Liquid-Liquid Phase Transition in Metallic Alloy Melts Detected by Nuclear Magnetic Resonance*

Liquid-liquid phase transition (LLPT) is thought to be exotic as it is not obvious what order parameters could be associated with qualitative changes in liquid structures. In some special cases where the valency of the element and bonding characters change substantially with pressure and temperature, such as the case of phosphorus and cerium, LLPT could take place accompanied by clear changes of the density. Here we show that non-density-driven LLPT can actually be common in metallic alloys, taking place above the liquidus temperature T_liq. High-temperature NMR is shown to be an effective tool for revealing such LLPT facilitated by its high sensitivity and high resolution. Subtle changes are detected that are difficult to detect by conventional thermodynamic measurement techniques. We will discuss the observation of LLPT in La-based alloys and Al-based alloys. We will show that the corresponding density change is exceedingly small. In addition, the kinetics of the transition process is also characterized systematically by NMR. This shows that the LLPT should have a profound influence on alloy solidifications.