Self-organization and stress in network glasses

Raman pressure measurements on GexSe1-x glasses using Diamond Anvil Cells show onset of a steady increase in the frequency of modes of corner-sharing GeSe4 tetrahedra when the external pressure P exceeds a threshold value (Pc). The threshold pressure Pc(x) decreases with x to nearly zero for 0.20 < x < 0.25, then increases up to x = 1/3. Pc indicates the presence of local stress at the Raman active units; so its vanishing suggests that these units are part of an isostatically rigid backbone. Isostaticity also accounts for the non-aging behavior of glasses observed in this same composition range.; Chalcohalide Ge25Se75-yIy glasses were synthesized over a wide range (0 < y < 0.40) and examined in Temperature Modulated Differential Scanning Calorimetry (MDSC) and Raman Scattering. The non-reversing enthalpy near the Tg shows an extremely narrow and deep minimum (intermediate phase) centered near yc = 16.0%. The intermediate phase location (yc) is in excellent agreement with mean-field theoretical predictions based on Extended Constraint Counting Algorithms. The observed Tg(y) decrease with I content indicates that iodine alloying lowers the global connectivity by replacing bridging Se atoms by dangling I ends. These observations in conjunction with previous ones on ternary Ge-S-I glasses show that one-fold coordinated atoms play an important role in determining the onset of rigidity.; Raman scattering on Ge-Se-I glasses permits a quantitative measurement of mixed tetrahedral Ge(Se1/2)4-mIm units. The results show the concentration of the iostatically rigid m = 1 units to display a step-like variation in the range 15.5% < y < 16.4%, that we identify with a narrow intermediate phase with glasses at y < 15.5% as stressed-rigid, while those at y > 16.4% floppy. The observed molecular structural rearrangement (populating the m = 1 units) observed in the intermediate phase provides a means to release network stress. Results on ternary Ge-S-I glasses did not show such a step, we suppose because network stress did not build up because of atomic size mismatch. These results show that there is an intrinsic difference in network stress distribution between ternary Ge-Se-I and Ge-S-I glasses. The stress induced by oversized iodine alloying is much greater in Ge-Se-I glasses than in Ge-S-I glasses because of the bigger size of Se atoms in relation to S atoms. Plus, because of the size and mass mismatch in the Ge-S based glass, iodine alloying induced stress is locally dissipated; on the other hand, in the equi-sized and equi-massed base glass Ge-Se, stress due to the iodine alloying is found to propagate globally.; Pronounced photo-polymerization effects are observed in floppy (partially polymerized) Ge25Se75-yIy (y > 30%) glasses and floppy GexS1-x (x < 20%) glasses. In the former glasses, GeI4 monomers are decoupled from the backbone can be opened and alloyed in the base glass network upon near-band-gap radiation. In the latter glasses S8 crowns decouple from the back bone and can be opened and alloyed in the base glass likewise.