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

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-solid-state-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)Si₅P and Li_10.1(2)Ge₅P 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)Si₅P and Li_10.1(2)Ge₅P contain aromatic Tt₅ pentagons with an environment of mostly ordered Li atoms and isolated P³⁻ anions with a coordination environment of predominantly disordered Li atoms. The five-membered ring subunits are stacked under formation of (Formula presented.) [Li₆Tt₅] 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)Si₅P and a=10.3599(11) Å, b=4.3072(2) Å, c=24.267(2) Å for Li_10.1(2)Ge₅P. The single crystal structure determination is supplemented by X-ray powder diffraction experiments and ⁷Li, ²⁹Si and ³¹P 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 Li₈MgSi₆ and the non-classical Zintl phase Li₁₂Si₇ having two extra electrons.