We first consider the α-Li 3 N and β-Li 3 N polymorphs within the harmonic approximation, to evaluate their relative thermodyamic stability as a function of temperature. Within the harmonic approximation, we calculate the free energy as a function of temperature of both αand β-Li 3 N using Equations 2 and 3. This analysis predicts that the βphase is more stable than the α-phase in the range 0 K to 300 K, with α-Li 3 N becoming the more stable polymorph above 300 K (Figure ). This phase-transition temperature is broadly consistent with previous neutron powder diffraction data , which showed a steady increase in proportion of α-Li 3 N with respect to β-Li 3 N in a α/β mixed phase sample with increasing temperature above 473 K, with only a small fraction, 2.5 wt%, of the β-phase detected at 673 K. Because β-Li 3 N is predicted to be thermodynamically favoured only below 300 K, which is well below the superionic transition temperature T s ≈ 678 K, we conclude that β-Li 3 N is not involved in the superionic transition behaviour of Li 3 N. We therefore solely focus on α-Li 3 N in the remainder of this study.