Polyanionic Frameworks in the Lithium Phosphidogermanates Li2GeP2 and LiGe3P3 – Synthesis, Structure, and Lithium Ion Mobility

Recently fast lithium ion conductors were discovered in compounds containing tetrahedral SiP48- and GeP48- 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. Li2GeP2 crystallizes in space group I4(1)/acd with unit cell parameters of a = 12.3069(1) angstrom and c = 19.0306(4) angstrom, consists of a framework of Ge4P10 supratetrahedra, and exhibits an ionic conductivity of 1.5(3)x10(-7) S center dot cm(-1) at 27 degrees C. LiGe3P3 crystallizes in Pbam with a = 9.8459(5) angstrom, b = 15.7489(7) angstrom, and c = 3.5995(2) angstrom. In LiGe3P3 Ge and P atoms form a two dimensional polyanion. The slabs consist of five- and six-membered heteroatomic rings comprising GeP4 and Ge(P3Ge) tetrahedra including homoatomic Ge-Ge bonds. A semiconducting behavior with an electronic conductivity of similar to 10(-4) S center dot cm(-1) and a remarkable stability vs. air and moisture is observed.