Ten questions on glassformers, and a real space 'excitations' model with some answers on fragility and phase transitions

We formulate ten questions, covering outstanding aspects of the phenomenolo gy of glassforming liquids, which we believe must be properly answered by a ny successful theory of structural glassformers. The questions range across thermodynamic, mass transport and vibrational dynamics phenomena. While th ese questions will only be addressed properly by a collective variables app roach (many aspects of which are reported in these proceedings) a number of them can be dealt with by use of simple physical models of appropriate for m. Here we discuss one such model in which the existence of elementary conf igurational excitations of the amorphous quasilattice is proposed. These st ates, which may range from broken bonds to packing defects, can be excited independently in the majority of cases, or cooperatively in others. We summ arize essential results of this model. These suggest that the source of the different fragilities in liquids (and the reason that structural glasses, alone among 'glassy' systems, have marked heat capacity jumps at T-g) may l ie largely in the way these configurational excitations couple to the vibra tional modes of the system. The generation of low frequency modes in the de nsity of vibrational stares, as a direct consequence of the excitation of c onfigurational states, explains why the quasi-elastic scattering from fragi le liquids is so much stronger near and above T-g than in the case of stron g liquids, and why the normal glass transition can be detected in picosecon d time scale experiments. Interactions among the 'excitations', or 'defects', are taken into account using the one component system equivalent of the binary system 'regular sol ution' model (which keeps only the first order term of the free energy of m ixing expansion). We show that a liquid-liquid first order transition must occur at sufficiently strong defect-defect interactions. The highly overcon strained amorphous silicon quasilattice is a strong candidate for such a tr ansition. We identify the 'first order melting' of amorphous silicon, and t he sudden, reproducible, termination of supercooling in experimental liquid silicon and germanium, with the phase transition predicted by the model. M any more cases of this phase transition may be anticipated, and a correspon ding range of glasses with low residual entropies-approaching the 'perfect' glass state-are predicted.