Firstprinciples study of configurational disorder in B4C using a superatomspecial quasirandom structure method
Abstract
Configurationally disordered crystalline boron carbide, with the composition B_{4}C, is studied using firstprinciples calculations. We investigate both dilute and high concentrations of carbonboron substitutional defects. For the latter purpose, we suggest a superatom's picture of the complex structure and combine it with a special quasirandom structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified lowenergy defects: (1) bipolar defects and (2) rotation of icosahedral carbon among the three polarup 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 an increase in temperature using a meanfield approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polarup 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 groundstate configuration and restore the rhombohedral system (R3m). The restoration of inversion symmetry yielding the full rhombohedral symmetry (R3̄m ) on average, corresponding to what is reported in the literature, is achieved after the second transition. Investigating the effects of high pressure on the configurational stability of the disordered B4C phases reveals a tendency to stabilize the ordered groundstate configuration as the configurationally ordering/disordering transition temperature increases with pressure exerted on B4C. The electronic density of states, obtained from the disordered phases, indicates a sensitivity of the band gap to the degree of configurational disorder in B4C.
 Publication:

Physical Review B
 Pub Date:
 July 2014
 DOI:
 10.1103/PhysRevB.90.024204
 arXiv:
 arXiv:1508.07786
 Bibcode:
 2014PhRvB..90b4204E
 Keywords:

 64.60.Cn;
 64.60.De;
 81.30.Hd;
 61.50.Ks;
 Orderdisorder transformations;
 statistical mechanics of model systems;
 Statistical mechanics of model systems;
 Constantcomposition solidsolid phase transformations: polymorphic massive and orderdisorder;
 Crystallographic aspects of phase transformations;
 pressure effects;
 Condensed Matter  Materials Science
 EPrint:
 Phys. Rev. B 90, 024204 (2014)