Catenation of carbon in LaC 2 predicted under high pressure

Carbon has the capability of forming various bonding states that affect the structures and properties of transition metal carbides. In this work, structural search was performed to explore the structural diversity of LaC 2 at pressures of 0.0–30.0 GPa. Five stable structures of LaC 2 reveal a variety of carbon structural units ranging from a dimer to bent C 3 , zigzag C 4 and armchair polymer chains. A series of pressure-induced structural transformations are predicted, I 4/ mmm ( i.e. experimental α phase) → C 2/ c → Pnma → Pmma , which involve the catenation of carbon from a dimer to zigzag C 4 units and further to armchair polymer chains. The bent C 3 unit appears in a novel Immm structure. This structure is the theoretical ground state of LaC 2 under ambient conditions, but is kinetically inaccessible from the experimental α phase. LaC 2 becomes thermodynamically metastable relative to La 2 C 3 + diamond above 17.1 GPa, and eventually decomposes into constituent elements above 35.6 GPa. The presented results indicate that catenation of carbon can be realized even in simple inorganic compounds under nonambient conditions.