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

Elasticity tensor components, Cij, 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, Ueff, are investigated. The GGA low energy structure, with wavevector along [110], disagrees with experiment, while the [100] structure from experiment is predicted as the 0 K structure in the GGA+U . Accurate estimation of the 20 K Néel temperature can only be achieved with small Ueff, 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 110 K.