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 accuratel y the distribution of the sodium atoms within the two available sites; In a greement 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 smalle r 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 stoi chiometric Na8Si46 clathrate, which is present as impurity in the studied s amples, has also been refined.