Local fivefold symmetry in liquid and undercooled Ni probed by x-ray absorption spectroscopy and computer simulations

Presence and significance of fivefold configurations in liquid metals are investigated by combining x-ray absorption spectroscopy and computer simulations (molecular dynamics and reverse Monte Carlo) in liquid and undercooled liquid nickel. We show that icosahedral short-range ordering (ISRO), probed by common-neighbor (CNA) and spherical invariant (${\widehat{W}}_6$) analysis, involves a limited fraction (14–18% in undercooled nickel for different structural models) of local atomic configurations. The emerging picture for the liquid structure is that of a mixture of nearly icosahedral structures embedded in a disordered network mainly composed of fragments of highly distorted icosahedra (40–45% of the total), structures reminiscent of the crystalline phase, and other configurations.

Figure 1

Left panel: X-ray absorption temperature scans of the Ni sample. The change of the absorption coefficient $\delta \alpha$ at $E^{*}=8.338$ keV (see XAS spectra in the inset) is plotted with respect to the c-Ni absorption level during repeated thermal cycles (see arrows). The hysteresis highlights an exceptional undercooling ($\approx$350 K) of the Ni droplets. Right panel: temperature dependence of the Ni nucleation rate from the cooling scan compared with the fitting model introduced in Ref. [5] within (solid curve) and outside (dashed curve) the previously accessed undercooling range.

Figure 2

Comparison of calculated (RMC simulation, green [gray]) and experimental EXAFS $\chi \left(k\right)$ signals for u-Ni (dots, red [gray]) and l-Ni (dots, blue [gray]) at 1493 K and 1733 K respectively. The flat residual confirms the quality of the refinement. The two experimental EXAFS signals are compared in the inset.

Figure 3

Normalized bond angle distributions $f\left(\theta \right)$ calculated from MD (dashed) and RMC configurations for u-Ni (blue [gray]) and l-Ni (red [gray]). Inset: first peak of the pair distribution function $g\left(r\right)$ obtained by RMC-XAS for l-Ni (red [gray]) and u-Ni (blue [gray]), compared with MD results (dashed) and previous XRD data [19] (dot-dashed).

Figure 4

Percentage of nearest-neighbors pairs with given symmetry (CNA analysis of MD and RMC configurations). ICOS, icosahedral (555 pairs); DICOS, distorted icosahedral (544 and 433 pairs); BCC, bcc (666 pairs); and FCC/HCP, fcc and hcp (421 and 422 pairs).

Figure 5

Normalized distribution of the spherical invariant ${\widehat{W}}_6$ calculated from MD and RMC configurations in u-Ni.