## 3 Results and Discussion
### Preliminary Material Characterization: Effects of Processing Parameters
#### 3.1.2 Macro- and Microstructure Phase Investigations  
The effects of aggregation on the macrostructure of pellets produced by cold isostatic pressing are displayed by the optical micrographs in Figure 4. Cross-sections revealed a packing order with visible anisotropy, manifesting as clusters of different orientations but with distinct grain boundaries within the pellet volume. The elevated applied pressures ensure high packing ratios of clusters and structural integrity of the assembled pellets. Another observation concerns the occurrence of miniscule dark spots within the matrix, albeit at low frequencies due to random carbonic impurities introduced by poor sample handling--this can be remedied by storing pre-sintered pellets in their source material. Investigations on the microstructure of NASICON need to commence with considerations of its crystal structure: Na\({}_{1+x}\)Zr\({}_{2}\)Si\({}_{x}\)P\({}_{3+}\)O\({}_{12}\) is an interstitial alloy of NaZr\({}_{2}\)P\({}_{3}\)O\({}_{12}\) and Na\({}_{4}\)Zr\({}_{2}\)Si\({}_{3}\)O\({}_{12}\) arranged into a 3D framework comprised of (Si/P)O\({}_{4}\) tetrahedra sharing pocket corners with ZrO\({}_{6}\) octahedra (Figure 5); its lattice conforms to a C2/c monoclinic phase (\(a\) = 15.6723 A; \(b\) = 9.0537 A; \(c\) = 9.2758 A; \(V\) = 1088.15 A\({}^{3}\)) at room temperature, transitioning to R\(\overline{3}\)c rhombohedral (\(a\) = \(b\) = 9.0535 A; \(c\) = 23.0677 A; \(V\) = 1637.45 A\({}^{3}\)) above approximately 160 \({}^{\circ}\)C [4; 7; 13; 14; 58; 66; 67]. Both phase structures present four Na sites (monoclinic: 1 Na\({}_{1}\), 2 Na\({}_{2}\), 1 Na\({}_{3}\); rhombohedral 1 Na\({}_{1}\), 3 Na\({}_{2}\)). In the monoclinic phase, both Na\({}_{1}\) and Na\({}_{2}\) sites are occupied, whereas in the rhombohedral phase, only Na\({}_{1}\) sites are fully occupied. The ionic conduction mechanism consists of Na\({}^{+}\) ions migrating from  
Figure 4: Optical micrographs of NASICON pellets (**a**) with pronounced clustering produced at low compacting pressure (cross-section); (**b**) with smoother packing produced by high compacting pressure (cross-section); and (**c**) riddled by random carbonic impurities introduced by improper handling and storage (peripheral surface). Scale bar at 500 \(\upmu\)m.  
Na\({}_{1}\) to Na\({}_{2}\) or to Na\({}_{3}\) and in reverse through the oxygen bottleneck within the crystal lattice. During this phase transition, the ZrO\({}_{6}\) and (Si/P)O\({}_{4}\) 3D framework remains intact, while one Na\({}_{2}\) and two Na\({}_{3}\) sites convert into Na\({}_{2}\) sites. Due to the higher symmetry of the rhombohedral phase, there are more diffraction peaks emanating from the monoclinic crystal, thus high-temperature in situ X-ray diffractometry (XRD) provides a viable means of detecting the two structures [58]. Being more symmetrical, the rhombohedral phase also displayed the higher Na\({}^{+}\) ion mobility, with lower activation energy \(E_{a}\) and higher bulk conductivity than the monoclinic phase [3; 4; 25; 53]. However, further elaboration on phase transitioning is beyond the scope of this work, especially since this transition is both temperature-dependent and reversible [7; 58; 66; 67]. The NASICON structure will permanently transform only if the Na:Si:P stoichiometry is altered--the end-members of Na\({}_{1+x}\)Zr\({}_{2}\)Si\({}_{x}\)P\({}_{3+}\)O\({}_{12}\) (i.e., \(x=0\) and 4) are rhombohedral at room temperature, but their lattice parameters change significantly as \(x\) approaches 2, where the lattice reverts to the monoclinic crystal structure [58; 61; 66; 67]. The critical temperature of this phase transition can be continuously lowered by incrementally increasing Na and the Si:P ratio in the general formula, until the rhombohedral phase is stabilized at room temperature [66].