Self-organization and stress in network glasses
Fei Wang · OhioLink ETD Center (Ohio Library and Information Network) · 2005
Raman pressure measurements on Ge x Se 1-x glasses using Diamond Anvil Cells show onset of a steady increase in the frequency of modes of corner-sharing GeSe 4 tetrahedra when the external pressure P exceeds a threshold value ( P c ).The threshold pressure P c (x) decreases with x to nearly zero for 0.20 < x < 0.25, then increases up to x = 1/3.P c 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 Ge 25 Se 75-y I y 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 T g shows an extremely narrow and deep minimum (intermediate phase) centered near y c = 16.0%.The intermediate phase location (y c ) is in excellent agreement with mean-field theoretical predictions based on Extended Constraint Counting Algorithms.The observed T g (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(Se 1/2 ) 4-m I m 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