Planar Si₅ and Ge₅ Pentagons beside Isolated Phosphide Anions in Lithium Phosphide Tetrelides Li_10+xSi₅P and Li_10+xGe₅P

Abstract

Dedicated to Professor Caroline Röhr on the Occasion of her 60 th Birthday.Binary lithium silicides and germanides as well as ternary lithium phosphidotetrelates have been intensively investigated with regard to their suitability as anode materials for all-solidstate-batteries. In this context we studied the ternary phase diagram Li/Tt/P on the phosphorus-poor side. The phosphide tetrelides Li 10.68(8) Si 5 P and Li 10.1(2) Ge 5 P are ternary compounds in the respective phase systems so far uniquely featuring discrete building blocks, which were previously found exclusively in binary alkali metal tetrelides and phosphides. Li 10.68(8) Si 5 P and Li 10.1(2) Ge 5 P contain aromatic Tt 5 pentagons with an environment of mostly ordered Li atoms and isolated P 3À anions with a coordination environment of predominantly disordered Li atoms. The five-membered ring subunits are stacked under formation of 1 ∞ [Li 6 Tt 5 ] chains (Tt = Si, Ge). chains. The compounds crystallize in the space group Pnma (63) (Z = 4) with cell parameters of a = 10.2434(3) Å, b = 4.2788(1) Å, c = 23.9767(3) Å for Li 10.68(8) Si 5 P and a = 10.3599(11) Å, b = 4.3072(2) Å, c = 24.267(2) Å for Li 10.1(2) Ge 5 P. The single crystal structure determination is supplemented by X-ray powder diffraction experiments and 7 Li, 29 Si and 31 P solid state MAS NMR spectroscopy. With respect to their electron count the compounds are placed between compounds comprising aromatic five-membered rings such as the electron-precise Zintl phase Li 8 MgSi 6 and the nonclassical Zintl phase Li 12 Si 7 having two extra electrons.