Li5−xSr2Al1−xSixP4 and the Influence of Void Filling with Li in the Hcp Array of P Atoms on the Ionic Conductivity for X = 0 and 1 as well as Li2Sr2Al2P4

Lithium-rich ternary lithium phosphides have proven to be good model systems for ionic conductivity, achieving lithium-ion conductivities up to 4.5 mS/cm (ω-Li₉GaP₄). Compounds in which lithium is partially substituted by alkaline-earth metals bear the possibility of vacancy generation and thus the possibility to reach better ion conductivity and also to gain further insight into structure-property relationships. Based on the recent report on the quaternary compound Li₄Sr₂SiP₄, we investigated the substitution of Si by Al and found the new compounds Li₅Sr₂AlP₄ and Li₂Sr₂Al₂P₄. The examination of the solid solution Li_5−xSr₂Al_1−x Si_xP₄ for x = 0.0, 0.125, 0.25, 0.5, 0.75, 0.875, and 1.0 resulted in a series of crystalline materials that adopt two different structure types in dependency of x. Compounds crystallize either in space group P$$;m1 (No. 164) (Li₅Sr₂AlP₄, Li_5−xSr₂Al_1−xSi_xP₄ with x < 0.5) or in space group P2₁/m (No. 11) (Li₂Sr₂Al₂P₄, Li_5−xSr₂Al_1−xSi_xP₄ with x ≥ 0.5). Li₅Sr₂AlP₄ is structurally related to Li₄Sr₂SiP₄ but exhibits an additional statistical disorder of Al and Li atoms, which is also found in Li₂Sr₂Al₂P₄.