In the liquid state, glass-forming ${\mathrm{Ni}}_{59.5}{\mathrm{Nb}}_{40.5}$ and ${\mathrm{Ni}}_{60}{\mathrm{Nb}}_{34.8}{\mathrm{Sn}}_{5.2}$ alloys exhibit an extraordinarily high packing fraction. The self-correlation functions measured using quasielastic neutron scattering clearly show the slowing down of microscopic dynamics with an increase in packing fraction. The self-diffusivity in liquid ${\mathrm{Ni}}_{60}{\mathrm{Nb}}_{34.8}{\mathrm{Sn}}_{5.2}$ decreases by about 2 orders of magnitude within a temperature range of 360 K. For these highly fragile systems, the critical packing fraction obtained form the analysis of incoherent data is in excellent agreement with the prediction made by mode-coupling theory. Our results provide the first experimentally observed value for the critical packing fraction in glass-forming metallic liquids.

• Received 23 August 2007

DOI:https://doi.org/10.1103/PhysRevLett.101.037801