Structure and Conductivity in LISICON Analogues within the Li₄GeO₄-Li₂MoO₄ System

New solid electrolytes are crucial for the development of all-solid-state lithium batteries with advantages in safety and energy densities over current liquid electrolyte systems. While some of the best solid-state Li⁺-ion conductors are based on sulfides, their air sensitivity makes them less commercially attractive, and attention is refocusing on air-stable oxide-based systems. Among these, the LISICON-structured systems, such as Li₂₊₂Zn_1-GeO₄ and Li₃₊V_1-GeO₄, have been relatively well studied. However, other systems such as the Li₄GeO₄-Li₂MoO₄ system, which also show LISICON-type structures, have been relatively little explored. In this work, the Li_4-2Ge_1-MoO₄ solid solution is investigated systematically, including the solid solution limit, structural stability, local structure, and the corresponding electrical behavior. It is found that a γ-LISICON structured solution is formed in the range of 0.1 ≤ x < 0.4, differing in structure from the two end members, Li₄GeO₄ and Li₂MoO₄. With increasing Mo content, the β-phase becomes increasingly more stable than the γ-phase, and at x = 0.5, a pure β-phase (β-Li₃Ge_0.5Mo_0.5O₄) is readily isolated. The structure of this previously unknown compound is presented, along with details of the defect structure of Li_3.6Ge_0.8Mo_0.2O₄ (x = 0.2) based on neutron diffraction data. Two basic types of defects are identified in Li_3.6Ge_0.8Mo_0.2O₄ involving interstitial Li⁺-ions in octahedral sites, with evidence for these coming together to form larger defect clusters. The x = 0.2 composition shows the highest conductivity of the series, with values of 1.11 × 10^-7 S cm^-1 at room temperature rising to 5.02 × 10^-3 S cm^-1 at 250 °C.