Molecular origin of aging of pure Se glass: Growth of inter-chain structural correlations, network compaction, and partial ordering.

Glass transition width W of pure Se narrows from 7.1(3) °C to 1.5(2) °C and the non-reversing enthalpy of relaxation (ΔHnr) at Tg increases from 0.23(5) cal/g to 0.90(5) cal/g upon room temperature aging for 4 months in the dark as examined in modulated differential scanning colorimetry (MDSC) at low scan rates. In Raman scattering, such aging leads the A1 mode of Sen-chains (near 250 cm-1) to narrow by 26% and its scattering strength to decrease as the strength of modes of correlated chains (near 235 cm-1) and of Se8 rings (near 264 cm-1) systematically grows. These calorimetric and Raman scattering results are consistent with the "molecular" chains of Sen, predominant in the fresh glass, reconstructing with each other to compact and partially order the network. Consequences of the aging induced reconstruction of the long super-flexible and uncorrelated Sen-chains are also manifested upon alloying up to 4 mol. % of Ge as revealed by a qualitative narrowing (by 25%) of the Raman vibrational mode of the corner-sharing GeSe4 tetrahedra and a blue-shift of the said mode by nearly 1 cm-1 in 194 cm-1. But, at higher Ge content (x> 6%), as the length of Sen chain-segments across Ge cross-links decreases qualitatively (⟨n⟩ < 8), these aging induced chain-reconstruction effects are suppressed. The width of Tg increases beyond 15 °C in binary GexSe100-x glasses as x> 10% to acquire values observed earlier as alloying concentration approaches 20% and networks become spontaneously rigid.

Structural singularities in Ge(x)Te(100-x) films.

Structural and calorimetric investigation of Ge(x)Te(100-x) films over wide range of concentration 10 < x < 50 led to evidence two structural singularities at x ∼ 22 at. % and x ∼ 33-35 at. %. Analysis of bond distribution, bond variability, and glass thermal stability led to conclude to the origin of the first singularity being the flexible/rigid transition proposed in the framework of rigidity model and the origin of the second one being the disappearance of the undercooled region resulting in amorphous materials with statistical distributions of bonds. While the first singularity signs the onset of the Ge-Ge homopolar bonds, the second is related to compositions where enhanced Ge-Ge correlations at intermediate lengthscales (7.7 Å) are observed. These two threshold compositions correspond to recently reported resistance drift threshold compositions, an important support for models pointing the breaking of homopolar Ge-Ge bonds as the main phenomenon behind the ageing of phase change materials.

Topology and glass structure evolution in (BaO)x((B2O3)32(SiO2)68)(100-x) ternary--evidence of rigid, intermediate, and flexible phases.

We examine variations in the glass transition temperature (T(g)(x)), molar volume (V(m)(x)), and Raman scattering of titled glasses as a function of modifier (BaO) content in the 25% < x < 48% range. Three distinct regimes of behavior are observed; at low x, 24% < x < 29% range, the modifier largely polymerizes the backbone, T(g)(x) increase, features that we identify with the stressed-rigid elastic phase. At high x, 32% < x < 48% range, the modifier depolymerizes the network by creating non-bridging oxygen (NBO) atoms; in this regime T(g)(x) decreases, and networks are viewed to be in the flexible elastic phase. In the narrow intermediate x regime, 29% < x < 32% range, T(g)(x) shows a broad global maximum almost independent of x, and Raman mode scattering strengths and mode frequencies become relatively x-independent, V(m)(x) show a global minimum, features that we associate with the isostatically rigid elastic phase, also called the intermediate phase. In this phase, medium range structures adapt as revealed by the count of Lagrangian bonding constraints and Raman mode scattering strengths.

Crucial effect of melt homogenization on the fragility of non-stoichiometric chalcogenides.

The kinetics of homogenization of binary AsxSe100 - x melts in the As concentration range 0% < x < 50% are followed in Fourier Transform (FT)-Raman profiling experiments, and show that 2 g sized melts in the middle concentration range 20% < x < 30% take nearly two weeks to homogenize when starting materials are reacted at 700 °C. In glasses of proven homogeneity, we find molar volumes to vary non-monotonically with composition, and the fragility index M displays a broad global minimum in the 20% < x < 30% range of x wherein M < 20. We show that properly homogenized samples have a lower measured fragility when compared to larger under-reacted melts. The enthalpy of relaxation at Tg, ΔHnr(x) shows a minimum in the 27% < x < 37% range. The super-strong nature of melt compositions in the 20% < x < 30% range suppresses melt diffusion at high temperatures leading to the slow kinetics of melt homogenization.

Topological origin of fragility, network adaptation, and rigidity and stress transitions in especially homogenized nonstoichiometric binary Ge(x)S(100-x) glasses.

Binary GexS100-x glasses reveal a richness of elastic and chemical phase transitions driven by network topology. With increasing Ge content (x), well-defined rigidity at xc(1) = 19.3(5)% and a stress transition at xc(2) = 24.9(5)% are observed in Raman scattering. In modulated DSC measurements, the nonreversing enthalpy of relaxation at Tg reveals a square-well-like minimum (reversibility window) with window walls that coincide with the two elastic phase transitions. Molar volumes show a trapezoidal-like minimum (volumetric window) with edges that nearly coincide with the reversibility window. These optical, thermal, and volumetric results are consistent with an isostatically rigid elastic phase (intermediate phase, IP) present between the rigidity (xc(1)) and stress (xc(2)) transitions. Complex Cp measurements show melt fragility index, m(x) to also show a global minimum in the reversibility window with m < 20, underscoring that melt dynamics encode the elastic behavior of the glass formed at Tg. The strong nature of melts formed in the IP has an important practical consequence; they lead to slow homogenization (over days not hours) of nonstoichiometric Ge-S batch compositions reacted at high temperatures. Homogenization of chalcogenide melts/glasses over a scale of a few micrometers is a prerequisite to observe the intrinsic physical properties of these materials.

Designing heavy metal oxide glasses with threshold properties from network rigidity.

Here, we show that a new class of glasses composed of heavy metal oxides involving transition metals (V2O5-TeO2) can surprisingly be designed from very basic tools using topology and rigidity of their underlying molecular networks. When investigated as a function of composition, such glasses display abrupt changes in network packing and enthalpy of relaxation at Tg, underscoring presence of flexible to rigid elastic phase transitions. We find that these elastic phases are fully consistent with polaronic nature of electronic conductivity at high V2O5 content. Such observations have new implications for designing electronic glasses which differ from the traditional amorphous electrolytes having only mobile ions as charge carriers.

Superstrong nature of covalently bonded glass-forming liquids at select compositions.

Variation of fragility (m) of specially homogenized Ge(x)Se(100-x) melts is established from complex specific heat measurements and shows that m(x) has a global minimum at an extremely low value (m = 14.8(0.5)) in the 21.5% < x < 23% range of Ge. Outside of that compositional range, m(x) then increases first rapidly and then slowly to about m = 25-30. By directly mapping melt stoichiometry as a function of reaction time at a fixed temperature T > Tg, we observe a slowdown of melt-homogenization by the super-strong melt compositions, 21.5% < x < 23%. This range furthermore appears to be correlated to the one observed between the flexible and stressed rigid phase in network glasses. These spectacular features underscore the crucial role played by topology and rigidity in the properties of network-forming liquids and glasses which are highlighted when fragility is represented as a function of variables tracking the effect of rigidity. Finally, we investigate the fragility-glass transition temperature relationship, and find that reported scaling laws do not apply in the flexible phase, while being valid for intermediate and stressed rigid compositions.

Elastic phases of Ge(x)Sb(x)Se(100-2x) ternary glasses driven by topology.

Topology offers a practical set of computational tools to accurately predict certain physical and chemical properties of materials including transformations under deformation. In network glasses with increased cross-linking three generic elastic phases are observed. We examine ternary Ge(x)Sb(x)Se(100-2x) glasses in Raman scattering, modulated DSC and volumetric measurements, and observe the rigidity transition, x = x(c)(1) = 14.9% that separates the flexible phase from the Intermediate phase, and the stress transition, x = x(c)(2) = 17.5% that separate the intermediate phase from the stressed rigid one. Raman scattering provides evidence of the structural motifs populated in these networks. Using size increasing cluster agglomeration, we have calculated the rigidity and stress transitions to occur near x(c)(1)(t) = 15.2% and x(c)(2)(t) = 17.5%, respectively. Theory predicts and experiments confirm that these two transitions will coalesce if edge-sharing Ge-tetrahedral motifs were absent in the structure, a circumstance that prevails in the Ge-deficient Ge7Sb(x)Se(93-x) ternary, underscoring the central role played by topology in network glasses. We have constructed a global elastic phase diagram of the Ge-Sb-Se ternary that provides a roadmap to network functionality. In this diagram, regions labeled A, B, and C comprise networks that are flexible, rigid but unstressed, and stressed-rigid, respectively.

Long term aging of selenide glasses: evidence of sub-Tg endotherms and pre-Tg exotherms.

Long term aging, extending from months to several years, is studied on several families of chalcogenide glasses including the Ge-Se, As-Se, and Ge-As-Se systems. Special attention is given to the As-Se binary, a system that displays a rich variety of aging behavior intimately tied to sample synthesis conditions and the ambient environment in which samples are aged. Calorimetric (modulated DSC) and Raman scattering experiments are undertaken. Our results show all samples display a sub-Tg endotherm typically 10-70 °C below Tg in glassy networks possessing a mean coordination number r in the 2.25 < r < 2.45 range. Two sets of As(x)Se(100-x) samples aged for eight years were compared, set A consisted of slow cooled samples aged in the dark, and set B consisted of melt-quenched samples aged at laboratory environment. Samples of set B in the As concentration range, 35% < x < 60%, display a pre-T(g) exotherm, but the feature is not observed in samples of set A. The aging behavior of set A presumably represents intrinsic aging in these glasses, while that of set B is extrinsic due to the presence of light. The reversibility window persists in both sets of samples, but is less well defined in set B. These findings contrast with a recent study by Golovchak et al (2008 Phys. Rev. B 78 014202), which finds the onset of the reversibility window moved up to the stoichiometric composition (x = 40%). Here we show that the up-shifted window is better understood as resulting due to demixing of As4Se4 and As4Se3 molecules from the backbone, i.e., nanoscale phase separation (NSPS). We attribute sub-Tg endotherms to compaction of the flexible part of the networks upon long term aging, while the pre-Tg exotherm is to NSPS. The narrowing and sharpening of the reversibility window upon aging is interpreted as the slow 'self-organizing' stress relaxation of the phases just outside the intermediate phase, which itself is stress free and displays little aging.

Elastic flexibility, fast-ion conduction, boson and floppy modes in AgPO(3)-AgI glasses.

Raman scattering, IR reflectance and modulated-DSC measurements are performed on specifically prepared dry (AgI)(x)(AgPO(3))(1-x) glasses over a wide range of compositions 0%37.8% are elastically flexible. Raman optical elasticity power laws, trends in the nature of the glass transition endotherms, corroborate the three elastic phase assignments. Ionic conductivities reveal a step-like increase when glasses become stress-free at x>x(c)(1) = 9.5% and a logarithmic increase in conductivity (σ∼(x-x(c)(2))(µ)) once they become flexible at x>x(c)(2) = 37.8% with a power law µ = 1.78. The power law is consistent with percolation of 3D filamentary conduction pathways. Traces of water doping lower T(g) and narrow the reversibility window, and can also completely collapse it. Ideas on network flexibility promoting ion conduction are in harmony with the unified approach of Ingram et al (2008 J. Phys. Chem. B 112 859), who have emphasized the similarity of process compliance or elasticity relating to ion transport and structural relaxation in decoupled systems. Boson mode frequency and scattering strength display thresholds that coincide with the two elastic phase boundaries. In particular, the scattering strength of the boson mode increases almost linearly with glass composition x, with a slope that tracks the floppy mode fraction as a function of mean coordination number r predicted by mean-field rigidity theory. These data suggest that the excess low frequency vibrations contributing to the boson mode in flexible glasses come largely from floppy modes.

Abrupt boundaries of intermediate phases and space filling in oxide glasses.

Modulated differential scanning calorimetry measurements on bulk (Na(2)O)(x)(GeO(2))(1-x) glasses show a sharp reversibility window in the 14%

Fast-ion conduction and flexibility of glassy networks.

We observe two thresholds in the variations of electrical conductivity of dry (AgI)_{x}(AgPO3)_{1-x} solid electrolyte glasses, when the AgI additive concentration x increases to 9.5% and to 37.8%. Raman scattering complemented by calorimetric measurements confirms that these thresholds are signatures of the rigidity phase transitions at x=9.5% from a stressed rigid to an isostatically (stress-free) rigid phase, and at x=37.8% from isostatically rigid to a flexible phase. In the flexible phase, the electrical conductivity seems to increase as a power of x. This is in good agreement with the theoretical prediction based on 3D percolation.

Visible light photocatalysis with platinized rutile TiO2 for aqueous organic oxidation.

Platinized rutile TiO2 samples containing varying concentrations of Pt were synthesized using Kemira (KE, BET surface area 50 m2/g, from Finland), and Toto HT0270 (HT, BET surface area 2.9 m2/g, from Japan) as the starting materials by solution mixing followed by sintering the precursors. Photocatalytic activities were established for phenol oxidation under visible light (wavelength >400 nm). Our results show optimal performance for 8 wt % platinized KE (8 wt % Pt/KE) and (1/2) wt % platinized HT rutile samples. The specific roles of O2 and visible light were examined using the 8 wt % Pt/KE sample in either N2 gas ambient or no illumination. Separately, 8 wt % platinized SiO2 was tested to compare its performance with that of platinized rutile TiO2. Several other chemicals containing different functional groups (formic acid, salicylic acid, 4-chlorophenol, 2,4,6-trichlorophenol, diethyl phosphoramidate) were selected for photooxidation tests with (1/2) wt % platinized HT rutile. X-ray diffraction reveals Pt metal clusters segregating on the surface of rutile TiO2 particles with increasing Pt weight percent. The Pt cluster surface area broadly increases, while the effective optical band gap steadily decreases with platinization of the rutile samples. These results suggest that Pt clusters on the surface of rutile TiO2 particles serve to mediate electron transfer from rutile to O2, thus facilitating photooxidation of organic chemicals.

Ageing, fragility and the reversibility window in bulk alloy glasses.

Non-reversing relaxation enthalpies (ΔH(nr)) at glass transitions T(g)(x) in the P(x)Ge(x)Se(1-2x) ternary display wide, sharp and deep global minima ([Formula: see text]) in the 0.09

Light-induced giant softening of network glasses observed near the mean-field rigidity transition.

The longitudinal acoustic (LA) mode of bulk GexSe1-x glasses is examined in Brillouin scattering (BS) over the 0.15

Mobile silver ions and glass formation in solid electrolytes.

Solid electrolytes are a class of materials in which the cationic or anionic constituents are not confined to specific lattice sites, but are essentially free to move throughout the structure. The solid electrolytes AgI and Ag2Se (refs 1, 2, 3, 4, 5, 6, 7) are of interest for their use as additives in network glasses, such as chalcogenides and oxides, because the resulting composite glasses can show high electrical conductivities with potential applications for batteries, sensors and displays. Here we show that these composite glasses can exhibit two distinct types of molecular structures-an intrinsic phase-separation that results in a bimodal distribution of glass transition temperatures, and a microscopically homogeneous network displaying a single glass transition temperature. For the first case, the two transition temperatures correspond to the solid-electrolyte glass phase and the main glass phase (the 'base glass'), enabling us to show that the glass transition temperatures for the AgI and Ag2Se phases are respectively 75 and 230 degrees C. Furthermore, we show that the magnitude of the bimodal glass transition temperatures can be quantitatively understood in terms of network connectivity, provided that the Ag+ cations undergo fast-ion motion in the glasses. These results allow us to unambiguously distinguish base glasses in which these additives are homogeneously alloyed from those in which an intrinsic phase separation occurs, and to provide clues to understanding ion-transport behaviour in these superionic conductors.