Previous X-ray data for α-Li 3 N indicate that this phase exhibits a superionic transition at T s ≈ 678 K . Below this temperature, the thermal mean-squareddisplacements of the Li ions and the ionic conductivity increase both linearly, suggesting Li 3 N exhibits a type-II superionic transition involving only the α-phase. Based on analysis of X-ray diffraction data, Schulz and Thiemann have proposed that the superionic transition is driven by the increasing amplitude of strongly-directional anharmonic vibrations between Li sites in the Li 2 N plane, and that these same anharmonic vibrations promote diffusion in the superionic phase .