High pressure synthesis and properties of intercalated silicon clathrates

In this paper, we review the synthesis of intercalated silicon clathrates and their physical properties. More particularly, their high pressure–high temperature synthesis and their superconductivity properties are detailed. Si-clathrates are cage-structures allowing for the ultra-degenerated intercalation of donor or acceptor atoms. Their physical properties are both governed by the sp3 tetrahedral bonding and by the guest–host interactions. The endohedral intercalation gives rise to the observation of a number of unique behaviors like their extreme high pressure stability as well as their unique type of isostructural phase transformations. Three types of Si-clathrates exist corresponding to different stoichiometries: type-I M8Si46 (with M=Na, K, Rb Ba, Sr, Ca or I), type-II MxSi136 with M=Na or Cs and type-III Ba24Si100 (sp2/sp3 system).The Si-clathrates are phonon-mediated BCS superconductors. Only type-I and type-III are superconducting with critical temperatures ranging from Tc=1.55 K (Ba24Si100) to 8 K (Ba8Si46). In the type-I, superconductivity is an intrinsic property of the sp3 silicon network. When Ba is replaced by Sr or Ca, the electronic density of states at the Fermi level decreases, involving the reduction of Tc. For Ba24Si100, the importance of the mixed sp2/sp3 character of the Si-bonds in the explanation of its lower Tc is still under debate.