This work illustrates the necessity of using a variety of techniques when characterizing these systems, which undergo complex structural changes that cannot be completely characterized using a single technique. The XRD results indicate a complete transition from α- to γ-Na\sb3PO\sb4 with no intermediate structures present. The DSC thermograms show the presence of a perturbed α- or γ-phase, but generally agree with the phases identified by the XRD patterns. Vibrational spectroscopy provides a richer description of the local structural changes occurring.

Na\sb3PO\sb4 has two solid phases: the tetragonal α phase, stable to 320\sp∘C, and the cubic γ phase, stable from 320\sp∘C to the melt at 1500\sp∘C. Initially, a factor group analysis of the expected modes and their symmetry-based assignments was performed for these two phases. A temperature-dependent Raman study was then performed for the pure compound. The bands change from sharp multiplets to broad bands with increasing temperature. The broad bands and collapsed band structure seen in γ-Na\sb3PO\sb4 are indicative of the high disorder present.

The stabilization of polymorphic compounds has been of interest for some time. Previous studies have shown it is possible to stabilize the high-temperature, fast-ion-conducting phase of sodium phosphate (Na\sb3PO\sb4), but these systems have not been thoroughly characterized. Therefore, a systematic study was conducted of pure Na\sb3PO\sb4 and solid solutions of Na\sb3PO\sb4 doped with Mg\sb3(PO\sb4)\sb2, Zn\sb3(PO\sb4)\sb2, Na\sb2SO\sb4 and ZnSO\sb4. The techniques used in this study were Raman and infrared spectroscopies, differential scanning calorimetry, and powder x-ray diffraction.

γ-Na\sb3PO\sb4 is stabilized in all four dopant systems. The percentage of dopant necessary for complete stabilization is different for anion, cation, and combined dopants. The two cation dopant systems, Na\sb3PO\sb4/Mg\sb3(PO\sb4)\sb2 and Na\sb3PO\sb4/Zn\sb3(PO\sb4)\sb2, exhibit similar behavior although the Raman spectra indicate that Zn\sp2+ has a stronger interaction than Mg\sp2+ with the PO\sbsp43− ion. The Raman spectra in the Na\sb3PO\sb4/ZnSO\sb4 system exhibit characteristic bands seen in both the Zn\sb3(PO\sb4)\sb2- and Na\sb2SO\sb4-doped systems. A remarkable characteristic of γ-Na\sb3PO\sb4 is the amount of aliovalent substitution that can be accommodated in the lattice before phase separation or compound formation occurs: 11% Mg\sb3(PO\sb4)\sb2 or Zn\sb3(PO\sb4)\sb2, more than 30% ZnSO\sb4, and over 60% Na\sb2SO\sb4.