Lattice dynamics and LDA calculations for LaFe/sub 4/Sb/sub 12/

We perform first principles calculations of the total energy and Hellmann Feynman forces generated by displacements of the La atom as well as by other atoms of the filled skutterudite LaFe/sub 4/Sb/sub 12/ within the LAPW method. Assuming short ranged interatomic forces we thereby evaluate the force constant matrix between La and Fe and between La and Sb. The strongest interactions are central bond stretching and that force constant is positive for the La-Sb interaction and negative for the La-Fe interaction. The potential seen by a La atom in the fixed environment of Sb and Fe atoms is found to be quite harmonic for energies even corresponding to T somewhat greater than room temperature, and the quartic term is positive and hence constraining. By incorporating the LDA information into a lattice dynamical model for CoSb/sub 3/ the two peak structure discovered in an analysis of inelastic neutron scattering data is explained. The specific heat is also calculated and found to disagree with previous experimental results, but those results have recently been superseded by other experimental results that agree with our calculations.