## Results
### Effect of the Binder Concentration on the Thermal Evolution and Structure of Membranes
Figure 4: In situ X-ray patterns obtained for the 100 PAA sample without the binder upon heating in the range 30–330 \({}^{\circ}\)C with steps of 10 \({}^{\circ}\)C.  
Figure 3: Pore size distribution in 90 PAA and 80 PAA samples. Coupling the observed changes in XRD patterns before and after treatment (see Figures 4 and 5) with STA data (Figure 1); one can conclude that up to 100 \({}^{\circ}\)C, water removal took place for some positions in the pyrochlore-type structure, which was responsible for the reflexes appearing in the XRD pattern. This process was accompanied by a slight unit cell shrinkage. Starting from 250 \({}^{\circ}\)C, further structural water removal from polyantimonic acid took place. The in situ thermo-XRD patterns obtained for 90 PAA and 80 PAA powders in the range 30-330 \({}^{\circ}\)C are shown in Figures 6 and 7. The introduction of a binder had no effect on phase compositions. The nature of reflex intensity changes found for the samples containing binder was almost identical to that observed for the 100 PAA membrane (with no binder added). The shift in the reflex's positions slightly decreased for the 90 PAA membrane, from 0.8 to 0.4\({}^{\circ}\) for the reflex at 2\(\Theta\) = 40.6\({}^{\circ}\), i.e., the lattice parameter shrank slightly less for the 10 wt.% binder content sample. Further increases in binder content to 20 wt.% had no effect on the XRD patterns obtained. Figure 5: Comparison of XRD pattens for 100 PAA, 90 PAA and 80 PAA before and after heat treatment. Figure 6: In situ X-ray patterns obtained for the 90 PAA sample upon heating in the range 30-330 \({}^{\circ}\)C with steps of 10 \({}^{\circ}\)C.  
All spectra obtained exhibit two distinct regions: signals at 0-1000 cm\({}^{-1}\) are attributed to antimony oxide vibration bands, whereas signals at 1000-4000 cm\({}^{-1}\) are ascribed to structural water bands (Figure 8a). The highest water band in the PAA spectrum after synthesis was located at around 3000 cm\({}^{-1}\). Its position continuously shifted towards lower wave numbers with increased temperature, and reached 2400 cm\({}^{-1}\) at 250 \({}^{\circ}\)C, being equal to the value calculated for the H\({}_{3}\)O\({}^{+}\) ion [42]. The enlarged area at 0-1000 cm\({}^{-1}\) for the PAA sample spectra, corresponding to antimony oxide, is presented in Figure 8b. The main features of the spectrum for PAA powder after synthesis are (i) the presence of a wide, intense band near 502 cm\({}^{-1}\) and (ii) a very weak, broad shoulder at 620 cm\({}^{-1}\), which is in accordance with previously reported data [43]. According to [PERSON] [44], any octahedral molecule XY\({}_{6}\), such as [SbO\({}_{6/2}\)]\({}^{-}\) octahedrons of PAA, has six normal vibration modes. Three of them are seen in the typical Raman spectra of antimony-based compounds, for example, in antimony (V) chloride [45]. However, the picture obtained for PAA differs significantly from the typical spectra for individual octahedron molecules, or crystalline Sb\({}_{2}\)O\({}_{5}\)[46], Sb\({}_{2}\)O\({}_{3}\) or Sb\({}_{2}\)O\({}_{4}\)[43]. The observable wide peak at 502 cm\({}^{-1}\) with a shoulder can be ascribed to the polymeric Sb-O network vibrations. Two additional broad weak bands, distinguishable at 150-200 cm\({}^{-1}\) and 200-250 cm\({}^{-1}\), are assigned to trace contents of the orthorhombic Sb\({}_{2}\)O\({}_{3}\) (valentinite phase). The peaks at 0-400 cm\({}^{-1}\), as well as the shoulder at 620 cm\({}^{-1}\), almost  
Figure 8: (**a**) Raman spectra of as-received PAA and after calcination at 150–250 \({}^{\circ}\)C; (**b**) detailed area of Sb\({}_{2}\)O\({}_{5}\) vibration bands. Figure 7: In situ X-ray patterns obtained for the 80 PAA sample without the binder upon heating in the range 30–330 \({}^{\circ}\)C with steps of 10 \({}^{\circ}\)C.  
disappear, indicating a gradual symmetry increase. The observed behavior is similar to that observed for other high-symmetry compounds, as is the case of BaZrO3 [47].