RAPID COMMUNICATIONS PHYSICAL REVIEW B 89, 060102(R) (2014) Local fivefold symmetry in liquid and undercooled Ni probed by x-ray absorption spectroscopy and computer simulations A. Di Cicco, 1 F. Iesari, 1 S. De Panfilis, 2 M. Celino, 3 S. Giusepponi, 3 and A. Filipponi 4 1 Physics Division, School of Science and Technology, Universit` a di Camerino, I-62032 Camerino (MC), Italy 2 Istituto Italiano di Tecnologia, Centre for Life Nanoscience–IIT@Sapienza,Viale Regina Elena 291, I-00161, Roma, Italy 3 ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, C.R. Casaccia, Via Anguillarese 301, I-00123 Roma, Italy 4 Dipartimento di Scienze Fisiche e Chimiche, Universit` a degli Studi dell’Aquila, I-67100 L’Aquila, Italy (Received 6 November 2013; revised manuscript received 17 January 2014; published 12 February 2014) 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 ( ˆ 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. DOI: 10.1103/PhysRevB.89.060102 PACS number(s): 61.25.Mv, 61.20.Ja The nature of local point symmetry in simple monoatomic liquids has been a fundamental open question for almost 40 years of computational and experimental studies, following Frank’s initial hypothesis [1] about the presence of icosahedral short-range ordering (ISRO) in liquids. The presence of such close-packed fivefold configurations, incompatible with trans- lational symmetry but favored by energetic considerations, was considered as a possible explanation for the peculiar undercooling properties of liquid metals studied by Turnbull in the early fifties [2]. Crystal nucleation [3] is hampered by the positive liquid-crystal interfacial energy, resulting in the possibility of bringing a liquid sample into a metastable (undercooled) molten state below the melting temperature T m . Experiments based on repeated temperature cycles on droplet specimens are commonly used to infer nucleation rate [4] and information on nucleation barriers [5]. The debate about the presence and amount of ISRO in undercooled liquids is still open, in part because of the experimental difficulties in accessing deeply undercooled states and the limited structural information supplied by x-ray diffraction experiments. In recent decades, several computational and experimental studies were devoted to investigating locally preferred struc- tures in simple atomic liquids (see, for example, Refs. [6–10]), and most of these works support the existence and importance of ISRO. X-ray absorption spectroscopy (XAS), which is suitable to investigate liquid matter [11] and sensitive to higher order distribution functions and local geometry, was previously exploited for undercooled Pd [12] and Cu [13]. For undercooled copper, it was shown that the fraction of nearly icosahedral configurations is around 10%, evidence later supported by molecular dynamics (MD) simulations [14,15]. In the case of liquid and undercooled nickel, an element very close to copper in its structural properties, MD simulations [16,17] and diffraction experiments [18,19] indicated that a large fraction of atoms show local icosahedral symmetry. However, the detailed structural analysis reported in Ref. [19], shows that the fraction of nearly icosahedral configurations is similar to that found in liquid Cu [13]. Stimulated by these recent results, we have performed an extensive investigation on liquid and undercooled nickel, combining state-of-the-art experimental and computational techniques, for the purpose of evaluating the amount of ISRO and establishing reliable criteria for assessing the nature of local geometries in close-packed liquids. In this work, the local order probed by XAS is analyzed using a reverse Monte Carlo (RMC) [20,21] approach, yielding structure models compatible with the measurements. Realistic models for the liquid structures are also obtained using advanced MD simulations. Reliable information about the presence and amount of ISRO in MD and RMC models is extracted using a suitable geometrical analysis. The XAS experiment was performed at the BM29 beamline [22] of the European Synchrotron Radiation Facility (ESRF, Grenoble). Samples for high-temperature measurements were prepared from submicrometric Ni and alumina (Al 2 O 3 ) powder mixtures with a 1:20 mass ratio that were suspended in alcohol, filtered, and pressed into 100-μm-thick, 13-mm-diameter pel- lets. The furnace consisted of a 130-mm-diameter cylindrical Pyrex glass vessel with a suitable window for simultaneous x-ray diffraction (XRD) and XAS data collection. The pellets were placed inside the crucible of the furnace and the heat treatments were performed under high-vacuum conditions of P ≃ 10 −5 mbar. Similar to the Pd case [12], alumina was not found to react with nickel at the temperatures involved in the experiment. Temperature measurements were performed using a high-temperature pyrometer (estimated uncertainty is 20 K). The temperature behavior of the sample and its phase transformations were monitored throughout the experiment by x-ray absorption temperature scans [22] and x-ray diffraction measurements, collected with an area detector. Temperature scans were performed while monitoring the x-ray absorption at the energy E ∗ = 8.338 keV on the rising part of the Ni K edge chosen to maximize the absorption contrast between solid and liquid Ni. A typical temperature scan is shown in Fig. 1, left panel. The hysteresis cycle through the solid-liquid phase transition is a proof of the deep undercooling of ≈350 K 1098-0121/2014/89(6)/060102(4) 060102-1 ©2014 American Physical Society