Our approach to investigating the glass transition lies in studies of the dynamics of supercooled liquids at temperatures above T_g using a dynamic light scattering technique known as photon correlation spectroscopy (PCS).  PCS provides a direct measure of the liquid's dynamic structure factor and for supercooled liquids near T_g exhibits a viscoelastic relaxation that is a non-exponential (parameterized by smallness of b) and whose average relaxation time displays a non-Arrhenius temperature dependence (parameterized by largeness of the fragility, m).

In this project, we plan to examine a series of phosphate glass forming liquids in which the chemical structure has been systematically altered through the addition of alkali oxides.  To date, we have succeeded in studying both neat P₂O₅, which forms a continuous random network of covalent bonds possessing three linkages per each PO₄ unit, and the two alkali metaphosphate liquids LiPO₃ and NaPO₃, for which the added alkali result in a reduction of linkages to only two per phophate unit.  In this way we have probed the two structural extremes: 3D network versus 2D chains.  Interestingly, we find stark differences in the liquid dynamics.  The fragility of P₂O₅ is only m = 20, while alkali addition produces greater non-Arrhenius behavior with m = 90 for the two metaphosphates.  Differences in the non-exponentiality of the relaxation are also puzzling.  For P₂O₅, b decreases from about 0.85 seen at high temperatures to around 0.55 near T_g.  In comparison, NaPO₃ (and to a lesser degree, LiPO₃) display b = 0.3 at high temperatures that increases to near 0.55 at T_g.

Perfecting the sample preparation of P₂O₅ turned out to be major hurdle in the previous year.  But now that we have succeeded with these two compositional endmembers, in the coming year we will examine all the compositions in between: (Na₂O)_x(P₂O₅)_1-x, for x = 0.1, 0.2, 0.3, 0.4.  Also, in collaboration with Dr. Sorensen at Kansas State University, we plan to attempt Raman measurements on the molten P₂O₅.  This interest stems from the similarity of the viscoelastic behavior of our P₂O₅ to that of molten B₂O₃ and the substantial development of intermediate range order (boroxyl rings) that occurs in the latter.  Could a similar IRO be developing in P₂O₅?

To date, two graduate students and one undergraduate have benefited from the financial support of the grant:  Ms. Jessica Changstrom, now writing up a M.S. thesis, Mr. Mark Durante, finishing his first year in our M.S. program, and Mr. Brandon Rodenburg, who graduated last May and is currently pursuing graduate work in optics at the University of Rochester.  Brandon's work on As₂O₃ glasses and Jessica's work on alkali metaphosphate glasses featured prominently in one publication and Brandon presented the collective work both as a poster at the most recent Glass and Optical Materials Division Meeting of the American Ceramic Society in Rochester and as an oral presentation at the regional meeting of the Nebraska Academy of Sciences.

Mark Durante's work is following up a thread of investigation on glassy dynamics in aqueous sugar solutions.  A study of aqueous glucose (published in Physical Review E) suggested the viscoelastic relaxation in these sugar glasses involves the formation of clusters and the percolation of a hydrogen-bonded network (in some ways akin to the networks occurring in the oxide glasses) with increasing concentration.  Mark is attempting to continue these studies to lower temperatures and also compare them with similar measurements in aqueous maltose (the dissacharide form of glucose).