Rietveld refinements of the X-ray diffraction patterns were conducted to evaluate in more detail the obtained crystal structures. All refinement parameters and constraints are provided in the Supporting Information Tables S3-S13. Initially, a small increase in the unit cell parameters of NAC can be observed on the Al-rich side (x from 0 to 0.045), which could be explained by the substitution of a small amount of Zn²⁺ in NAC. However, there is no linear behavior to confirm the existence of an extended solid solution, as all unit cell parameters remain constant when x increases further. Moreover, the Zn occupancy on the Al site was refined and is shown in Figure , demonstrating that the Zn occupancy remains near 0 for all compositions in the NAC phase. Substituting a Zn²⁺ ion (74 pm) into an Al³⁺ site (53.5 pm) requires significant distortion. Additionally, to maintain charge balance in the crystal, an extra Na⁺ ion would need to be intercalated into the structure. This is unfavorable for the stability of the NAC phase, as indicated by our previous DFT calculations of the substituted structure. On the Zn-rich side, the diffractograms for nominal substitution degrees x = 0.75 to 1 show no additional reflections aside from those of the NZC phase. Linear behavior of the lattice parameters a and b and unit cell volume is observed for x = 0.75 to 1, corroborating the formation of solid solutions.