Here, n represents the number of charge carriers (Na + ) per unit volume, q is their charge and 𝜇 is the mobility of Na + . When the x value decreases from x = 0.75 to x = 0.625, more Na + , particularly in the Na1 site, will be replaced by vacancies, possibly activating the participation of adjacent Na + ions in the transport mechanism, thereby increasing the density of charge carries 𝑛 %& # . Moreover, due to the two-phase behavior observed when x is between 0 and 0.69, no typical trade-off relationship between n and 𝜇, as often seen in ionic conductors, can be established in this case. In summary, introducing a sufficient amount of vacancies within the NZC-type phase, while maintaining a high phase fraction is key to achieving the highest ionic conductivity in NZAC samples.