Catenation of carbon in LaC sub(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 sub(2) at pressures of 0.0-30.0 GPa. Five stable structures of LaC sub(2) reveal a variety of carbon structural units ranging from a dimer to bent C sub(3), zigzag C sub(4) and armchair polymer chains. A series of pressure-induced structural transformations are predicted, I4/mmm(i.e.exper imental alpha phase) arrow right C2/c arrow right Pnma arrow right Pmma, which involve the catenation of carbon from a dimer to zigzag C sub(4) units and further to armchair polymer chains. The bent C sub(3) unit appears in a novel Immmstructure. This structure is the theoretical ground state of LaC sub(2) under ambient conditions, but is kinetically inaccessible from the experimental alpha phase. LaC sub(2) becomes thermodynamically metastable relative to La sub(2)C sub(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.