Charge density in disordered boron carbide:B 12 C 3 . An experimental and ab-initio study.
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V7.4.1
Charge density in disordered boron carbide:B12C3. An experimental and ab-initio study. Gianguido Baldinozzi, Michaël Dutheil, David Simeone1 and Andreas Leithe-Jasper2 SPMS, CNRS Ecole Centrale Paris, F 92295 Châtenay-Malabry, France 1 LM2E, CEA, CE Saclay, F 91191 Gif-sur-Yvette, France 2 NIRIM, Namiki 1-1, Tsukuba Ibaraki 305-0044, Japan ABSTRACT A charge density study of boron carbide B12C3 single crystals at different temperatures allows a quantitative description of the electron density responsible for chemical bonding. These results, based on direct observations, are compared to previous models of bonding. This description points out that structural features are more complex than suggested by simple qualitative models. The effects of chemical substitution, the characteristics of the interatomic bonding, the charge transfer and the bond strength are discussed and compared to the information obtained from previous conventional refinements and ab-initio calculations in disordered B12C3.
INTRODUCTION Among the members of the boron carbide family the most interesting and perhaps more studied compound is B12C3, which possesses remarkable properties: the melting temperature is very high and the mechanical properties (hard as corundum, stronger than steel, lighter than aluminium) are outstanding [1]. These properties are related to the unusual bonds occurring between boron and carbon atoms [2]. The average structure of B12C3 is trigonal R-3m. This structure consists of a molecular-like sub-unit based on 12 atoms forming an icosahedron sitting at the origin of the primitive trigonal cell and a linear chain extending along the threefold axis (Fig. 1).
Figure 1. Structure of boron carbide. The existence of a threefold axis and of an inversion centre at the origin requires two independent atoms, B(2) and B(3), to describe the icosahedra. It is therefore possible to distinguish between 6 equatorial sites B(2) (forming bonds with the linear chain) and 6 polar sites B(3) (forming bonds with the neighbouring icosahedra). The actual location of carbon substitution for boron is still an open issue though several clues [3-16] suggest a C-B-C chain and a C atom somewhere on an icosahedral site. Our aim is to elucidate these bonding features through an experimental study of the charge density on a single crystal of B12C3. All the previous structural determinations of B12C3 were performed on powders or polycrystalline samples. Here,
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V7.4.2
we present various structural refinements on a single crystal using different models of increasing complexity and discussing the validity of the approximations. We start from a standard structure refinement using spherical atomic factors, then we discuss the static and dynamic disorder in the structure before refining the structure within the
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