**4. Conclusions**

The results of a comprehensive 1H NMR study of three different forms of the layered perovskite-like niobate HCa2Nb3O10·*y*H2O can be summarized as follows:


We expect that the results obtained will clarify the relationship between the hydration level (and hence the type and, possibly, localization of hydrogen-containing species) and the photocatalytic activity of this class of layered materials. Currently, studies of the photocatalytic activity of the different forms of HCa2Nb3O10·*y*H2O are under evaluation.

**Author Contributions:** Conceptualization, M.G.S., O.I.S. and I.A.Z.; methodology, M.G.S. and O.I.S.; investigation, D.Y.N., A.O.A., O.I.S., S.A.K. and A.M.; data curation, M.G.S., O.I.S. and E.A.A.; writing—original draft preparation, M.G.S., O.I.S. and E.A.A.; writing—review and editing, M.G.S., O.I.S. and I.A.Z.; visualization, M.G.S. and E.A.A.; supervision, I.A.Z.; funding acquisition, M.G.S., O.I.S. and I.A.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was financially supported by Russian Science Foundation (NMR study of hydrated forms was carried out within the framework of the project No. 19-13-00184, synthesis of a number of hydrated compounds, and their basic characterization was carried out within the framework of the project No. 20-73-00027).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** The samples were studied at the Research Park of Saint Petersburg State University, Centre for X-ray Diffraction Studies, Centre of Thermal Analysis and Calorimetry and Interdisciplinary Resource Centre for Nanotechnology, and Centre for Diagnostics of Functional Materials for Medicine, Pharmacology and Nanoelectronics.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.

#### **References**

