Bond analysis of phosphorus skutterudites: Elongated lanthanum electron buildup in LaFe4P12

Electron transfer and bonding characteristics of a selected set of phosphorus skutterudites are investigated using density functional calculations. Electron transfer for CoP3, LaFe4P12 and the hypothetical FeP3 compounds are calculated using procrystals as the non-binding references. Spherical integration and Bader analysis are performed to quantify electron transfer differences between these compounds. The results are in good qualitative agreement with simple electronegativity considerations. The calculations confirm that the transition metal–phosphorus and the phosphorus–phosphorus bonds are covalent, while the lanthanum–phosphorus bond is of a more polar and complex nature. In addition, we describe a unique bonding scheme between lanthanum and phosphorus where elongated electron buildup along the crystal axes surround the lanthanum ion. This is identified as a likely reason for the special phonon-scattering properties of the lanthanum filler ion.