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 [(SiP_4/2 )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 .