Does Osmium Carbide Exist? Ab initio Investigation
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Does Osmium Carbide Exist? Ab initio Investigation M. Zemzemi1, M. Hebbache1, D. Zivkovic2, and L. Stuparevic2 1 Matériaux et Phénomènes Quantiques, Université Paris 7, CNRS UMR 7162 2 place Jussieu, Paris, Cedex 05, F-75251, France 2 Department of Metallurgy, University of Belgrade, VJ12, Bor, Serbia, 19210, Yugoslavia Abstract Osmium carbide has been synthesized by Kempter and Nadler forty six years ago. According to the authors, OsC crystallizes in WC-type structure and has a hardness equal to 2000 kg mm-2. Up to date, almost nothing is known about the physical properties of this carbide. We studied electronic, elastic, plastic and mechanical properties of this carbide using an approach based on the density-functional theory. We found that the work of the above mentioned authors is sound. The calculated lattice parameters are in good agreement with that given by Kempter and Nadler and a rough estimate also showed that the measured hardness is reasonable. However, we found that the hexagonal structure of osmium carbide is unstable.
1 Introduction Carbon forms binary compounds with almost all the transition metals. The most stable carbides are obtained with transition metals of groups IV, V and VI which have very high melting points [1]. Transition metal carbides possess various other exceptional properties which makes them very interesting for technical applications. Important properties are their great hardness, wear resistance, good thermal shock resistance, thermal conductivity and weak oxidation.
It is believed that the late transition metals of the second and third row of group VIII, namely the platinum group (Os, Ir, Pt, Ru, Re, Rh), do not form solid carbide phases, nor nitride phases. Nevertheless, nitrides of three noble metals belonging to the platinum group, namely platinum, iridium and osmium have been synthesized recently[2][3][4]. The pressure-temperature stability range of platinum metal carbides should also be investigated. Platinum metals are usually used as the major constituents in cemented carbide cutting tools. The alloying of cemented carbides with these noble metals increase significantly their hardness, compressive and yield strengths. Actually, osmium carbide has been synthesized by Kempter and Nadler forty six years ago [5]. They also measured its hardness. According to them, OsC crystallizes in the tungsten carbide structure with a microhardness equal to 2000 kg mm-2. Almost nothing is known about this carbide.
The aim of the present work is to investigate the structural stability and the physical properties of OsC, in particular its hardness. Our computation was carried out within the density functional theory [6]. We used the full-potential linearized augmented-plane-wave (FP-LAPW) method as implemented in WIENk2k package [7]. The generalized gradient approximation for the exchange-correlation energy (GGA), as parameterized by Perdew et al. [8], has been employed. The 5s, 6p and 4f states of osmium were treated as semi-core states and the 1s state of carbon as a core s
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