Synthesis, Crystal structure, electronic structure, and Raman spectra of Li₄Sr₂SiP₄

Recently, ternary lithium phosphides have been studied intensively owing to their high lithium ion conductivities. During investigations aiming for derivatives with less lithium content the new compound Li₄Sr₂SiP₄ was obtained. It crystallizes in the monoclinic space group P2₁/m (N°11) with lattice parameters of a=7.2919(8) Å, b=8.3679(6) Å, c=6.9965(8) Å, and β=90.061(9)°. The crystal structure contains isolated [SiP₄]⁸⁻ tetrahedra that are charge balanced by two Sr²⁺ and four Li⁺ counter ions. The structure can be derived as a superstructure of the layered CaAl₂Si₂ structure type comprising a hexagonal closest packing of P atoms and tetrahedral voids filled with Si and Li atoms, whereas octahedral voids are filled with Sr and Li atoms. DFT calculations confirm the structure with ordered atom positions as a global minimum and predict an indirect band gap of 2.23 eV. In addition, the experimental Raman spectrum is in good agreement with the calculated one.