To understand the origin of the increased conductivity, we refer to our findings on the material's composition. Previously, we concluded that NaAlCl4 is undoped with Zn²⁺. Interestingly, the highest conductivities are achieved near the edge of the miscibility gap, where the sample is Zn-rich with an approximate composition Na1.625Zn0.625Al0.375Cl₄ and a minor side phase of NaAlCl4 (as determined by refinement, 4% ± 2% molar fraction). Since NaAlCl4 does not show any improvement in conductivity even at 35 MPa, one may hypothesize that the increase in conductivity arises from: i) the introduction of sufficient vacancies in the Na₁ sites of the NZC-type structure, allowing for effective Na hopping, and/or ii) the influence of the amorphous phase in the samples resulting from the harsh ball-milling process. Previous studies have demonstrated that the amorphous phase may play a role in ionic conduction in halide metal electrolytes. As discussed above, all samples contain 30 ~ 50% amorphous content (Figure ). The amorphous content gradually increases with x up to x = 0.5 and x = 0.625, where the NZAC samples exhibit optimal ionic conduction. When x further increases to x = 0.75, the amorphous fraction remains comparable, but the ionic conductivity declines by an order of magnitude. In the pure NZC phase, which also exhibits a slightly higher amorphous content, the ionic conductivity essentially disappears. Therefore, we can conclude that there is no correlation between ionic conductivity and amorphous content in our samples (Figure ). Instead, the mechanism responsible for the observed conductivity improvements lies within the crystalline domain. To further investigate the underlying mechanisms, EIS measurements were conducted at a fixed pressure (140 MPa) while varying the temperature. Arrhenius plots for all Na1+xZnxAl1-xCl4 compositions are presented in Figure . As expected, ln(σT) vs. 1/T exhibits a linear behavior within the probed temperature range. The activation energy Ea was calculated from the following Arrhenius relation : )