P. Boolchand

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 °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.

Role of network connectivity on the elastic, plastic and thermal behavior of covalent glasses

Abstract Experiments on chalcogenide glasses show that local elasticity as measured by Raman scattering and the kinetic heat-flow near T g as established from temperature modulated differential scanning calorimetry each display a threshold behavior associated with network connectivity as defined by the mean coordination, 〈 r 〉, near 〈 r 〉 = 〈 r 〉 c = 2.40. Network plasticity of amorphous group IV networks as deduced from nanometer-indentation hardness measurements varies linearly with 〈 r 〉 once 〈 r 〉 > 2.40. Consequences of these results connecting physical behavior of covalent networks with their connectivity are discussed.

Rigidity transitions in binary Ge-Se glasses and the intermediate phase

Abstract Raman scattering measurements, undertaken on bulk GexSe1−x glasses at 0 (ν ES ) Ge ( Se 1/2 ) 4 mode frequencies. A second-order transition from a floppy to an unstressed rigid phase occurs near xc(1)=0.20(1) where both νCS(x) and νES(x) show a kink. A first-order transition from an unstressed rigid to a stressed rigid phase occurs near xc(2)=0.26(1), where νCS2(x) displays a step-like discontinuity between x=0.25 and 0.26 and a power-law behavior at x>xc (2). In sharp contrast, earlier micro-Raman measurements that use at least three orders of magnitude larger photon flux to excite the scattering, showed only one rigidity transition near xc=0.23, the mid-point of the intermediate phase (xc(1)

Self-organization and the physics of glassy networks

Network glasses are the physical prototype for many self-organized systems, ranging from proteins to computer science. Conventional theories of gases, liquids and crystals do not account for the strongly material-selective character of the glass-forming tendency, the phase diagrams of glasses or their optimizable properties. A new topological theory, only 25 years old, has succeeded where conventional theories have failed. It shows that (probably all slowly quenched) glasses, including network glasses, are the result of the combined effects of a few simple mechanisms. These glass-forming mechanisms are topological in nature and have already been identified for several important glasses, including chalcogenide alloys, silicates (window glass and computer chips) and proteins.

Molecular phase separation and cluster size in GeSe2 glass

The local vibrational modes and the microscopic nature of Te sites in GeSe2-xTex alloy glasses have been examined using Raman and129I Mossbauer emission spectroscopy. Two distinct types (A, B) of chalcogen (Te) sites are observed. The Mossbauer site intensities IB/IA(x) reveal a power-law variation which on statistical grounds requires that these sites be formed in a cluster. The characteristic size of the cluster is found to be 60–75 A.

Variation of glass transition temperature, Tg, with average coordination number, 〉m〈, in network glasses: evidence of a threshold behavior in the slope |dTg/d〉m〈 | at the rigidity percolation threshold (〉m〈 = 2.4)

Abstract Alkali oxide (Ak2O) addition to telluria lowers glass transition temperature, Tg, of (Ak2O)x(TeO2)1−x glasses systematically, with the slope dTg/dx displaying a local maximum at xc ≅ 0.18 corresponding to 〉m〈 ≅ 2.4, the rigidity percolation threshold. In covalent network glasses, as in the present alkali tellurate glasses, Tg is found to increase with 〉m〈 with the slope |dTg/d〉m〈| displaying a maximum near 〉m〈≅2.4. It is recognized that this threshold behavior can be traced to a qualitative increase of molecular relaxation time near 〉m〈≅2.4, where a condition for mechanical equilibrium is locally satisfied. This increase leads to a local Tg(〉m〈) enhancement at 〉m〈 = 2.4 due to a kinetic effect, which is superposed on a quasi-linear Tg(〉m〈) variation with 〉m〈 due to chemical effects.

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<x<0.145 range, within which T(g) s become thermally reversing. In this reversibility window, glasses are found not to age, in contrast to ageing observed for fragile glass compositions outside the window. Thermal reversibility and lack of ageing seem to be paradigms of self-organization which molecular glasses share with protein structures which repetitively and reversibly change conformation near T(g) and the folding temperature respectively.

Processing of iron-doped titania powders in flame aerosol reactors

A flame aerosol reactor was used to synthesize Fe(III)-doped titania powders. The processing conditions were controlled to obtain varying ratios of Fe:Ti in the as processed powders. The iron was incorporated into the titania lattice and promoted the conversion of the anatase to the rutile phase. With an increase in the iron dopant concentration, a decrease in the crystal size of the resultant titania particles was observed, along with a conversion to the amorphous state. The defect structure was further explored by Raman spectroscopy, revealing an increased shift and broadening of the anatase peaks with an increasing iron dopant concentration, and was attributed to shrinkage in the grain size. Absorption spectra revealed a shift of the absorption band toward the visible frequencies. Powders with Fe:Ti ratio exceeding 0.8 resulted in a binary mixture that had superparamagnetic characteristics.

Pressure Raman effects and internal stress in network glasses

Raman scattering from binary

Intrinsic nanoscale phase separation of bulk As2S3 glass

{\mathrm{Ge}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}