  Due to a large discrepancy between theory and experiment, the electronic
character of crystalline boron carbide B$_{13}$C$_{2}$ has been a controversial
topic in the field of icosahedral boron-rich solids. We demonstrate that this
discrepancy is removed when configurational disorder is accurately considered
in the theoretical calculations. We find that while ordered ground state
B$_{13}$C$_{2}$ is metallic, configurationally disordered B$_{13}$C$_{2}$,
modeled with a superatom-special quasirandom structure method, goes through a
metal to non-metal transition as the degree of disorder is increased with
increasing temperature. Specifically, one of the chain-end carbon atoms in the
CBC chains substitutes a neighboring equatorial boron atom in a B$_{12}$
icosahedron bonded to it, giving rise to a B$_{11}$C$^{e}$(BBC) unit. The
atomic configuration of the substitutionally disordered B$_{13}$C$_{2}$ thus
tends to be dominated by a mixture between B$_{12}$(CBC) and
B$_{11}$C$^{e}$(BBC). Due to splitting of valence states in
B$_{11}$C$^{e}$(BBC), the electron deficiency in B$_{12}$(CBC) is gradually
compensated.
