*2.1. X-ray and TG Analysis*

Figure 1a shows the X-ray Diffraction (XRD) patterns of the studied HCa2Nb3O10·*y*H2O samples with different hydration levels. The XRD shows that the samples are practically monophase. All the samples can be described by the *P*4/*mmm* space group. The unit cell is shown in Figure 1b. The lattice parameters are listed in Table 1.

**Figure 1.** XRD powder patterns for the studied forms of HCa2Nb3O10·*y*H2O with different hydration levels (**a**) and the unit cell of HCa2Nb3O10·*y*H2O (**b**) with possible sites for the water oxygen Ow.


**Table 1.** Lattice parameters (*a*, *c*) and unit cell volume (*V*) for the studied forms of HCa2Nb3O10·*y*H2O.

The thermogravimetric (TG) curves that represent the mass decay due to the water release are shown in Figure 2a. As can be seen from the thermogravimetric curves, α-HCa2Nb3O10·*y*H2O exhibits behavior typical for a low-stable highly hydrated protonated form. Its thermal decomposition proceeds in two main steps, which is typical of protonated layered perovskite-like oxides [40–42]. The first step (T < 373 K) is associated with the release of the intercalated water and the formation of a dehydrated protonated compound:

$$\text{HCa}\_2\text{Nb}\_3\text{O}\_{10} \cdot y\text{H}\_2\text{O} \to \text{HCa}\_2\text{Nb}\_3\text{O}\_{10} + y\text{H}\_2\text{O}.\tag{1}$$

**Figure 2.** (**a**) TG curves for the studied forms of HCa2Nb3O10·*y*H2O with different hydration levels; (**b**) number of H2O molecules per formula units (*y*) versus the interlayer space (*d*).

The second step of the mass loss that occurs at about 525 ÷ 550 K is related with thermal degradation, or so-called topochemical condensation:

$$\rm{HCa\_2Nb\_3O\_{10}} \rightarrow \rm{Ca\_2Nb\_3O\_{9.5}} + 0.5 \rm{H\_2O}.\tag{2}$$

The thermal decomposition of the γ-form demonstrates similar trends, but the mass loss at the first step is essentially low due to the much lower content of the intercalated water. Thermolysis of the β-form appears to be a more complex process, including gradual evolution of interlayer water in the temperature range of 373 ÷ 525 K, with the subsequent topochemical condensation of the protonated compound. The absence of the mass loss for the β-form at the beginning part of the TG curve indicates its greater thermal stability in comparison with the α-form.

According to the TG analysis, all the studied forms are fully protonated compounds with a substitution degree of K<sup>+</sup> cations for protons H+ close to 100%. The water content as determined from TG curves results in 1.6, 0.8, and 0.1 H2O molecules per formula unit for α-, β-, and *γ*-forms, respectively. When describing layered structures, an important parameter is the interlayer distance *d*, the distance between the center of the adjacent perovskite slabs. For the studied structures, *d* = *c*; Figures 1b and 2b show the correlation between the *d* parameter and the water content, which confirms that water molecules are located within the interlayer space.
