Topological origin of glass formation, rigidity and stress transitions, conductivity and fragility in specially homogeneous Heavy Metal Oxide and Chalcogenide systems

We examine the physical properties of (B2O3) 5(TeO2)95-x(V2O5)x , a Heavy Metal Oxide (HMO) glass and the GexS100-x binary glass using calorimetric (MDSC), optical (FT and Dispersive Raman), volumetric (Molar Volume) and Electrical conductivity measurements. Calorimetric data provides direct evidence of square-well like reversibility windows (RWs) in the 23.5% < x < 26.5% range for the HMO and in the 19.3% < x < 24.9% range for Ge-S glass systems. The space filing nature of networks formed in these windows, the existence of optical or electrical conductivity thresholds near the walls of these windows confirm that RW glasses represent rigid but unstressed networks representing Intermediate Phases in respective glass systems, identified in the present work for the first time.;The microscopic origin of glass formation in HMOs is for the first time shown to be topological (could be designed from the basics of Rigidity Theory), a result facilitated by the present experimental results and previous NMR studies. These data afford an atomic scale structure based understanding of the functionality of these glasses. The three regimes of behavior in the electrical conductivity measurements underscore the polaronic nature of conduction in these glasses. Variations in melt-fragility as a function of composition show square-well like minima that coincides with the RW. The finding has permitted one to understand that slow melt-homogenization can be traced to hindered diffusion at high temperature. For these reasons, special care was exercised in synthesis of the bulk glasses to homogenize them.;In Ge-S binary, we observe a volumetric window, a fragility window and Raman optical thresholds that coincide with rigidity (xc(1) = 19.3%) and stress (xc(2) = 24.9%) transitions defining the walls of the RW. Melt fragility index (m) variation with composition display a broad minimum (m<20) in the RW, with a global minimum near x = 25% of m = 15.2. The super-strong character of melts near x = 25%, causes the binary Ge-S melts to undergo slow homogenization for which direct evidence is obtained by Raman profiling of melts. Aging related compaction of molar volumes along with segregation of S8 rings in S-rich glasses is elucidated directly from Raman scattering and molar volume measurements. Glass molecular structure evolution with Ge content, in the 0 < x < 33.3% range, shows evidence of the onset of two elastic- and chemical- phase transitions, in harmony with an ordered bond network model but not stochastic network. Raman scattering data reveals the concentration of ES/CS tetrahedra = 0.43(5) for the stoichiometric GeS 2 glass to be in excellent agreement with two previous neutron scattering results. However, the ES/CS concentration ratios for the non-stoichiometric glasses deduced from present Raman scattering differ from a recent neutron scattering measurement. These differences are traced to the challenges in synthesizing homogeneous non-stoichiometric Ge-S glasses. Comparison of the present Sulfide with corresponding Selenide glasses reveal broad similarities in many structure related properties including ES/CS tetrahedral fraction, RWs and the Ordered Bond Network (OBN) evolution with topology despite differences in chalcogen atom (S, Se) sizes.