Layer Alignment via Short Strong Hydrogen Bonding in the Dion–Jacobson Perovskite Solid Acid HLaNb2O7

The Dion–Jacobson perovskite solid acid, HLaNb2O7, is an important intermediate in many topochemical reactions such as those involving ion exchange, intercalation, exfoliation, and grafting. Rietveld structure refinement of time-of-flight neutron diffraction data (7.66 K, P4/mmm, a = b = 3.8907(3) Å, c = 10.5068(9) Å) shows that protons bridging across the HLaNb2O7 interlayer serve to align LaNb2O7 blocks through NbO6 apical oxygens. The oxygens are 2.54 Å apart, with the intermediate proton distributed over two half-occupied sites, and 1.15(10) Å from the closest oxygen. Such short strong hydrogen bonds are shorter (1.40 Å) than are typical for many known mixed metal oxide solid acids (ca. 2.8 Å–3.5 Å). Anisotropic refinement of the hydrogen thermal parameter reveals an elongation suggesting an active hydrogen that moves between the two oxygens, O–H…O ↔ O…H–O; DFT modeling supports this behavior, finding a minimal activation energy between two states; and infrared studies (FTIR) exhibit a broad low-energy band at ~1600 cm−1, associated with the hydrogen resonance between the apical oxygens. These results are discussed in terms of structure, strong low-barrier hydrogen bonding, and proton mobility.