Polymorphic phase transitions are common in crystalline solids. Recent studies suggest that phase transitions may also exist between two liquid forms with different entropy and structure. Such a liquid-liquid transition has been investigated in various substances including water, Al 2 O 3 -Y 2 O 3 and network glass formers. However, the nature of liquid-liquid transition is debated due to experimental difficulties in avoiding crystallization and/or measuring at high temperatures/pressures. Here we report the thermodynamic and structural evidence of a temperature-induced weak first-order liquid-liquid transition in a bulk metallic glassforming system Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 characterized by non-(or weak) directional bonds. Our experimental results suggest that the local structural changes during the transition induce the drastic viscosity changes without a detectable density anomaly. These changes are correlated with a heat capacity maximum in the liquid. Our findings support the hypothesis that the 'strong' kinetics (low fragility) of a liquid may arise from an underlying lambda transition above its glass transition.

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Using high energy synchrotron x-ray radiation combined with electrostatic levitation, in situ structural analysis of a bulk metallic glass forming liquid is performed from above the liquidus temperature down to the glass transition. The data indicate a liquid-liquid transition (LLT) in the deeply undercooled state at T /T g ∼ 1.2 which manifests as a maximum in the heat capacity and an abrupt shift in the first peak position of the total structure factor in the absence of a pronounced density change. Analysis of the corresponding real-space data shows that the LLT involves changes in short-and medium-range order. The structural changes on the length scale of medium-range order imply a fragile-strong transition in agreement with experimental viscosity data.