Pressure-induced polyamorphism in a main-group metallic glass

The mechanism of pressure-induced amorphous-to-amorphous transitions (AATs) in metallic glasses (MGs) has been a subject of intense research. Most AATs in MGs were found in lanthanide-based alloys and shown to originate from $4f$ orbital delocalization. Recently, evidence of an unexpected AAT in the main-group Ca-Al MGs was reported without a satisfactory explanation. Here, based on the results of first-principles molecular dynamics calculations, the suggested AAT at 12–15 GPa in the $\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$ MG is confirmed. Contrary to the common belief that the coordinate of metallic glasses with close packing cannot be increased further, the coordination around Al atoms is found to increase suddenly at the transition as a consequence of atomic migration and the aggregation of Al atoms. This transition originates from pressure-enhanced bonding between Ca $3d$ and Al $3p$ orbitals and is confirmed by the good agreement on the predicted and measured electrical conductivities. The theoretical analysis not only uncovers a mechanism of pressure-induced AAT in main-group MGs, but it can be generalized to establish a different perspective to guide the understanding of transformation phenomena in compressed MGs.

Figure 1

(a) Equation of states of $a\text{−}\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$. The inset shows the comparison of $\lambda /\left(2sin\theta \right)$ between experiment and theoretical results. (b) Partial PCFs of $a\text{−}\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$ under pressure. The inset shows Al-Al partial PCFs at selected pressures. The red star shows the development of a broad shoulder at $\sim 3.1\AA$ between 15 and 18 GPa.

Figure 2

Coordination number and Bader analysis. (a) Total and partial coordination number of $a\text{−}\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$ under pressure. The subscript “C” represents the central atom. (b) Selected Ca- and Al-centered cluster before and after the AAT. The white spheres are the central atoms labeled separately. Note the intrusion of a second Al into the first coordination sphere of Al after AAT. (c) Cutoff radius-dependent integrated coordination number of $\mathrm{A}{\mathrm{l}}_{\mathrm{C}}\text{-Al}$ at different pressures. (d) Bader atomic volume and charge of $a\text{−}\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$.

Figure 3

PDOS of $a\text{−}\mathrm{C}{\mathrm{a}}_{72.}\mathrm{A}{\mathrm{l}}_{27.3}$ at different pressures. The Fermi level is set to 0.0 eV.

Figure 4

Pressure-dependent relative electrical resistivity of $a\text{−}\mathrm{C}{\mathrm{a}}_{72.7}\mathrm{A}{\mathrm{l}}_{27.3}$. The red and black circles represent experimental data and theoretical results, respectively. The inset shows the relative reciprocal number of states at the Fermi level as a function of pressure.