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

Abstract

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-(Zr 1/2 Ti 1/2 )O 3 (SZT)], which are isoelectronic Pb-free analogues of Pb(Zr 1/2 Ti 1/2 )O 3 (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(Zr 1/2 Ti 1/2 )O 3 (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, ZrO 2 , and TiO 2 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. Mossbauer spectroscopic data ( 119 Sn) 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 5s 2 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.

Switching Lead for Tin in PbHfO3: Noncubic Structure of SnHfO3**

“…The synthesis of Sn(Zr,Ti)O 3 shells on the surfaces of BZT particles (BSZT) demonstrated that cation-exchange and kinetic stabilization of Sn(II) is feasible in the 3D perovskite-type structure, and providing the first evidence that these thus-far only theorized phases do indeed have intriguing properties such as visible-light driven water oxidation. [27] Further developments of these synthetic techniques have led to the first-reported successful synthesis of a fully Sn(II)-substituted perovskite oxide, SnHfO 3 , stabilized as nanoeggshells. [28] These observations demonstrate the effectiveness of topotactic ion-exchange in metastable oxide synthesis, and suggesting at least two criteria for the successful synthesis of a desired phase; (I) the underlying substructure must have a strong cohesive energy, and (II) an absence of lower energy polymorphs easily accessible via ion-diffusion.…”

“…Segregation is proposed to occur more readily since the interlayer‐Sn(II) ions can more freely diffuse to an energetically favorable coordination environment, although the hypothetical Sn 5 Nb 4 O 15 phase is not expected to be nearly as metastable as perovskite‐type SnTiO 3 . The synthesis of Sn(Zr,Ti)O 3 shells on the surfaces of BZT particles (BSZT) demonstrated that cation‐exchange and kinetic stabilization of Sn(II) is feasible in the 3D perovskite‐type structure, and providing the first evidence that these thus‐far only theorized phases do indeed have intriguing properties such as visible‐light driven water oxidation [27] . Further developments of these synthetic techniques have led to the first‐reported successful synthesis of a fully Sn(II)‐substituted perovskite oxide, SnHfO 3 , stabilized as nano‐eggshells [28] …”

Technological Challenges and Advancement in Proton Conductors: A Review

“…The quantitative estimations of structural deviations from the ideal behavior in perovskites is denoted using Goldschmidt’s tolerance factor ( t ) in eq : italict = italicr normalA + italicr normalO 2 ( r B + r O ) where r A , r B , and r O represent the ionic radii of the A-site cation, B-site cation, and oxygen anion, respectively. Perovskites with lower t values crystallize in ilmenite polymorphic structures. , Alongside the structural viewpoint, chemical coordination of individual cations in perovskite PCs is also equally significant to preserve perovskite properties. One of the intriguing materials is BaGeO 3 with four coordination B-site Ge atoms.…”

“…Perovskites with lower t values crystallize in ilmenite polymorphic structures. 46,47 Alongside the structural viewpoint, chemical coordination of individual cations in perovskite PCs is also equally significant to preserve perovskite properties. One of the intriguing materials is BaGeO 3 with four coordination B-site Ge atoms.…”

Switching Lead for Tin in the ‘De-Leadification’ of PbHfO3: Noncubic Structure of SnHfO3

“…The synthesis of Sn(Zr,Ti)O3 shells on the surface of BZT particles (BSZT) demonstrated that cationexchange and kinetic stabilization of Sn(II) is feasible in the 3D perovskite-type structure and provided the first evidence that these thus-far only theorized phases do indeed have intriguing properties such as visible-light driven water oxidation. [27] Further investigations into these synthetic techniques lead to the first-reported successful synthesis of a fully Sn(II)-substituted perovskite oxide, SnHfO3, stabilized as nano-eggshells. [28] These observations demonstrate the effectiveness of topotactic ion-exchange for metastable oxide synthesis and suggests at least two criteria for the successful synthesis of a desired phase; (I) the underlying substructure must have strong lattice energy, and (II) there must be an absence of lower energy polymorphs easily accessible via ion-diffusion.…”