The Ca 3d state is a necessary and sufficient condition to produce an interlayer band in CaC6 according to model and first-principles studies, and this band cannot be considered to be a free-electron band. Involvement of the Ca 3d state in the chemical bonding and its tight-binding character explain the unusual Ca isotope effect in CaC6. The picture shows the orbital topology of one of the Fermi states of CaC6.
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Deng et al. (2008) studied this question.
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