Supertetrahedral Networks and Lithium-Ion Mobility in Li2 SiP2 and LiSi2 P3.

The new phosphidosilicates Li2 SiP2 and LiSi2 P3 were synthesized by heating the elements at 1123 K and characterized by single-crystal X-ray diffraction. Li2 SiP2 (I41 /acd, Z=32, a=12.111(1) Å, c=18.658(2) Å) contains two interpenetrating diamond-like tetrahedral networks consisting of corner-sharing T2 supertetrahedra [(SiP4/2 )4 ]. Sphalerite-like interpenetrating networks of uniquely bridged T4 and T5 supertetrahedra make up the complex structure of LiSi2 P3 (I41 /a, Z=100, a=18.4757(3) Å, c=35.0982(6) Å). The lithium ions are located in the open spaces between the supertetrahedra and coordinated by four to six phosphorus atoms. Temperature-dependent 7 Li solid-state MAS NMR spectroscopic data indicate high mobility of the Li+ ions with low activation energies of 0.10 eV in Li2 SiP2 and 0.07 eV in LiSi2 P3 .