Effects of the 10 B/ 11 B isotopic substitution on shear relaxation in supercooled B 2 O 3 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 B 2 O 3 liquids using boron isotope substitutions. The molar volume decreases and T g and isothermal viscosity increase on the substitution of lighter 10 B isotopes with the heavier 11 B 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.