**1. Introduction**

In recent years, layered perovskite-like oxides have attracted much attention because of their outstanding physical and chemical properties, including high-temperature superconductivity [1,2], colossal magnetoresistance [3], the capability of photocatalytic water decomposition under sunlight irradiation for further hydrogen storage [4,5], and ionic conductivity due to high mobility of interlayer cations [6,7]. The majority of ion-exchangeable layered perovskite-like oxides can be converted into their protonated forms, which, besides being proton conductors [6,8] and photocatalysts for water splitting [9–11], exhibit the ability to intercalate water [10–14] and other molecules [15,16] and/or to form graft derivatives [16–19] susceptible to further exfoliation [10,20,21].

The hydrated form of HCa2Nb3O10 (usually referred to as HCa2Nb3O10·1.5H2O in the literature) belongs to the Dion-Jacobson phase and can be obtained from KCa2Nb3O10 oxide by ion-exchange in acid solutions [22]. It was shown that HCa2Nb3O10·1.5H2O enables the intercalation of amines by an acid-base mechanism [23] and may be later exfoliated into nanolayers [24,25]. Both KCa2Nb3O10 and HCa2Nb3O10·1.5H2O, as well as their exfoliated and restacked forms, exhibit photocatalytic properties [26–28]. Along with this form, there may be others with a lower water content. The ability to intercalate water molecules often plays a crucial role in other intercalation reactions and photocatalysis [9,29–31]. Hydrated protonated forms may comprise protons [13,30] or charged complexes like H+ ... *<sup>n</sup>*·H2O in their interlayer slab [32–34]. Obviously, water content and its state and localization should affect both the pathway and efficiency of chemical or photocatalytic reactions. From this perspective, an identification of proton-containing species and a comprehensive study of their motion in the interlayer slab is required.

Proton Nuclear Magnetic Resonance (NMR) is one of the most versatile experimental methods. It enables the identification of the proton-containing species and provides insight on the local structure [13,32,35–37] and information at the microscopic level on the dynamics of intercalated species [13,16,32,35,36,38]. In particular, by using 1H NMR, it was shown that both the local environment and the dynamics of hydrogen in these materials are affected by the stacking sequence of the perovskite-like slabs [39].

Here, we report on the results of the proton NMR spectroscopy and relaxation studies of the layered perovskite-like niobate HCa2Nb3O10 with different hydration levels: hydrated—α-form, dehydrated—*γ*-form, and intermediate—β-form. The details of their synthesis can be found in Section 3.

## **2. Results and Discussion**
