Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci Symmetry transformation in Pd quasicrystals upon heating and hydrogenation Vladimir Dubinko a, ⁎ , Denys Laptiev b , Dmitry Terentyev c , Sergey Dmitriev d,e , Klee Irwin f a NSC Kharkov Institute of Physics and Technology, Kharkov 61108, Ukraine b B. Verkin Institute for Low Temperature Physics and Engineering, Kharkiv 61103, Ukraine c SCK•CEN, Nuclear Materials Science Institute, Boeretang 200, Mol 2400, Belgium d Institute for Metals Superplasticity Problems RAS, Khalturin St. 39, Ufa 450001, Russia e National Research Tomsk State University, Tomsk 634050, Russia f Quantum Gravity Research, Los Angeles, USA ARTICLE INFO Keywords: Quasicrystals Symmetry transformation Icosahedral structure Palladium ABSTRACT In this work, the structural transformation from a crystalline to quasicrystalline symmetry in palladium (Pd) and palladium-hydrogen (Pd-H) atomic clusters upon thermal annealing and hydrogenation has been addressed by means of atomistic simulations. A structural analysis of the clusters was performed during the heating up to the melting point to identify the temperature for the phase transformation. It has been demonstrated that nano- metric pure Pd clusters transform from cuboctahedral to icosahedral structures under heating. This transfor- mation is thermally activated process and the activation barrier depends on the cluster size. The activation energy of the cubo-ico symmetry transformation was measured using the variable heating rate method and was found to increase with the cluster size from 0.05 eV for 55 atomic cluster up to 0.66 eV for 147 atomic cluster. Hydrogenation of the nanometric Pd clusters yields to the modification of the transformation barrier in a non- monotonic form. At low H concentration, the transformation barrier decreases, while by increasing H con- centration above a certain threshold, the barrier grows again thus making a minimum around a specific hy- drogen concentration. This behaviour was rationalized as a competition between two processes, namely: the structure symmetry breaking at low H concentrations and stabilization of cuboctahedral phase of the clusters at high H concentration. The obtained results provide an estimation of the temperature range at which the sym- metry transformation should occur under thermal annealing with experimentally achievable heating rates. 1. Introduction With the development of modern computational tools, there is a rapid increase of atomistic simulations of nanoscale materials due to their emerging in numerous applications in science and innovative technologies. The properties of nanoclusters with sizes up to few nan- ometers and smaller are somewhat peculiar as they partially resemble the properties of isolated atoms and partly the properties of bulk solids. In addition, multi-component nanoclusters may exhibit an extra degree of freedom related to its stoichiometry and local chemical arrangement. As a consequence, the practical interest in studying nanoclusters is driven by the possibility to engineer their physical-chemical properties up to specific needs by adjusting a nanoscale dimension, certain che- mical composition and crystallographic structure [1–3]. Palladium hydrides (Pd-H) and in particular Pd nano-films have attracted a lot of attention due to their unusual structural, thermodynamic and mechanical properties [4]. In particular, the in- teraction of hydrogen with palladium has technological applications in catalysis such as in the synthesis of ammonia or olefin hydrogenation [5]. Catalysis is a particularly important field in nanoscale materials science, because the large surface/volume ratio of nanocrystals often enhances the reactivity compared to bulk samples. In this respect, it is interesting to investigate dynamics of symmetry and phase transfor- mation in Pd nanocrystals upon thermal annealing and hydrogenation, which involves collective rearrangement of atoms that can mediate heterogeneous catalysis in the cluster or at its surface. The first study of hydrogen absorption by Pd brings us back to 1866 [6]. Over the last two decades many more experiments were performed to provide an insight to the nanoscale size-dependent phenomena oc- curring in Pd-H system [7–10]. These studies have revealed significant thermodynamic deviations from the bulk behavior of Pd-H system, potentially attributable to a high surface-to-volume ratio, modified https://doi.org/10.1016/j.commatsci.2020.109582 Received 17 November 2019; Received in revised form 30 January 2020; Accepted 31 January 2020 ⁎ Corresponding author. E-mail address: vdubinko@hotmail.com (V. Dubinko). Computational Materials Science 177 (2020) 109582 0927-0256/ © 2020 Elsevier B.V. All rights reserved. T