Dynamics of a glass‐forming system: 11B NMR of B2O3

Effects of the 10B/11B isotopic substitution on shear relaxation in supercooled B2O3 liquid: A validation of the elastic model of viscous flow

The effects of atomic mass in terms of its zero-point vibrational energy, on molar volume, glass transition temperature T_g, and viscosity are studied in glassy and supercooled B₂O₃ liquids using boron isotope substitutions. The molar volume decreases and T_g and isothermal viscosity increase on the substitution of lighter ¹⁰B isotopes with the heavier ¹¹B isotopes. These effects are argued to be a manifestation of the higher zero-point vibrational energy of the lighter isotope, which along with the anharmonicity of the potential well, results in a longer equilibrium inter-atomic distance and larger mean-square displacement with respect to that for the heavier isotope. The isotope effect on viscosity is increasingly enhanced as the temperature approaches T_g, which is shown to be consistent with the prediction of the elastic models of viscous flow and shear relaxation.

Reorientational dynamics of trimethoxyboroxine: A molecular glass former studied by dielectric spectroscopy and 11B nuclear magnetic resonance

In this work, trimethoxyboroxine (TMB) is identified as a small-molecule glass former. In its viscous liquid as well as glassy states, static and dynamic properties of TMB are explored using various techniques. It is found that, on average, the structure of the condensed TMB molecules deviates from threefold symmetry so that TMB's electric dipole moment is nonzero, thus rendering broadband dielectric spectroscopy applicable. This method reveals the super-Arrhenius dynamics that characterizes TMB above its glass transition, which occurs at about 204 K. To extend the temperature range in which the molecular dynamics can be studied, ¹¹B nuclear magnetic resonance experiments are additionally carried out on rotating and stationary samples: Exploiting dynamic second-order shifts, spin-relaxation times, line shape effects, as well as stimulated-echo and two-dimensional exchange spectroscopy, a coherent picture regarding the dynamics of this glass former is gained.

Liquid B2O3 up to 1700 K: x-ray diffraction and boroxol ring dissolution

Using high energy x-ray diffraction, the structure factors of glassy and molten B2O3 were measured with high signal-to-noise, up to a temperature of T  =  1710(20) K. The observed systematic changes with T are shown to be consistent with the dissolution of hexagonal [B3O6] boroxol rings, which are abundant in the glass, whilst the high-T (>~1500 K) liquid can be more closely described as a random network structure based on [BO3] triangular building blocks. We therefore argue that diffraction data are in fact qualitatively sensitive to the presence of small rings, and support the existence of a continuous structural transition in molten B2O3, for which the temperature evolution of the 808 cm−1 Raman scattering band (boroxol breathing mode) has long stood as the most emphatic evidence. Our conclusions are supported by both first-principles and polarizable ion model molecular dynamics simulations which are capable of giving good account of the experimental data, so long as steps are taken to ensure a ring fraction similar to that expected from Raman spectroscopy. The mean thermal expansion of the B-O bond has been measured directly to be αBO  =  3.7(2)  ×  10−6 K−1, which accounts for a few percent of the bulk expansion just above the glass transition temperature, but accounts for greater than one third of the bulk expansion at temperatures in excess of 1673 K.

Universality of the dynamic crossover in glass-forming liquids: a "magic" relaxation time

Analysis of experimental data on the structural relaxation time tau(alpha) in various glass formers revealed its universality at the critical temperature T(c) of the mode-coupling theory. In most glass formers studied ln tau(alpha)(T(c))=-(6.5-7.5). Possible reasons for such a universality are discussed.