We wish to determine with highest precision the structural properties of two skutterudite compounds (LaFe4Sb12 and YbFe4Sb12) at three different temperatures 2, 100 and 300 K by single-crystal diffraction at D9. Skutterudites (like clathrates, beta-pyrochlore and V2Al20 compounds) are representatives of crystals formed by a fully bonded and connected network of nano-meter sized polyhedra. A common feature of many of the nano-cage compounds is an anomaly in the elastic constants when cooled below 100K. It is believed that this anomaly is linked to a change in structural properties of the crystals. Specifically, it is discussed that cations occupying the polyhedra move from the centres of the polyhedra at high T to off-centre positions. We wish to examine this possibility.