The phase diagram Li₄GeO₄–LiAlGeO₄ has been shown to contain a short range of Li₄GeO₄ solid solutions, a range of γ solid solutions, Li_{4 – 3x}Al_xGeO₄[where (0.04–0.13)<x<0.30] and a line phase, α, with x= 0.375. The γ solid solutions are stable up to melting at 1140–1160°C and the α phase is stable between 1080 and 1160°C only. X-Ray powder data are given for α and γ solid solutions.

The γ solid solutions exhibit a remarkable temperature-independent conductivity of Li⁺ ions above ca. 700 °C which is particularly noticeable at low values of x. With decreasing temperature, Li⁺ ion motion is increasingly thermally activated, with an activation energy of 0.45–0.60 eV at temperatures below 300°C. The highest room-temperature conductivity was ca. 2 × 10^–5Ω^–1 cm^–1 in freshly quenched γ samples of x= 0.10, but the conductivity gradually decreased with time owing to ageing. There is evidence from differential scanning calorimetry (DSC) studies of a sluggish order–disorder transition at 200–250°C involving the Li⁺ ion site occupancies, which has many of the characteristics of a glass transition. An additional second-order transition occurs at 480–500°C which is attributed to a change from disordered to liquid-like properties of the mobile Li⁺ ions.