## 3 Results and Discussion
### Preliminary Material Characterization: Effects of Processing Parameters
#### 3.1.2 Macro- and Microstructure Phase Investigations
In this work, XRD was employed to study the evolution of the SSR over selected temperature levels that mark the different stages of thermal treatment and also investigate any possible changes in the structure induced by the different isostatic pressure levels applied during pellet formation (Figure 6). Primary peak signatures revealed the high crystallinity of the calculated and sintered materials, with NASICON-associated signatures increasing both in crystallite size and intensity (Table 2). SSR is completed at the final sintering stage at 1250 \({}^{\circ}\)C, with the primary NASICON peaks also appearing at the calcination stage at 600 \({}^{\circ}\)C. The untreated sample presented most of its zirconia content in unreacted form, transforming from its initial tetragonal to the monoclinic phase (PDF 00-037-1484) at elevated temperatures--these traces of monoclinic zirconia persisted as significant secondary phases throughout the thermal treatments, most possibly due to the  
Figure 5: 3D models of the crystal structure of NASICON, displayed as top views of (**a**) linked ZrO\({}_{6}\) octahedra (gold) and (P/Si)O\({}_{4}\) tetrahedra (purple), oxygen atoms (red), and free conduction channels through the structure; (**b**) sodium ions (cyan) within the channels. (**c,d**) Corresponding front views of the lattice and the ions within the channels, respectively. loss of volatile elements during the high-temperature SSR [5; 53]. The peak around 21.5\({}^{\circ}\), present only in the untreated material, corresponded to an unreacted cubic \(\gamma\)-Na\({}_{3}\)PO\({}_{4}\) phase, which vanished over elevated temperatures [51]. Another important observation concerning the XRD spectra over the investigated thermal treatment stages is the marked deviation between the characteristic signatures observed for the in-lab produced sample sintered at 1250 \({}^{\circ}\)C and the commercial reference one, which is instead more closely mirrored by the calculated sample at 600 \({}^{\circ}\)C. This becomes more evident by the close inspection of the (104), (110), and (116) peaks, as seen in the side-panels of Figure 6a, suggesting that the commercial sample was sintered at low temperature (below 1100 \({}^{\circ}\)C) to avoid zirconia contamination, which is a common negative effect occurring over elevated sintering temperatures [46; 48; 50; 52; 53; 58]. Here, it needs to be underlined that the commercial reference sample was treated as a \"black-box\" (there was no information provided by the industrial manufacturer on its synthesis method and parameters), as the aim of this study was to focus on the intrinsic characteristics--both positive and negative--of the classic (i.e., non-optimized) SSR synthesis route. Figure 6: XRD spectra of NASICON signatures (**a**) over increasing processing temperatures and (**b**) over selected levels of isostatic pressure. Miller indices given above their corresponding 2\(\theta\) ranges; zirconia and other secondary phases marked as black \(\Diamond\). Side-panels correspond to focused displays of characteristic peaks of interest (scaling not proportional for readability). Zirconia contamination was further investigated by examining XRD spectra of samples obtained after applying different levels of isostatic pressure (245, 345, and 540 MPa) to compact the 1100 \({}^{\circ}\)C sintered samples into pellet form (Figure 6b, Table 3). Prominent zirconia signatures included: (i) the trace at 20.1\({}^{\circ}\) appearing as a shoulder of the (110) peak (top side-panel of Figure 6a); (ii) the trace at 27.0\({}^{\circ}\) before the (024) peak (top side-panel of Figure 6b) that has been identified as zirconium orthosilicate (ZrSiO\({}_{4}\)) contaminant [58]; and (iii) the traces around 35.7\({}^{\circ}\) following the (330) peak (bottom side-panel of Figure 6b). Increasing pressure levels did not necessarily cause the above traces to recede.