Wayne Joseph Bresser

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)

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.

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.

Thermally reversing window and stiffness transitions in chalcogenide glasses

Abstract Raman scattering and temperature-modulated differential scanning calorimetry measurements on SixSe1−x glasses show that onset of rigidity occurs over a wide compositional window 0.20

Identification of the intercalant species in SbCl5-graphite using Mössbauer spectroscopy

Abstract 121Sb Mossbauer spectra of SbCl5-graphite have been obtained. The spectra provide clear evidence for the presence of the acceptor molecular ion SbCl6-(Sb5+ and neutral SbCl3(Sb3+ in the intercalate layers, with weak evidence for neutral intercalated SbCl5. Lineshape analyses of the Sb3+ region of the spectra further indicate that two chemical Sb3+ sites fit the data significantly better than one site. We attribute the second Sb3+ site to SbCl4- and find the concentration ratio [SbCl4-]/[SbCl3] ∼ 0.25. Samples prepared by the reaction of graphite with vapor mixtures of SbCl5 and SbCl3 were found to exhibit dramatically higher concentrations of SbCl3 than those samples prepared from a reaction with SbCl5 alone. This latter result is interesting in view of the fact that the reaction of graphite with SbCl3 vapor does not lead to an intercalation compound.

Lamb-Mössbauer factors as a local probe of floppy modes in network glasses

Abstract The temperature dependence of the Lamb-Mossbauer factor, f(T), in a solid provides the first inverse, 〈 1 ω 〉 , and second inverse, 〈 1 ω 2 〉 , moments of the vibrational density of states. In network glasses, these moments serve as local probes of low-frequency vibrational excitations, such as floppy modes, and provide a means to establish the rigidity percolation threshold. Lamb-Mossbauer results on prototypical chalcogenide glasses (GexSe1−x) correlate well with those of Raman scattering, inelastic neutron scattering and Mossbauer hyperfine structure experiments in indicating that the rigidity percolation threshold occurs near 〈r〉c = 2.46(4). These observations provide experimental support for predictions of the Phillips-Thorpe constraint theory, when provision is made for a small but finite concentration of broken bond-bending constraints around chalcogen sites.

Onset of Rigidity in Steps in Chalcogenide Glasses

The starting point to understand the physical behavior of crystalline solids is their crystallographic structure. Near phase transitions usually a change of structure occurs, an underlying crystal symmetry is broken, and an order parameter displays a specific power-law. Such effects in oxides have a rather rich history [1]. We are also beginning to recognize that phase separation on a nanoscale in doped crystalline semiconductors and oxides apparently is not that unusual, and that such delicate aspects of structure could play a role to understand the metal insulator transition [2] in Si and Ge and possibly the origin of high-T superconductivity [3] in the oxides.