On Dynamic and Elastic Stability of Lanthanum Carbide

The elastic stability up to 100 GPa and dynamic stability up to ∼ 150 GPa of LaC have been investigated in NaCl (B1 phase) type and CsCl (B2 phase) structures, by first-principles calculations using the plane-wave pseudopotential method. The analysis of elastic moduli shows that the shear elastic modulus C' (=(C11-C12)/2) of B1 phase is negative up to ∼47 GPa, suggesting that the B1 phase is elastic unstable up to this pressure. The theoretically calculated phonon frequencies in various directions of the Brillouin zone are imaginary even up to 150 GPa, indicative of dynamic instability of B1 phase in this compound up to 150 GPa. These analyses in B2 phase of LaC suggest that this phase will also remain unstable elastically and dynamically up to 150 GPa.