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

Abstract

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 I41/acd with unit cell parameters of a = 12.3069(1) Å and c = 19.0306(4) Å, consists of a framework of Ge4P10 supratetrahedra, and exhibits an ionic conductivity of 1.5(3)×10–7 S·cm–1 at 27 °C. LiGe3P3 crystallizes in Pbam with a = 9.8459(5) Å, b = 15.7489(7) Å, and c = 3.5995(2) Å. 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 ∼10–4 S·cm–1 and a remarkable stability vs. air and moisture is observed.

Aliovalent substitution in phosphide‐based materials – Crystal structures of Na10AlTaP6 and Na3GaP2 featuring edge‐sharing EP4 tetrahedra (E=Al/Ta and Ga)

“…The (Al 0.5 Ta 0.5 )P 4 tetrahedra in Na 10 AlTaP 6 are slightly distorted with a narrow P3À (Ta/Al)À P3 angle of 101.468 (7)°due to the repulsion between the highly charged central atoms; the remaining angles range from 108.037(8)°to 114.167 (8)°and are closer to the ideal tetrahedron angle. The (Al/Ta)À P bond lengths range from 2.4055(2) to 2.4410(2) Å and are between TaÀ P distances in compounds with isolated TaP 4 tetrahedra like Na 7 TaP 4 , [41] Na 5 SrTaP 4 (2.3856-2.4108 Å), [42] Na 5.15 Eu 0.85 TaP 4 (2.378(5)-2.399(2) Å) [43] or K 5 BaTaP 4 (2.3980(1)-2.4115(1) Å), [44] and AlÀ P distances in compounds with isolated AlP 4 tetrahedra like Li 9 AlP 4 (2.425(1) and 2.433(1) Å).…”

“…Illustrative examples of this manifold structural variety are Li 10 Si 2 P 6 , which features edgesharing SiP 4 double tetrahedra, [6] Li 2 SiP 2 /Li 2 GeP 2 and LiSi 2 P 3 , [7][8][9] where SiP 4 and GeP 4 tetrahedra are condensed to a network of super-tetrahedra, Li 3 Si 3 P 7 with double layers formed by vertexsharing SiP 4 tetrahedra, [6] and LiGe 3 P 3 with a two-dimensional polyanion comprising GeP 4 and Ge(P 3 Ge) tetrahedra. [8] A formally aliovalent substitution of the tetrel by a triel element leads to the lithium phosphidotrielates Li 3 AlP 2 and Li 3 GaP 2 which contain two-dimensional layers of corner-and edge-sharing tetrahedra, [4,10] whereas the corresponding Li 3 InP 2 shows a polyanionic framework of corner-sharing InP 4 supertetrahedra. [11] While lithium-based phosphides have already been studied intensively, much less is reported about the corresponding sodium species.…”

Planar Si5 and Ge5 Pentagons beside Isolated Phosphide Anions in Lithium Phosphide Tetrelides Li10+xSi5P and Li10+xGe5P

“…In other ternary Li/Ge/P compounds with tetrahedral units, crystallographically different Li atoms are not distinguishable in the Li NMR spectra. [30][31]33,[37][38][39]45] Compared to the upfield signal with a half width of only 281 Hz the much wider constituents of the larger signal show half widths of 1627, 634 and 861 Hz from downfield to upfield, respectively, indicating a larger disorder in these sites. However, since distinct signals can be observed, an exchange of Li atoms between the different sites seems to be unlikely or slow.…”

“…They exhibit low electronic conductivities, but Li-ionic conductivities of up to 10 À 3 S/ cm. [30][31][32][33][37][38][39] These differences can be explained by different structural motifs occurring in these compounds. While the herein reported compounds form isolated, homoatomic anions as they occur in binary Li-rich silicides, the previously reported compounds of these element combinations form almost exclusively heteroatomic phosphidotetrelate units with covalent SiÀ P bonds.…”

Synthesis, Crystal and Electronic Structure of La2SiP4

“…Metal tetrel‐pnictides ( A‐TtPn , A =electropositive metal, Tt =Si, Ge and Sn, Pn =P and As) are a large family of compounds showing a variety of crystal structures and atom connectivity, and exhibiting exciting transport, magnetic, and optical properties [1–9] . The underlying Tt‐Pn anionic sublattice can range from isolated TtPn 4 tetrahedra (Ca 4 SiP 4 , Sr 4 SiP 4 , Ba 4 SiP 4 , Li 8 SiP 4 and Li 8 GeP 4 ), 1D‐chains (Ca 3 Si 2 P 4 and K 2 SiP 2 ), 2D‐layers (LiGe 3 P 3 , LiGe 3 As 3 , Cs 0.16 SiAs 2 , Ca 2 Si 2 P 4 , Li 1‐x Sn 2 As 2 ), to 3D‐frameworks (Li 2 GeP 2 , SrSi 7 P 10 , BaSi 7 P 10 , Li 2 SiP 2 , LiSi 3 As 6 , MgSiAs 2 , Mg 3 Si 6 As 8 , and Ca 3 Si 8 P 14 ) [2,7,10–24] . The tetrel‐pnictide tetrahedral unit can be connected to its neighbors via sharing vertices and edges, or by P−P bonds [1,13,25–27] …”