A molecular dynamics investigation of the structure of PbO---PbF2 glasses*1

The structure of PbO---PbF2 glasses has been studied using molecular dynamics (MD). The existence of [OPb4] structural units is observed over the entire glass-forming range, in conformity with a model proposed earlier based on various structural investigations of this system. Various other features of the structural model are also supported by the MD calculations.

ESR and optical studies of Mo5+ and W5+ ions in phosphomolybdate and phosphotungstate glasses

ESR and optical studies of phosphomolybdate and phosphotungstate glasses are discussed. Both the ESR and optical results indicate that molybdenum or tungsten ions are present in distorted octahedral environments in these glasses. In addition, ESR spectra of Mo5+ and W5+ ions show that the d electrons are localized on molybdenum and tungsten sites respectively. The variation of gperpendicular and gshort parallel values has been examined using appropriate structural models of these glasses.

MAS NMR as a probe for investigating the distribution of Si---O---Si angles in lithium silicate glasses

Magic-angle-spinning NMR has been used to study Si---O---Si bond-angle distributions associated with various structural elements, Qn, present in lithium silicate glasses of different compositions. It is shown that glasses contain a plurality of structural elements with a broad distribution of Si---O---Si bond angles, and that the width of the distribution is characteristic of a particular Qn species

Magnetic susceptibility studies of lead oxyhalide glasses containing transition metal oxides

Magnetic susceptibility studies of lead oxyhalide glasses containing high concentrations of transition metal oxides such as MnO and Fe2O3 have been performed. While they exhibit predominantly antiferromagnetic interactions, the low temperature (<100K) region is dominated by paramagnetic contributions. The behaviour in these glasses is found to be similar to that of covalent oxide glasses and is different from that of purely ionic sulphate glasses.

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Investigations Of Glasses In The System Pbo-Pbf2

Glass formation in the system PbO–PbF2 has been investigated. The structure of these glasses has been studied using X-ray diffraction. Densities, heat capacities, glass-transition and crystallization temperatures and Vicker's microhardnesses have been measured. D.c. conductivities of these glasses have also been measured as a function of temperature. A structural model has been developed which suggests the existence of [PbO2F4]-type units over the entire composition range. It is suggested that covalent linkages of the type—O—Pb—O— play a crucial role in determining the composition limits to glass formation. The structural model has been shown to be consistent with other physical properties of the glasses.

A composition dependent thermal behavior of GexSe35-xTe65 glasses

Alternating differential scanning calorimetric (ADSC) studies have been performed to understand the thermal behavior of bulk GexSe35-xTe65 glasses (17 <= x <= 25); it is found that the glasses with x <= 20 exhibit two crystallization exotherms (T-c1 & T-c2). On the other hand, those with x >= 20.5, show a single crystallization reaction upon heating. The exothermic reaction at T-c1 has been found to correspond to the partial crystallization of the glass into hexagonal Te and the reaction at T-c2 is associated with the additional crystallization of rhombohedral Ge-Te phase. The glass transition temperature of GexSe35-xTe65 glasses is found to show a linear but not-steep increase, indicating a progressive, but a gradual increase in network connectivity with Ge addition. It is also found that T-c1 of GexSe35-xTe65 glasses with x <= 20, increases progressively with Ge content and eventually merges with T-c2 at x approximate to 20.5 (< r > = 2.41); this behavior has been understood on the basis of the reduction in Te-Te bonds of lower energy and increase in Ge-Te bonds of higher energy, with increasing Ge content. Apart from the interesting composition dependent crystallization, an anomalous melting behavior is also exhibited by the GexSe35-xTe65 glasses.

Glass transition. A new approach based on cluster model of glasses

The structure of real glasses has been considered to be microheterogeneous, composed of clusters and connective tissue. Particles in the cluster are assumed to be highly correlated in positions. The tissue is considered to have a truly amorphous structure with its particles vibrating in highly anharmonic potentials. Glass transition is recognized as corresponding to the melting of clusters. A simple mathematical model has been developed which accounts for various known features associated with glass transition, such as range of glass transition temperature,T g, variation ofT g with pressure, etc. Expressions for configurational thermodynamic properties and transport properties of glass forming systems are derived from the model. The relevence and limitations of the model are also discussed.

Thermal properties of 3BaO-3TiO(2)-B2O3 glasses

Transparent glasses in the system 3BaO-3TiO2-B2O3 (BTBO) were fabricated via the conventional melt-quenching technique. The as-quenched samples were confirmed to be non-crystalline by differential thermal analysis (DTA). Thermal parameters were evaluated using non-isothermal DTA experiments. The Kauzmann temperature was found to be 759 K based on heating-rate-dependent glass transition and crystallization temperatures. A theoretical relation for the temperature-dependent viscosity is proposed for these glasses and glass-ceramics.

A neutron diffraction investigation of the structure of PBO-PBCL2 glasses

Neutron diffraction techniques have been employed to investigate the structure of PbO-PbCl2 glasses as a function of composition in the nominal range PbO.PbCl2 to 9PbO.PbCl2. It is concluded that, whereas the first Pb-O distance is well defined, the distribution of Pb-Cl distances is much broader, in agreement with a previous EXAFS study.

Electron spin resonance studies of d5 ions (Fe3+ and Mn2+) in lead oxide-lead halide glasses

Electron spin resonance (ESR) of d5 ions (Fe3+ and Mn2+) has been investigated in PbO---PbF2 and PbO---PbCl2 glasses in wide ranges of composition. ESR spectra of d5 ions in these glasses exhibit significant differences which we have attributed to at least three important causes: (i) The ionic potentials of Fe3+ and Mn2+ are different. Hence Fe3+ ions tend to acquire their own environment while Mn2+ ions take up substitutional (Pb2+ ion) positions. (ii) The sizes and nephelauxetic behaviours of O2- and F- ions are similar. Thus even when there is a mixed anionic coordination, the environment of Mn2+ ions is highly symmetrical in oxyfluoride glasses. The Mn2+ spectra in oxychloride glasses are considerably different. (iii) Increase in halide ion concentration increases the ionicity of lead-ligand bonding and favours a more symmetrical environment around dopant ions in halide-rich glasses. The features in ESR spectra have been interpreted in the light of known behaviour of d5 ions in glasses and also in the context of known structural features of PbO---PbX2 glasses. Dopant ions appear to cluster at high concentrations although isolated low-symmetry sites are still observed. Effects of crystallization and annealing upon ESR spectra have also been investigated.