A New n = 4 Layered Ruddlesden–Popper Phase K2.5Bi2.5Ti4O13 Showing Stoichiometric Hydration

Abstract

A new highly ordered Bi-containing layered perovskite of the Ruddlesden-Popper phase, K2.5Bi2.5Ti4O13, has been prepared by solid state synthesis. It has been shown to hydrate to form stoichiometric K2.5Bi2.5Ti4O13 · H2O. Diffraction data show that the structure consists of a quadruple stacked (n = 4) perovskite layer with K-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 dehy- drated sample. Computational methods show that the hydrated phase consists of intact water molecules in a vertical “pillared” arrangement bridging across the inter- layer space. Rotations of water molecules about the c-axis were evident in molecular dynamic calculations, which increased in rotation angle with increasing temperature. In situ diffraction data point to a broad structural phase transition consistent with relaxor behaviour from orthorhombic to tetragonal at TC ∼600 °C. A corresponding broad increase in the dielectric constant was observed in dielectric property measure- ments. The relative Bi-rich composition in the perovskite block results in a higher TC compared to related perovskite structures. Water makes a significant contribution to the dielectric constant, which disappears after dehydration.