Unraveling the structural features of Dion-Jacobson-type layered perovskite-related material HCa₂Nb₃O₁₀·1.5H₂O

Hydrated layered oxides are widely encountered, yet the presence of disordered interlayer water often complicates crystal structure determination from laboratory X-ray diffraction. Here, we report the crystal structure of the Dion-Jacobson-type layered perovskite-related material HCa₂Nb₃O₁₀·1.5H₂O, solved from synchrotron X-ray diffraction data by combining direct methods in reciprocal space, Le Bail whole-pattern fitting, and Rietveld refinement. The hydrate crystallizes in a tetragonal structure with space group P4₂2₁2 (a = 7.7070(5) Å, c = 32.4870(3) Å). Incorporation of partially occupied interlayer water-oxygen sites on the (110) plane at z = 0 and 1/2 successfully reproduces the low-angle 00l reflections while preserving the Ca₂Nb₃O₁₀ framework. The resulting crystallographic model explicitly resolves the arrangement of interlayer water molecules and provides a robust structural foundation for band-structure calculations as well as for the rational design of hydration-controlled intercalation, exfoliation, and composite materials based on layered perovskite-related materials.