Polyanionic frameworks in the lithium phosphidogermanates Li${}_2$GeP${}_2$ and LiGe${}_3$P${}_3$ - synthesis, structure, and lithium ion mobility

Recently fast lithium ion conductors were discovered in compounds containing tetrahedral SiP${}_4$${}^{8-}$ and GeP${}_4$${}^{8-}$ units. In the context of material development for all solid state batteries the ternary Li/Ge/P phase system has been further investigated and two new lithium phosphidogermanates were discovered on the lithium poor side of the ternary composition diagram. Li${}_2$GeP${}_2$ crystallizes in space group I4${}_1$/acd with unit cell parameters of a = 12.3069(1) Å and c = 19.0306(4) Å, consists of a framework of Ge${}_4$P${}_{10}$ supratetrahedra, and exhibits an ionic conductivity of 1.5(3)×10${}^{-7}$ S·cm${}^{-1}$ at 27 °C. LiGe${}_3$P${}_3$ crystallizes in Pbam with a = 9.8459(5) Å, b = 15.7489(7) Å, and c = 3.5995(2) Å. In LiGe${}_3$P${}_3$ Ge and P atoms form a two dimensional polyanion. The slabs consist of five- and six-membered heteroatomic rings comprising GeP${}_4$ and Ge(P${}_3$Ge) tetrahedra including homoatomic Ge–Ge bonds. A semiconducting behavior with an electronic conductivity of ∼10${}^{\text{â}\text{€}\text{“}4}$ S·cm${}^{-1}$ and a remarkable stability vs. air and moisture is observed. (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)