Polyamorphism is critical to our fundamental understanding of amorphous solids and the corresponding elusive liquid-liquid transitions, an intriguing but controversial topic over the last decades. Traditional polyamorphous systems have open network structure at ambient pressure, which could evolve into more densely-packed structure under high pressure. Metallic glasses have close-packed atomic structure with non-directional metallic bonding. Therefore, it was very surprisingly to report pressure-induced polyamorphism in some Ce-based alloys (1, 2). The mechanism is attributed to the electronic transition of 4f electrons in the lanthanide solvent element. This novel phenomenon brings new insight into the mechanism of polyamorphism and metallic glasses themselves. Therefore, pressure-induced polyamorphism in metallic glasses soon becomes a research focus (3-5). However, whether polyamorphism only exists in metallic glasses with 4f electron elements as the solvent or not remains unclear.

In our works, by combining in-situ high-pressure x-ray diffraction (XRD), in-situ high-pressure Ce L₃-edge x-ray absorption spectroscopy (XAS), and molecular dynamics (MD) simulations, a transition from a low-density amorphous state to a high-density amorphous state is confirmed in a lanthanide-solute metallic glass, Al₉₃Ce₇, with an extremely low Ce concentration. The transition starts from ~0.8 GPa and completes at ~1.8 GPa with a volume collapse of ~0.78% from XRD results. The delocalization of 4f electrons in the dilute Ce atoms, which accounts for the volume collapse, is verified by the XAS experiment. The MD simulation results confirm that the Ce atoms are fully isolated by the Al solvent, and a volume collapse of ~0.78% is also reproduced by the simulation. Our results provide explicit evidence for the existence of polyamorphism in dilute 4f element-bearing metallic glasses, extending the compositional space from the solvent end to the very dilute solute end for the first time. This finding could provide new insights into the chemical effect on the polyamorphism in MGs and also highlights the critical role of “minor alloying” elements in affecting properties of MGs.
 

metallic glass,polyamorphic transition,high pressure