A comparison of DFT-calculated and experimental mean-squared displacements, elastic constants, and phonons in skutterudites

We discuss interatomic forces for four skutterudites that have received attention in the literature, LaFe4Sb12, BaCo4Sb12, FeSb3 and CoSb3. We employ the PAW-VASP density functional implementation and a least square procedure to derive quadratic, cubic and quartic Taylor expansion coefficients from LDA- and GGA-PBE-computed atomic forces generated by displacements of single atoms within both conventional and 2X2X2 conventional cells of the skutterudite structure. We compute--and compare with available experiment and theory--second order elastic constants, phonon frequencies, and the mean square atomic displacement tensor as a function of temperature, We are not aware of any published calculations of the full mean square displacement tensor for any skutterudite. A surprising result is that a filler-filler force constant, namely the interaction between atoms on cube-corners, can have a non-negligible (few percent) effect in the phonon dispersion curves: This occurs for LaFe4Sb12 but not for BaCo4Sb12, as the corresponding Ba-Ba interaction is negligibly small.