Prediction and Kinetic Stabilization of Sn(II)-Perovskite Oxide Nanoshells

The synthesis of kinetically stabilized, i.e., metastable, dielectric semiconductors, represents a major frontier within technologically important fields as compared to thermodynamically stable solids that have received considerably more attention. Of long-standing theoretical interest are Sn(II) perovskites [e.g., Sn(Zr1/2Ti1/2)O3 (SZT)], which are isoelectronic Pb-free analogues of Pb(Zr1/2Ti1/2)O3 (PZT), a commercial piezoelectric composition that is dominant in the electronics industry. Herein, we describe the synthesis of this metastable SZT dielectric through a low-temperature flux reaction technique. The SZT has been found, for the first time, to grow and to be stabilized as a nanoshell at the surfaces of Ba(Zr1/2Ti1/2)O3 (BZT) particles, i.e., forming as BZT-SZT core-shell particles, as a result of Sn(II) cation exchange. In situ powder X-ray diffraction (XRD) and transmission electron microscopy data show that the SZT nanoshells result from the controlled cation diffusion of Sn(II) cations into the BZT particles, with tunable thicknesses of ∼25-100 nm. The SZT nanoshell is calculated to possess a metastability of approximately -0.5 eV atom-1 with respect to decomposition to SnO, ZrO2, and TiO2 and cannot currently be prepared as stand-alone particles. Rietveld refinements of the XRD data are consistent with a two-phase BZT-SZT model, with each phase possessing a generally cubic perovskite-type structure and nearly identical lattice parameters. Mössbauer spectroscopic data (119Sn) are consistent with Sn(II) cations within the SZT nanoshells and an outer ∼5-10 nm surface region comprised of oxidized Sn(IV) cations from exposure to air and water. The optical band gap of the SZT shell was found to be ∼2.2 eV, which is red-shifted by ∼1.2 eV compared to that of BZT. This closing of the band gap was probed by X-ray photoelectron spectroscopy and found to stem from a shift of the valence band edge to higher energies (∼1.07 eV) as a result of the addition of the Sn 5s2 orbitals forming a new higher-energy valence band. In summary, a novel synthetic tactic is demonstrated to be effective in preparing metastable SZT and representing a generally useful strategy for the kinetic stabilization of other predicted, metastable dielectrics.