Mobile M -ions occupy the tetrahedral voids within this cubic structure, which belongs to the space group I43m, as illustrated in Fig. . They typically undergo a transition from a normal to a superionic phase, specifically from a tetragonal to a cubic structure. Interestingly, the cubic phase remains stable at RT for compounds like M 3 PO 4 (M = Li, Na, and K). The conductivity of M 3 PO 4 materials is on the order of 10 -3 S cm -1 at 873 K . In contrast, Na 3 PS 4 and Na 3 PSe 4 exhibit lower conductivity values of 0.46 × 10 -3 and 0.10 × 10 -3 S cm -1 , respectively, at RT . It is worth noting that Famprikis et al. made a significant discovery regarding a new highly conductive γ phase in Na 3 PS 4 with an activation energy of 0.11 eV. They proposed that the exceptional Na + ion conductivity in this phase is linked to the rotation of PS 4 tetrahedra. Their findings were corroborated by ab initio MD simulations, which provided additional insights into this phenomenon.