Superconducting High-Pressure Forms of LiSi and LiGe Featuring Square Planar Nets

High-pressure forms of LiX (X = Si or Ge) that adopt the simple tetragonal P4/mmm CuAu structure were synthesized by reacting stoichiometric Li$_{12}$Si$_7$/5Si mixtures and by transforming I41/a-LiGe (MgGa structure) at ∼12.5 GPa and 410 and 265 °C, respectively. P4/mmm-LiGe was recovered in quantitative yield as a metastable phase at ambient pressure, whereas P4/mmm-LiSi was partially converted into a hitherto unknown, kinetically more stable polymorph. The structures of the P4/mmm phases consist of alternately stacked square planar nets of X (d$_{Si–Si}$ = 2.595 Å, and dGe–Ge = 2.761 Å) and Li atoms. Density functional theory (DFT)-based electronic structure calculations reveal pronounced polarity, Li$^{0.83+}$Si$^{0.83–}$ and Li$^{0.84+}$Ge$^{0.84–}$, together with strong covalent bonding between X atoms. Electron–phonon coupling calculations within the Migdal–Eliashberg framework predict superconducting transition temperatures of ∼7 K for P4/mmm-LiGe and ∼6 K for P4/mmm-LiSi. For LiGe, magnetic susceptibility measurements show a sharp diamagnetic transition at 6.3 K, in support of the theoretical result.