9604921 Capelletti Technical Abstract This is a new project in which a joint experimental and theoretical program to study compound chalcogenide glasses is proposed using a combination of thermal neutron scattering and ab initio molecular dynamics. In particular, we propose to study alloys in the family Se-As-Ge. A remarkably simple feature has emerged from our previous studies of glasses in this family: the dependence of some properties only on network topology as characterized by one parameter, the average atomic coordination number . Recently we have made the first ab initio molecular dynamics calculations in a compound chalcogenide glass, GeSe_2. This work demonstrates the power of a combined experimental and computational approach in gaining understanding of amorphous systems. In this proposal we address the following issues: atomic structure and dynamics and their relation to network topology and chemical order/disorder; "floppy" modes and their relation to network properties and to photo-induced phenomena; and the "Bose peak." We also theoretically investigate holeburning in glassy As_2S_3 :H. %%% Non-Technical abstract This a new project of study on chalcogenide glasses. Unlike crystals, glasses are difficult to study because the atoms are not arrayed in a nice regular pattern. For this reason, even obvious properties like the excellent transmission of light through ordinary window glass are not very well understood. Nor is it understood why some materials form glasses easily while others don't. In this work we study a family of glasses (chalcogenides) which display a strikingly simple feature: some properties apparently depend only on the average number of atoms to which any given atom is linked up (bonded.) Among these properties, for example, is how the atoms vibrate in the glass. We make measurements which provide some information about this, but which cannot reveal the atomic motions in detail. This is where computations come in. By making a reliable computer model of the same glass (which we can tell by comparing its predictions with measurements) we can use it to see just how the atoms vibrate. But the model then provides much more information about these glasses. It gives details about atomic structure which can be compared to experiment, and about how these glasses will respond to light. In this way we hope to come to a better understanding of this glass family, and of glasses generally. ***
