Vibrational properties of the filled tetrahedral compounds LiCdP and LiCdAs

We have studied the dynamical properties of the filled tetrahedral compounds LiCdP and LiCdAs and the binary analogs InP and InAs by employing the plane-wave pseudopotential method within density functional theory. The calculated lattice constants for these compounds are consistent with other local density approximation calculations and they reproduce the trend encountered in the experimental data. The phonon dispersion curves and density of states are calculated by using density functional perturbation theory. The high frequency optic modes in LiCdP and LiCdAs are assigned to Li-P and Li-As pairs, respectively. The splitting of the LO and TO branches at Γ point for each pair suggests that the Li-P and Li-As bonds have high ionicity. The force constants of Cd-P and Cd-As are about 56% and 59% higher than those of Li-P and Li-As, respectively, showing the relatively high covalency of the former bonds.