Nanoscale phase separation effects near (r)over-bar=2.4 and 2.67, and rigidity transitions in chalcogenide glasses

Nanoscale phase separation effects are of general interest in glass science. Such structural effects produce usually pronounced changes in glass physical properties which can mask the more subtle elastic effects related to rigidity transitions. The glass-transition temperature, T-g, is an intrinsic measure of network connectivity. It can be expected to increase or decrease as a network polymerizes or nanoscale phase separates. Compositional trends in T-g(x) in binary AsxSe1-x and GeySe1-y glasses show thresholds near the stoichiometric compositions, x = 2/5 or a mean coordination number (r) over bar = 2.4, y = 1/3 or (r) over bar = 2.67. These T-g trends in conjunction with spectroscopic (Raman, Mossbauer) evidence of broken chemical order suggest that the stoichiometric glasses As2Se, and GeSe2 consist of a Se-rich majority phase that is separate from a compensating Ge- or As-rich minority phase, i.e., they are nanoscale phase separated. On the other hand, ternary Ge-x(As or P)(x)Se1-2x. glasses containing equal proportions of the group IV and V elements reveal compositional trends in T-g(x) that increase monotonically with x; they appear to polymerize increasingly over a wide 2 < r < 2.8 range of connectivities and are homogeneous. In these ternary glasses, experimental evidence for rigidity transitions is observed near (r) over bar = 2.40. Previous claims of a rigidity transition near (r) over bar = 2.67 can be traced to nanoscale phase separation effects.