First-principles study of elastic and phonon properties of the heavy fermion compound CeMg

Elasticity tensor components, C_ij, the crystallographic dependence of Poisson's ratio, the phase stability, and vibrational spectra are computed for nonmagnetic and magnetic CeMg (1:1 Ce:Mg) structures using density functional theory. Results from both the generalized gradient approximation (GGA), and the GGA+U, based upon an effective on-site Coulomb potential, U _eff, are investigated. The GGA low energy structure, with wavevector along [110], disagrees with experiment, while the [100] structure from experiment is predicted as the 0K structure in the GGA+U. Accurate estimation of the 20K Néel temperature can only be achieved with small U _eff, which suggests that CeMg is not a strongly correlated system. For all CeMg structures investigated, we find C₁₁≈C₄₄; this is consistent with the near equivalency of transverse and longitudinal sound speeds. The origin of this behavior is the negative stretching force constants for the interaction between the second-and third-nearest-neighbor Mg and Ce ions, respectively. Results are compared with neutron scattering experiments at 30 and 110K.