Structural characterisations of the NaxSi136 and Na8Si46 silicon clathrates using the Rietveld method

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 less than or equal to 8, the alkali atoms occupy exclusively, and not only preferentially the eight larger Si-28 sites. For 8 <x < 24, the filling of the sixteen smaller Si-20 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.