Bond analysis of cobalt and iron based skutterudites: elongated   lanthanum bonds in LaFe4P12
  Motivated by the possibility of further improving the thermoelectric properties of skutterudites we investigate charge transfer and bonding in this class of materials using density functional calculations. Results for the CoP3, CoSb3, LaFe4P12 and the hypothetical FeP3 compounds are presented using the procrystal as the non-binding reference. Spherical integration and Bader analysis are performed to illustrate charge transfer differences between these compounds. The results are in good qualitative agreement with simple electronegativity considerations. The calculations confirm that the transition metal-pnictogen and the pnictogen-pnictogen bonds are covalent, while the filler atom-pnictogen bond is of a more polar and complex nature. The success of the "rattling" cage as phonon inhibitor is explained by a unique semi-correlated bonding scheme between lanthanum and phosphorus. Elongated bonds along the crystal axes surrounds the lanthanum ion and generate a dodecahedra grid. Vibrations along the crystal axes are then closely connected to and scatter from the phosphorus rings. In the other directions, a more uncorrelated vibration is possible. This duality widens the phonon dampening possibilities. 
Espen Flage-Larsen, Ole Martin Loevvik, Oeystein Prytz and Johan   Taftoe