**3. Materials and Methods**

KCa2Nb3O10, a precursor for further synthesis of the protonic form, was synthesized by the standard ceramic method at normal conditions (air atmosphere and pressure) using CaO, Nb2O5, and K2CO3 as parent materials. CaO and Nb2O5 were taken in amounts to satisfy the stoichiometry of the reaction:

$$4\text{CaO} \, + 3\text{Nb}\_2\text{O}\_5 + \text{K}\_2\text{CO}\_3 = 2\text{K}\text{Ca}\_2\text{Nb}\_3\text{O}\_{10} + \text{CO}\_2\uparrow\,. \tag{10}$$

To compensate for losses during calcination, potassium carbonate was taken with a 30% excess. All the components were mixed and ground in a planetary ball mill in n-heptane. The obtained powder was pelletized and then calcined at 800 ◦C for 12 h. After that, it was ground in an agate mortar, pelletized again, and calcined at 1100 ◦C for 24 h.

The first hydrated protonated form of KCa2Nb3O10, named as α-form of HCa2Nb3O10· *y*H2O, was prepared by acid processing of KCa2Nb3O10, with an excess of 12 M HNO3 (50 mL per 2.5 g of the oxide), at room temperature for 24 h. The product was centrifuged, washed with 50 mL of water three times to remove acid residues, and dried over CaO for 24 h. Subsequently, it was stored in a humid air atmosphere.

The second hydrated protonated form of KCa2Nb3O10, named as β-form of HCa2Nb3O10· *y*H2O, was prepared by hydrothermal treatment of the α- form. For this, 0.5 g of the latter was placed in the laboratory autoclave with 35 mL of water (volume filling approximately 80%) and processed at 150 ◦C for 7 d. The product obtained was centrifuged and dried over CaO for 24 h.

To obtain the dehydrated protonated form of KCa2Nb3O10, named as γ-form of HCa2Nb3O10·*y*H2O, the α-form was dried in a desiccator with a vacuum pump (about <sup>1</sup> × <sup>10</sup>−<sup>4</sup> atm) for 24 h.

Powder XRD analysis was done on a Rigaku Miniflex II diffractometer using monochromatic CuK<sup>α</sup> radiation (λ = 0.154056 nm). Diffractograms were recorded in the 2*θ* range of

3–120◦ (step width 0.02◦). The lattice parameters were calculated in the tetragonal system on the basis of all the reflections observed using DiffracPlus Topas 4.2 software.

TG analysis was carried out using a Netzsch TG 209 F1 Libra thermobalance. Analysis of samples was carried out in the temperature range 30–900 ◦C at a heating rate of 10 ◦C/min in an argon stream at a rate of 90 mL/min.

1H NMR experiments were done using a Bruker Avance IIITM 400 MHz solid-state NMR spectrometer (operating with Topspin version 3.2) using a double-resonance 4 mm low-temperature MAS probe. The temperature was changed within a temperature range of 139 to 297 K and controlled with an accuracy of 0.5 K. 1H NMR MAS spectra were recorded at rotor frequency 12 kHz. To measure *T*<sup>1</sup> relaxation times, the spin-locking technique was applied, with the rf frequency of the locking pulse equal to 40 kHz. Tetramethylsilane (TMS) was used as an external standard.

The 1H magnetic shielding tensor for H2O, H3O+, and H5O2 <sup>+</sup> was calculated using the Gauge-Independent Atomic Orbital (GIAO) method [65] for the geometries optimized at the B3LYP/6-311G level of theory, as implemented in Gaussian 09 [66]. The theoretical isotropic chemical shift was estimated relative to the magnetic shielding tensor in TMS, calculated at the same level of theory.
