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

Abstract

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 (W6 ) 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

The structure of liquid metals probed by XAS

“…The peak fitting approach, however, can be adopted as a preliminary and effective tool to investigate the information content in a signal, and this analysis is presented here for the case of undercooled liquid Ni at T u = 1493 K [14]. The experimental spectrum is slightly weaker and shifted with respect to the one of the f.c.c.…”

“…All the above signals can be calculated for a reference configuration and the effect of atomic vibrations accounted for by a suitable complex damping term. The fit of the solid Ni kχ(k) spectrum, measured at T = 1493 K [14], including all pair contributions. The relatively strong triplet signal from equilateral triangles can be appreciated.…”

“…The presence of a signature from close packed equilateral triangular configurations has been previously emphasized in the Cu case [13]. In this paper Ni K-edge XAS spectra at T = 1493 K in the solid and undercooled liquid phases [14] are analyzed with a peak fitting approach.…”

“…Even if this environment is not as clean as in the case of levitation techniques, a deep undercooling can be usually achieved because of the small droplet size, in particular when the supporting framework does not act as a strong heterogeneous nucleant. Our contribution to the field includes a number of structural investigations on undercooled liquid elemental metals with high melting points such as Pd [11], Rh [12], Cu [13], Ni [14] at high temperature and low melting point metals such as Sn [15] and Ga [16] at high pressure.…”

“…Recent Ni data [14] were further investigated to assess the possible occurrence of homogeneous nucleation [19] with the support of kinetic Monte Carlo simulations [20].…”