The Intermediate Phase in Network Glasses

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This project concerns the properties of glasses and their phase transformations. The physical nature of the Intermediate phase (IP) that separates the Floppy phase (FP) from the Rigid Phase (RP) in select chalcogenide glass systems will be examined. These properties will be investigated in four different sets of experiments. First, temperature-dependent Raman scattering studies of Ge-Se, As-Se and Ge-As-Se bearing bulk glasses will be performed over the range from 78K to Tg+200 K to examine the nature of molecular structure changes taking place across Tg in the FP, IP and the RP. Second, the same glass systems, but now synthesized as obliquely deposited thin-films (obliqueness angle in the range from 0 to 80 degrees) will be examined in their virgin and photo-illuminated state in Raman, 119Sn Mossbauer effect and photo-contraction studies. The columns formed in the porous (obliquely) deposited films may represent the elements of medium range structure responsible for the rather unique physical properties of glasses in the IP. Third, pnictide based glass systems, such as As-Se, As-S and As-Ge-Se bearing glasses will be examined in Raman, NQR, EXAFS and 129I Mossbauer spectroscopic studies to elucidate new aspects of molecular structure recently inferred from the discovery of the IP in Modulated Differential Scanning Calorimetry (MDSC) measurements. Lastly, 119Sn Lamb-Mossbauer factors in select glasses will be established as a function of mean coordination number to contrast the local elastic response of glasses in the IP from those in the FP and the RP. Students will participate in this research and will thereby be prepared to enter the scientific technical work force of the 21st Century.This research concerns the properties of glasses and glass systems. Glasses are characterized by progressive cross-linking of a floppy network that will render it rigid at a critical connectivity. This is often realized by systematically changing the chemical composition in a glass system. Experiments on sulfur- and selenium-bearing glasses have shown however that a floppy phase (FP) and a rigid phase (RP) are separated by an intermediate phase (IP). Four different sets of experiments will be performed to establish the physical nature of the IP in select glass systems. Different experimental techniques will be brought to bear on selected glass systems over a range of glass compositions that encompass the three different phases. The experiments will be performed over a wide temperature range that embraces the transitions among the phases. Furthermore, specially fabricated thin films of these glasses will be investigated. These experiments are expected to shed light on the intricacies of glass systems under various conditions of temperature and composition and the results will be of use for the incorporation of new glasses in advanced technological devices. Students will participate in these investigations. They will thereby acquire training in one of the important contemporary areas of condensed matter physics and materials science and will be prepared to enter the scientific/ technical work force in industry, academe, and the government.