The viscosity and the relaxation time of a glass-forming liquid vary over 15 orders of magnitude before the liquid freezes into a glass. The rate of the change with temperature is characterized by liquid fragility. The mechanism of such a spectacular behavior and the origin of fragility have long been discussed, but it remains unresolved because of the difficulty of carrying out experiments and constructing theories that bridge over a wide timescale from atomic (ps) to bulk (minutes). Through the x-ray diffraction measurement and molecular dynamics simulation for metallic liquids we suggest that large changes in viscosity can be caused by relatively small changes in the structural coherence which characterizes the medium-range order. Here the structural coherence does not imply that of atomic-scale structure, but it relates to the coarse-grained density fluctuations represented by the peaks in the pair-distribution function (PDF) beyond the nearest neighbors. The coherence length is related to fragility and increases with decreasing temperature, and it diverges only at a negative temperature. This analysis is compared with several current theories which predict a phase transition near the glass transition temperature.

在液体冻结成玻璃之前，玻璃形成液的粘度和松弛时间变化超过15个数量级。随温度变化的速率以液体脆性为特征。这种引人注目的行为的机制和易碎性的起源已被讨论了很长时间，但由于难以进行实验和构建从原子（ps）到体积（分钟）的宽时间尺度上进行桥接的理论，这一问题仍未得到解决。通过对金属液体进行X射线衍射测量和分子动力学模拟，我们认为，粘度的较大变化可能是由于结构相干性相对较小的变化所引起的，而结构相干性是中等范围内的特征。这里的结构一致性并不意味着原子级结构，但它与以对分布函数（PDF）中的峰值表示的粗粒度密度波动超出了最近的邻居。相干长度与易碎性有关，并且随着温度的降低而增加，并且仅在负温度​​下发散。将该分析结果与几种当前的理论进行了比较，这些理论预测了接近玻璃化转变温度的相变。