Nb₂O₅ and Ta₂O₅ Thin Films from Polyoxometalate Precursors: A Single Proton Makes a Difference

Thin film materials from water-based precursors follow the principals of green chemistry, leading to a more sustainable future in the energy intensive era in which we currently reside. While simple in practice, aqueous metal-oxide chemistry is complex at the molecular level. Here we develop the first water-based formation of Nb₂O₅ and Ta₂O₅ thin films; utilizing tetramethylammonium salts of [H₂Ta₆O₁₉]^6– and [H₃Nb₆O₁₉]^5– polyoxometalates. Although the clusters are structurally identical group V analogues and differ only by a single proton, this difference has a considerable influence on the quality of the films that are obtained. Through characterization of the solid-state precursor (single-crystal X-ray diffraction), the aqueous precursor solution (X-ray scattering), and the thin films (atomic force and scanning electron microscopies, X-ray diffraction, and reflectivity), we rationalize the important roles of cluster protonation that carry through all chemical processes from the precursor to the metal oxide coating

Dielectric and Ferroelectric Properties in Highly Substituted Bi₂Sr(A)TiNb₂O₁₂ (A = Ca²⁺, Sr²⁺, Ba²⁺) Aurivillius Phases

Structure–property relationships were determined for the family of three-layer Aurivillius materials Bi₂Sr­(A)­TiNb₂O₁₂ (A = Ca²⁺, Sr²⁺, Ba²⁺). X-ray and neutron diffraction along with selected area electron diffraction indicate that Bi₂SrBaTiNb₂O₁₂ crystallizes in the nonpolar <i>I</i>4/<i>mmm</i> space group, whereas the polar <i>B</i>2<i>cb</i> space group best describes Bi₂SrCaTiNb₂O₁₂ and Bi₂Sr₂TiNb₂O₁₂. Despite the different space groups, all three compositions show relaxor behavior as evidenced through <i>P</i>(<i>E</i>) and dielectric measurements. These relaxor properties are derived from the extensive amount of disorder in each composition that is found at every cationic crystallographic site and do not depend on the space group. This disorder is so extensive that it disrupts the ferroelectric properties allowed by symmetry in the <i>B</i>2<i>cb</i> space group. This work demonstrates the important role of cation substitution and site disorder in these three-layered Aurivillius materials and its significant effect on both ferroelectric and dielectric properties