possibility to produce within glass a graded refractive index single crystal (GRISC) optically active waveguide.

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.

Tellurium speciation, connectivity, and chemical order in As(x)Te(100-x) glasses: results from two-dimensional 125Te NMR spectroscopy.

The short-range structure, connectivity, and chemical order in As(x)Te(100-x) (25 ≤ x ≤ 65) glasses are studied using high-resolution two-dimensional projection magic-angle-turning (pjMAT) (125)Te nuclear magnetic resonance (NMR) spectroscopy. The (125)Te pjMAT NMR results indicate that the coordination of Te atoms obeys the 8-N coordination rule over the entire composition range. However, in strong contrast with the analogous glass-forming As-S and As-Se chalcogenides, significant violation of chemical order is observed in As-Te glasses over the entire composition range in the form of homopolar As-As (Te-Te) bonds, even in severely As (Te)-deficient glasses. The speciation of the Te coordination environments can be explained with the dissociation reaction model As2Te3 → 2As + 3Te(II), characterized by a dissociation constant that is independent of glass composition. These structural characteristics can be attributed to the high metallicity of Te and the strong energetic similarity between the Te-Te, Te-As, and As-As bonds, and they are consistent with the monotonic and often nearly linear variation of physical properties observed in telluride glasses as a function of the Te content.