The Phosphidosilicates AE₂ Li₄ SiP₄ (AE=Ca, Sr, Eu) Ba₄ Li₁₆ Si₃ P₁₂

The quaternary phosphidosilicates AE₂ Li₄ SiP₄ (AE=Ca, Sr, Eu) and Ba₄ Li₁₆ Si₃ P₁₂ were synthesized by heating the elements and Li₃ P under argon atmosphere. Their crystal structures were determined by single crystal X-ray diffraction. AE₂ Li₄ SiP₄ crystallize in a new layered structure type (P2₁ /m, Z=2) with CdI₂ -analoguos layers. Edge sharing CaP₆ octahedra are separated by layers of vertex-sharing SiP₄ and LiP₄ tetrahedra, which contain additional chains of LiP₆ octahedra. Ba₄ Li₁₆ Si₃ P₁₂ forms likewise a new structure type (P2₁ /c, Z=16) with a three-dimensional network of SiP₄ , Si₂ P₆ and LiP₄ entities as well as one phosphorus site not bonded to silicon. Barium is located in capped trigonal prisms of phosphorus which form strongly corrugated layers. ³¹ P and ²⁹ Si solid-state NMR spectra confirm the crystal structures of the compounds AE₂ Li₄ SiP₄ . ⁷ Li spectra show only one signal in spite of quite different crystallographic positions, which indicate possible Li⁺ mobility. However, this signal is much broader compared to the known Li⁺ conducting phosphidosilicates. Accordingly, electrochemical impedance measurements show low Li⁺ conductivities.