Structural stability and phase transition in OsC and RuC.

The structural stability and phase transition of osmium and ruthenium carbides (OsC and RuC) were investigated by first principles. Nine structures were considered for each carbide. Zinc blende structure has the lowest energy among the considered structures at ambient conditions for both carbides. For OsC at elevated pressures, the most stable phase is zinc blende structure from 0 to 10 GPa, FeSi from 10 to 32 GPa. In these two structures, Os atom is fourfold coordinated. From 32 to 40 GPa, tungsten carbide (WC) and NiAs are energetically competitive with Os atom sixfold coordinated. NiAs becomes energetically the most stable structure above 40 GPa. For RuC, zinc blende structure is the most stable from 0 to 20 GPa. From 20 to 100 GPa, WC structure is the most stable.