Mechanical, electronic and optical properties of antifluorites semiconductors X 2 C (X = Mg, Be) S. Laref a,b, * , A. Laref b a Departement of Chemical Engineering, Universiteit van Amsterdam, Nieuwe Achtergracht 166, NL-1018 WV Amsterdam, The Netherlands b Computational Materials Science Laboratory, Physics Department, University of Sidi Bel Abbes, Sidi Bel Abbes 22000, Algeria article info Article history: Received 25 March 2008 Received in revised form 1 May 2008 Accepted 6 May 2008 Available online 7 July 2008 Keywords: Antifluorite Electronic and mechanical properties Ab initio calculations abstract First-principles calculations have been performed to investigate structural, mechanical, electronic, and optical properties of antifluorites Mg 2 C and Be 2 C in the framework of the full-potential linear aug- mented-plane wave (FPLAPW) method. The exchange–correlation effects are treated in the local-density approximation (LDA). We have evaluated the ground-state quantities such as equilibrium volume, bulk modulus and its pressure derivative as well as the elastic constants. The results of band structure and density of states, show a wide indirect band gap. In addition, we have computed the imaginary and real parts of dielectric function in order to extract the optical properties for these compounds. The results were in favorable agreement with previous theoretical works and the existing experimental data. In order to complete the fundamental characteristics of antifluorites compounds, we have analyzed their bonding character by plotting the charge densities. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Wide band-gap semiconductors have expanded the scope of de- vice applications beyond those of silicon and gallium arsenide. Exploitation of wide gap semiconductors holds promise for revolu- tionary improvement in the cost, size, weight and performance of a broad range of military and commercial microelectronic and opto- electronic systems. The inherent material properties of antifluor- ites have a small band gap semiconductors hence most of the interesting optical transition is make them ideal candidates operat- ing in the visible region of the spectrum and consequently very attractive for various industrial applications such as optical disc storage. Recently, many interesting studies have been made to shed light on the structural and electronic properties of antifluorites. The interest in solids formed from atoms with low atomic numbers has recently increased greatly [1], because of the potential unique properties of these materials. Some of these properties are associ- ated with the relatively strong chemical bonding characteristic of the first row atoms, others with the frequent occurrence of large band gap. The former leads to high melting points, which implies potential use as refractories and large elastic constants which in turn, are related to hardness, high sound velocities and good ther- mal conductivities. The band gaps could lead to potential interest- ing optical properties. Mg 2 C and Be 2 C, which forms in so-called antifluorite structure, has to date received relatively few attention. In part, this is due to the fact that these materials tend to hydrolyze to Be(OH) 2 and to react with O and N. This study of the mechanical and electronic structure of these materials is part of a larger theo- retical effort to explore the nature and properties of low Z materi- als. In contrast to the previous subjects of inquiry, which had relatively strongly covalent character, Mg 2 C and Be 2 C are appar- ently more strongly ionic. We also note that Be 2 C has been found to occur in the epitaxial growth of diamond on Be 2 O [2–3], and it is also of interest as a possible starting material for synthesis of BeCN 2 [4–6]. At present, there is limited empirical data on the properties of these materials in general, and, in the mechanical and optical prop- erties in particular. To our knowledge, there have been four previ- ous calculations of the electronic structure of Mg 2 C and Be 2 C. The first, by Herzig and Redinger [7], was a self-consistent augmented- plane wave (APW) calculation. That calculation was carried out at the empirical lattice constant and thus did not investigate the ‘‘energetic” properties of the materials. Also, this calculation em- ployed the muffin-tin approximation, the validity of which for these materials is open to question (Note that 28% of the valence charge is found to be inside the ‘‘interstitial” region). The second was a simple semiempirical tight-binding calculation that accom- panied the energy-loss measurements [8]. This approach is cer- tainly limited by the paucity of relevant empirical data, as well as, by the intrinsic limitations of such calculations. The third calcu- lation, by Corkill and Cohen [9], employed the first-principles 0927-0256/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2008.05.013 * Corresponding author. Address: Computational Materials Science Laboratory, Physics Department, University of Sidi Bel Abbes, Sidi Bel Abbes 22000, Algeria. E-mail address: Iaref_s@yahoo.fr (S. Laref). Computational Materials Science 44 (2008) 664–669 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci