Perfect LFP crystals are known to possess high Li conductivity along the b direction. Likewise, the polyhedral network in the NZC structure provides a potential continuous Na⁺ conduction pathway, as indicated by black arrows in Figure . A chain of Na1 octahedra on 4a sites connects through faceshared vacant tetrahedral sites, allowing Na⁺ to hop neighboring octahedral sites along the [010] direction. However, due to the fully occupied Na1 sites in NZC, no free volume is available for Na⁺ conduction. Therefore, as expected, when measuring the Na ionic conductivity in NZC we find a very low value below 10 -10 S/cm (Figure ). Consequently, NZC itself is not a promising SE. One could wonder, however, how the creation of vacancies along the Na channels could impact the conductivity, which we indeed will show next.