Structure of the Al 2 Cu(001) and Al 9 Co 2 (001) surfaces: role of the covalent-like bonding network and off-stoichiome
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Structure of the Al2Cu(001) and Al9Co2(001) surfaces: role of the covalent-like bonding network and off-stoichiometric effects Émilie Gaudry1, Sebastián Alarcón Villaseca1, Julian Ledieu1, Jean-Marie Dubois1 and Vincent Fournée1 1 Institut Jean Lamour, UMR 7198 CNRS-Université de Lorraine, Parc de Saurupt, 54011 Nancy, France. ABSTRACT The Al2Cu and Al9Co2 intermetallic compounds share structural similarities: they are both described in terms of coordination polyedra with a tetragonal symmetry and covalent-like bonding occur in both compounds. In this paper, the (001) surface structure of Al2Cu and Al9Co2 is described based on a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) and density functional theory (DFT) study. Surface models are elaborated from stoichiometric ideal compounds, leading to a pure aluminium plane as the surface termination in both cases. The nature of the constitutional defect in Al9Co2 is determined using density functional theory calculations. The influence of the surface atomic density, of the surface composition and of off-stoichiometric effects on the (001) surface structures of Al2Cu and Al9Co2 are discussed. INTRODUCTION The nature of the bonding that links the atoms in a solid determines its physical properties. Metallic solids present a high density of delocalized electrons, resulting in the metallic bonding responsible for the well known physical properties of metals (conduction, ductility, low melting points...). Although complex metallic alloys (CMAs) consist only of metal elements, covalent-like bondings was identified in many intermetallic compounds by quantum chemical calculations. Within the stable Al-rich intermetallic phases of the Al-Co phase diagram, this mainly concerns the Al13Co4 [1,2], Al5Co2 [3] and Al9Co2 [4] intermetallic compounds, while the Al2Cu compound is relevant within the Al-rich part of the Al-Cu system [5]. The above intermetallic compounds present unusual physical properties compared to the corresponding pure elements or disordered solid solutions, which may be attributed partly to the existence of bonds with a covalent character in the structure. Two examples are given in the following. First, the microhardness of the considered intermetallic compounds is much higher than the one of the pure metallic elements: within the Al-Co system, the Vickers microhardness of Al13Co4 (743 kg.mm-2), Al5Co2 (927 kg.mm-2 ) and Al9Co2 (644 kg.mm-2) is much higher than the one of pure aluminium (23.5 kg.mm-2) or α-Co(Al) solid solution (206 kg.mm-2) [6]. Similarly, the Vickers Hardness of Al2Cu is quite high (550 HV, load 0.5 N) [7]. Second, the electrical resistivity is generally much higher than the one of the pure metals: it falls n the range 69-180 μΩ.cm for Al13Co4 [8] and 2.82 μΩ.cm for pure aluminium [9]. This is linked to the reduced density of states at the Fermi energy observed for the considered intermetallic compounds, which is generally interpreted by a combined
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