NaAlCl4 has been applied as a catholyte in Zebra Batteries due to its Na + ionic conductivity in the molten state. As a solid, NaAlCl4 (NAC) adopts an orthorhombic space group (P212121) at room temperature. Mechanochemically prepared NAC with such crystal structure exhibits a Na⁺ conductivity of 3.9 × 10⁻⁶ S/cm at 30 °C. According to the work of Jung et al., Ball-milled NAC contains two Na positions with occupancies of Na1 = 0.673 and Na2 = 0.327. However, first-principles calculations from previous work revealed that the structural model with all Na⁺ ions occupying the Na1 site is energetically more favorable than the model where Na⁺ ions occupy both Na1 and Na2 sites, with only a minimum difference of 5 meV/atom. We synthesized NaAlCl4 mechanochemically and verified its crystal structure via XRD. The measured pattern is gathered in Figure . Despite the synthesis route, the material appears well crystallized, and the reflections can be well indexed with the P212121 space group, with merely additional peaks from unreacted NaCl impurities below 1 wt.%. The presence of minor NaCl in ball-milled sodium halide SSES was also found in previous work but was identified as non-detrimental to ionic conduction. The crystal structure is based on Al³⁺ occupying the Wyckoff 4a site, forming AlCl₄⁻ tetrahedra due to its small ionic radius (53.5 pm). The XRD patterns can be refined in two ways: using either a single sodium site (Na1 model) or with two as recently reported (the Na1+2 model). Figures and show the Rietveld refinement of these two structural models. The Rietveld refinement results based on both models show no significant differences in terms of RBragg and Rwp. However, the refinement results for the Na1+2 model (Figure ) demonstrate that the Na2 occupancy in the Na1+2 model is effectively zero within error, suggesting that Na⁺ ions strongly prefer not to occupy the Na2 sites. To further verify the potential occupancy of the Na2 site, the same NAC was measured via synchrotron XRD with high angular resolution and counting statistics. Figure depicts the refinement results (details are listed in Table ). Based on the calculated and observed XRD pattern, difference Fourier maps were calculated to verify potential mismatches in the electronic density. Figure demonstrates the positive(yellow) and negative(blue) electron density iso-surface in the structure. When the existence of Na + in Na2 is ignored in the refinement based on the Na1 model, a positive(yellow) isosurface shall appear. However, the Na2 sites are not adjacent to any electron density iso-surface, which implies no potential occupation of Na + in the Na2 site.