Constraint theory and roken bond bending constraints in oxide glasses.

摘要

The molecular structure of sodium tellurate glasses was established using {dollar}sp{lcub}125{rcub}{dollar}Te absorption and {dollar}sp{lcub}129{rcub}{dollar}I emission Mossbauer spectroscopies, differential scanning calorimetry (DSC), molar volume measurements and powder x-ray diffraction (XRD). The local atomic arrangement in these glasses is found to be different from that in corresponding crystals. This picture does not follow the usual thinking (Ioffe-Regel rule) about glass structure. The experimental evidence for this conclusion derives not only from Mossbauer spectroscopy but also from time-temperature-transformation curve and powder XRD measurements used to examine the crystallization of the bulk glasses. The TTT-curve exhibits both nucleation and growth branches, while XRD scans reveal growth of metastable phases before forming the stable crystalline phases. These results are in harmony with {dollar}sp{lcub}23{rcub}{dollar}Na solid state NMR results that reveal that sodium local environment in a x = 0.20 glass differs qualitatively from that of the crystalline counterpart.; Results from DCS and XRD measurements reveal that at x = 0.18 several observables such as, dT{dollar}rmsb{lcub}g{rcub}{dollar}/dx, activation energy for enthalpy relaxation, molar volume and Lamb-Mossbauer f factor, each display a threshold behavior. We believe that the physical origin of this threshold behavior comes from the rigidity percolation threshold.; The constraint theory has recently been extended to include one-fold coordinated species and broken bond bending ({dollar}beta{dollar}) constraints. The latter was developed and has been applied successfully to many glass systems including the oxides, as we did for the first time in our Science paper, but also to chalcogenides and chalcohalides, etc.. In the experiments, the observed threshold apparently shifts to the over-constrained regime, i.e. {dollar}langle{dollar}r{dollar}rangle{dollar} {dollar}>{dollar} 2.4 in many glass systems. This shift is largely due to broken {dollar}beta{dollar}-constraint at some two-fold coordinated atoms, e.g. Se/S in chain segments and oxygen atoms. An example is g-Ge{dollar}rmsb{lcub}x{rcub}Sesb{lcub}1-x{rcub}{dollar} where one can understand the rigidity percolation threshold shift from x = 0.20 to x = 0.23, if one assumes a fraction of 20% chalcogen atoms have their bond angle constraints broken. A simple interpretation is that these chalcogen atoms (with broken bond bending constraints) represent short floppy chain-segments connecting the more rigid tetrahedral Ge(Se{dollar}sb{lcub}1/2{rcub}{dollar}){dollar}sb4{dollar} units. Thus the concept of broken bond bending constraints plays an important role in promoting glass forming tendency of materials.; The extended constraint theory has also found application in aspect of mechanical property of hydrogenated diamond like carbon, silicon carbide and silicon thin films. We have established for the first time a linear relationship between measured hardness and hardness index, a geometric parameter derived from constraint theory. The slopes of such linear functions for different type materials are determined by chemical effect that reflect bonding type and interaction strength among atoms.