Edouard Reny, Pierre Gravereau, Christian Cros and Michel Pouchard
The crystal structure of the non-stoichiometric NaxSi136 silicon clathrate has been refined using the Rietveld method, in order to determine accurately the distribution of the sodium atoms within the two available sites. In agreement with the previous data, it was found that for x≤8, the alkali atoms occupy exclusively, and not only preferentially the eight larger Si28 sites. For 8<x<24, the filling of the sixteen smaller Si20 cages occurs gradually with increasing x, and a slight increase of the unit cell parameter is then observed. The crystal structure of the stoichiometric Na8Si46 clathrate, which is present as impurity in the studied samples, has also been refined.