Search for the Ground State of C 60 B 10
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ABSTRACT Recent experimental discovery of C6 0BI0 has motivated the present search for the possible atomic structures of this system. We have already studied the electronic properties of C5 sBN within the density functional theory, and are interested in the effects of other substitutions of carbon atoms by boron and nitrogen in fullerenes. In our all-electron calculation, we adopt the mixed-basis approach in which Is and 2p orbitals in addition to about 3000 plane waves are included. The total energy and the band gap of many possible configurations are calculated in order to predict the most stable structure of this system. The density of states is presented as well for the ground state configuration.
INTRODUCTION The discovery of fullerenes as fundamentally new stable structures of carbon clusters(') has stimulated much interest in the existence of similar type of clusters constructed with different species. For example, by laser vaporization supersonic cluster beam studies, Guo et al.P2 ) suggested that several carbon atoms in C 60 were replaced by boron atoms, forming C 60 -nBn • Miyamoto et al(3) and Esfarjani et al.(4 ) calculated the electronic structures of hypothetical fcc C5 9B and C 58 BN , respectively. In the first case it was found that there is an acceptor level due to the boron atom in the band gap of C5 9 B which has an open shell structure. In the second case, for N and B far from each other, a donor as well as an acceptor level were found in the gap, whereas for N nearest neighbor to B, very similar properties to C6 0 were discovered (i.e. same band gap with no donor nor acceptor levels). The structure was closed-shell and therefore stable. Recently, we were informed(5) of the synthesis of a new doped fullerene, namely C60 B10 for which no structural data was available. We have therefore performed electronic structure calculations for several possible structures to find the most stable one. Going from one structure to another requires breaking many bonds: typically to form an isomer of C6 0 with two neighboring pentagons, one needs to overcome an energy barrier of about 7 eV( 6). The energy required to break only one bond is at least 3 eV and that corresponds to a temperature of 33,000 K! This temperature being much higher than the experimental temperature at which these molecules are formed, there can not be an activated transition from one structure to another of lower energy. Therefore, both structures can a priori be formed and would be thermodynamically stable; however their kinetic stability would depend on other parameters such as the band gap. Since the energy barrier between these structures is very high, a molecular dynamics simulation would not be able to predict systematically the lowest energy structure. Accordingly, one can find many metastable states, i.e. local minima in each class of considered structures (i.e. the minimum would depend on the starting point, and would have a similar geometric structure to it.) In this study, we concentrate on the search of local or global minima
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