Configurationally disordered crystalline boron carbide, B_4C, is studied using first-principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatom's picture of the complex structure and combine it with a special quasi-random structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: 1) Bipolar defects and 2) Rotation

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... f icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon increasing temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites, meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground state configuration and restore the rhombohedral system (R3m). The restoration of inversion symmetry yielding the full rhombohedral symmetry (R-3m) on average, corresponding to what is reported in the literature, is achieved after the second transition. The electronic density of states, obtained from the disordered phases indicates a sensitivity of band gap to the degree of configurational disorder in B_4C.