Structural and thermodynamic similarities of phases in the Li–Tt (Tt = Si, Ge) systems: Redetermination of the lithium-rich side of the Li–Ge phase diagram and crystal structures of Li₁₇Si_4.0−xGe_x for x = 2.3, 3.1, 3.5, and 4 as well as Li_4.1Ge

A reinvestigation of the lithium-rich section of the Li–Ge phase diagram reveals the existence of two new phases, Li₁₇Ge₄ and Li_4.10Ge (Li_16.38Ge₄). Their structures are determined by X-ray diffraction experiments of large single crystals obtained from equilibrated melts with compositions Li₉₅Ge₅ and Li₈₅Ge₁₅. Excess melt is subsequently removed through isothermal centrifugation at 400 °C and 530 °C, respectively. Li₁₇Ge₄ crystallizes in the space group F4 3m (a = 18.8521(3) Å, V = 6700.1(2) ų, Z = 20, T = 298 K) and Li_4.10Ge (Li_16.38Ge₄) in Cmcm (a = 4.5511(2) Å, b = 22.0862(7) Å, c = 13.2751(4) Å, V = 1334.37(8) ų, Z = 16, T = 123 K). Both phases are isotypic with their Si counterparts and are further representative of the Li₁₇Pb₄ and Li_4.11Si structure types. Additionally, the solid solutions Li₁₇Si_4−xGe_x follows Vegard's law. A comparison of the GeLi_n coordination polyhedra shows that isolated Ge atoms are 13- and 14-coordinated in Li₁₇Ge₄, whereas in Li_16.38Ge₄ the Ge atoms possess coordination numbers 12 and 13. Regarding the thermodynamic stability, Li_16.38Ge₄ is assigned a high-temperature phase existing between ∼400 °C and 627 °C, whereas Li₁₇Ge₄ decomposes peritectically at 520–522 °C. Additionally, the decomposition of Li_16.38Ge₄ below ∼400 °C was found to be very sluggish. These findings are manifested by differential scanning calorimetry, long-term annealing experiments and the results from melt equilibration experiments. Interestingly, the thermodynamic properties of the lithium-rich tetrelides Li₁₇Tt₄ and Li_4.1Tt (Li_16.4Tt₄) are very similar (Tt = Si, Ge). Besides Li₁₅Tt₄, Li₁₄Tt₆, Li₁₂Tt₇, and LiTt, the title compounds are further examples of isotypic tetrelides in the systems Li–Tt.