A New n = 4 Layered Ruddlesden-Popper Phase K_2.5Bi_2.5Ti₄O₁₃ Showing Stoichiometric Hydration

A new bismuth-containing layered perovskite of the Ruddlesden-Popper type, K_2.5Bi_2.5Ti₄O₁₃, has been prepared by solid-state synthesis. It has been shown to hydrate to form stoichiometric K_2.5Bi_2.5Ti₄O₁₃·H₂O. Diffraction data show that the structure consists of a quadruple-stacked (n = 4) perovskite layer, with potassium ions occupying the rock salt layer and its next-nearest A site. The hydrated sample was shown to remove the offset between stacked perovskite layers relative to the dehydrated sample. Computational methods show that the hydrated phase consists of intact H₂O molecules in a vertical "pillared" arrangement bridging across the interlayer space. Rotations of H₂O molecules about the c axis were evident in molecular dynamic calculations, which increased in rotation angle with increasing temperature. In situ diffraction data for the dehydrated phase point to a broad structural phase transition from orthorhombic to tetragonal at 600 °C. The relative bismuth-rich composition in the perovskite block results in a higher transition temperature compared to related perovskite structures. Water makes a significant contribution to the dielectric constant, which disappears after dehydration.