Hierarchical densification and negative thermal expansion in Ce-based metallic glass under high pressure.

Qiang Luo,Yue Zhang,Jin Jiao,Norbert Mattern,Zhi Wang,Gaston Garbarino,Jürgen Eckert,Pengshan Li,Baoan Sun,Xiaodong Li,Dawei Fan,Yajuan Sun,Jun Shen

doi:10.1038/ncomms6703

Abstract

The polyamorphsim in amorphous materials is one of the most fascinating topics in condensed matter physics. In amorphous metals, the nature of polyamorphic transformation is poorly understood. Here we investigate the structural evolution of a Ce-based metallic glass (MG) with pressure at room temperature (RT) and near the glass transition temperature by synchrotron X-ray diffraction, uncovering novel behaviours. The MG shows hierarchical densification processes at both temperatures, arising from the hierarchy of interatomic interactions. In contrast with a continuous and smooth process for the low- to medium-density amorphous state transformation at RT, a relatively abrupt and discontinuous transformation around 5.5 GPa is observed at 390 K, suggesting a possible weak first-order nature. Furthermore, both positive and abnormal-negative thermal expansion behaviours on medium-range order are observed in different pressure windows, which could be related to the low-energy vibrational motions and relaxation of the weakly linked solute-centred clusters.

Highlights

The polyamorphsim in amorphous materials is one of the most fascinating topics in condensed matter physics
Little is known about how the medium-range order (MRO), short-range order (SRO) and free-volume zones react to external pressure, intensive research has been carried out on the effects of MRO, SRO and free-volume zones on the mechanical and relaxation behaviours in various MGs15–17
The Ce65Co25Al10 metallic glass (MG) shows abnormal-negative thermal expansion (NTE) under a pressure range of 0.6–9.0 GPa, which could arise from a combination of the relaxation and tilting or rocking motions of the tightly bonded polyhedral clusters linking to weakest atom zones

Summary

Introduction

The polyamorphsim in amorphous materials is one of the most fascinating topics in condensed matter physics. In contrast with a continuous and smooth process for the low- to medium-density amorphous state transformation at RT, a relatively abrupt and discontinuous transformation around 5.5 GPa is observed at 390 K, suggesting a possible weak first-order nature Both positive and abnormal-negative thermal expansion behaviours on medium-range order are observed in different pressure windows, which could be related to the low-energy vibrational motions and relaxation of the weakly linked solute-centred clusters. Since the work of Sheng et al.[6], it has been realized that polyamorphism is possible in metallic glasses (MGs) despite their non-directional nature of metallic bonds and atom dense-packing characteristic It was proposed, from ab initio calculation and the genetic isostructural g–a transition of Ce metal, that the mechanism was due to the pressure-induced 4f electron delocalization[6,7,8]. Pressure induced bond shorting and local atomic rearrangement, which are assisted significantly from the 4f electron localization, play the dominate role on the polyamorphic transformations and the abnormal thermal expansion

Methods

Results

Conclusion