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

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.

Polyamorphism of a Ce-based bulk metallic glass by high-pressure and high-temperature density measurements

“…Secondly, the temperature of our study is relatively lower (by 50 degrees), say not as close to T g as in the Luo et al study. However, the present work does not clearly discriminate a possible weak first-order nature as argued in Ref [19]. The discontinuous volume change measured in our work between 4 and 6…”

“…Also using x-ray techniques, Duarte et al [15] reported a low-and high-density amorphous phases at different pressure ranges, with the occurrence of a hysteresis at the transition, and of an intermediate density region expected to coincide with a mixture of both phases, or with a new metastable phase. This last hypothesis has been recently reported by Luo et al [19], through an exploration of the LDA-HDA transformation path at the temperature of 390 K.…”

“…It suggests a transition related to a collapse of the main architecture of the first atomic shells, with shorter average atomic distance, apparent at high pressure. Note that this is a quite unusual behavior since in the majority of glassy oxides where polyamorphism has been demonstrated, the LDA-to-HDA transition goes along with a critical change of the mean distance order [11,19].…”

“…It must be emphasized that the iterative procedure to extract the density from the experimental signal did not converge in this pressure range. This could be due either to the presence of an intermediate local structure between the LDA and HDA phases [19], or to the coexistence between the two phases, as expected in a first-order phase transition.…”

“…GPa at 340 K, as it occurs for a first-order transformation, can actually be hidden by the coexistence of the low-and high-pressure phases between 4 and 6 GPa, or by the presence of an intermediate medium-density amorphous state [19].…”