  Citation: Ojovan, M.I.; Louzguine-Luzgin, D.V. On Structural Rearrangements during the Vitrification of Molten Copper. Materials 2022, 15, 1313. https:// doi.org/10.3390/ma15041313 Academic Editors: George Wardeh and Michele Bacciocchi Received: 26 December 2021 Accepted: 7 February 2022 Published: 10 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article On Structural Rearrangements during the Vitrification of Molten Copper Michael I. Ojovan 1,2, * and Dmitri V. Louzguine-Luzgin 3,4 1 Department of Materials, South Kensington Campus, Imperial College London, Exhibition Road, London SW7 2AZ, UK 2 Department of Radiochemistry, Faculty of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1, Bd. 3, 119991 Moscow, Russia 3 Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan; dml@wpi-aimr.tohoku.ac.jp 4 MathAM-OIL, National Institute of Advanced Industrial Science and Technology (AIST), Sendai 980-8577, Japan * Correspondence: m.ojovan@imperial.ac.uk Abstract: We utilise displacement analysis of Cu-atoms between the chemical bond-centred Voronoi polyhedrons to reveal structural changes at the glass transition. We confirm that the disordered congruent bond lattice of Cu loses its rigidity above the glass transition temperature (T g ) in line with Kantor–Webman theorem due to percolation via configurons (broken Cu-Cu chemical bonds). We reveal that the amorphous Cu has the T g = 794 ± 10 K at the cooling rate q = 1 × 10 13 K/s and that the determination of T g based on analysis of first sharp diffraction minimum (FDSM) is sharper compared with classical Wendt–Abraham empirical criterion. Keywords: amorphous copper; vitrification; glass transition; molecular dynamic simulation; Voronoi polyhedrons; congruent bond lattice; configuron; percolation; Hausdorff–Besicovitch dimensionality; set theory 1. Introduction Spatial distributions and displacement analysis through and between Voronoi poly- hedrons are widely used to reveal structural changes in various materials in wide ranges of temperature and composition utilising molecular dynamic (MD) simulations [1,2]. The Voronoi polyhedrons are constructed using tessellations (honeycombs)-atomic centred polyhedrons which are regular or semiregular polyhedrons in crystalline materials and irregular polyhedrons in amorphous materials which are characterised by topological disorder. MD simulations are particularly effective in detecting changes that occur on vitrification of melts or melting of glasses (glass transition). Pure metals having only one sort of atoms are attractive as they enable a simplified approach compared to multiatomic compounds or metallic alloys. There are several ways in which amorphous materials including metallic systems can be produced. These include enough rapid cooling of melts, physical vapor deposition, solid-state reactions, irradiation- and pressure-induced amorphization. Amorphous Cu was produced both by extra-rapid cooling and ultrahigh pressure 14 GPa [3,4]. Many researchers utilise MD simulations to investigate its behaviour and properties [5]. The most intriguing question is about structural changes at the glass transition which was analysed using MD simulations for Fe [6,7], Ni [8,9], Cu [10], Al [11] as well as for other metals [12]. We aim here to analyse the glass transition (vitrification) of Cu revealing the structural differences below and above the glass transition temperature (T g ) and utilising the pair distribution functions, mainly their first sharp diffraction minimum (FSDM) as a tool to identify the T g . Materials 2022, 15, 1313. https://doi.org/10.3390/ma15041313 https://www.mdpi.com/journal/materials