Solid phases of phosphorus carbide: An ab initio study

The thermodynamic stability and properties of different phosphorus carbide phases are studied as a function of composition with first-principles periodic density-functional theory calculations. Calculations are reported for P4C3, PC, and P3C4 and a range of possible structures examined for each. For P4C3, the favored structures are defect zinc-blende structures, while for PC GaSe-like and beta-InS-like structures are lowest in energy. For P3C4, the lowest-energy structure is a beta-C3N4-like structure, where phosphorus and carbon occupy sites occupied by carbon and nitrogen, respectively, in the nitride. The importance of the local environments of carbon and phosphorus is emphasized and trends in the preferred environments of C and P are analyzed as a function of composition. For carbon-rich structures, a bonding arrangement with four-coordinate hypervalent phosphorus and sp(2) carbon is preferred, while, for higher phosphorus content, structures containing three-coordinate phosphorus and sp(3) carbon are lower in energy. Comparisons are made with the molecular chemistries of carbon and phosphorus, and also with the corresponding nitrogen analogs. Energies of formation, bulk moduli, and band gaps are calculated for the predicted low-energy phases. The stoichiometry with the lowest formation energy (for x(P)>0.15) is PC.