  Bulk glass formation occurs over a very small part of phase space, and "good"
glasses (which form even at low quench rates ~ 10K/sec) select an even smaller
part of that accessible phase space. An axiomatic theory provides the physical
basis of glass formation, and identifies these sweet spots of glass formation
with existence of rigid but stress-free networks for which experimental
evidence is rapidly emerging. Recently, theory and experiment have come
together to show that these sweet spots of glass formation occur over a range
of chemical compositions identified as Intermediate Phases. These ranges appear
to be controlled by elements of local and medium range molecular structures
that form isostatically rigid networks. Intermediate Phase glasses possess
non-hysteretic glass transitions (Tgs) that do not age much. Raman scattering
has played a pivotal role in elucidating molecular structure of glasses in
general, and in identifying domains of Intermediate Phases. Experiments reveal
these phases to possess sharp phase boundaries and to be characterized by an
optical elasticity that varies with network mean coordination number, r, as
power-law. In this review, we provide examples in chalcogenide and oxide glass
systems where these phases along with optical elasticity power-laws have been
established. Intermediate Phase glasses represent self-organized nanostructured
functional materials optimized by nature.
