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

Abstract

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.