Renaissance of Topotactic Ion‐Exchange for Functional Solids with Close Packed Structures

Abstract

Recently, many new, complex, functional oxides have been discovered with the surprising use of topotactic ion‐exchange reactions on close‐packed structures, such as found for wurtzite, rutile, perovskite, and other structure types. Despite a lack of apparent cation‐diffusion pathways in these structure types, synthetic low‐temperature transformations are possible with the interdiffusion and exchange of functional cations possessing ns2 stereoactive lone pairs (e. g., Sn(II)) or unpaired ndx electrons (e. g., Co(II)), targeting new and favorable modulations of their electronic, magnetic, or catalytic properties. This enables a synergistic blending of new functionality to an underlying three‐dimensional connectivity, i. e., [‐M−O‐M‐O‐]n, that is maintained during the transformation. In many cases, this tactic represents the only known pathway to prepare thermodynamically unstable solids that otherwise would commonly decompose by phase segregation, such as that recently applied to the discovery of many new small bandgap semiconductors.

Kinetic Stabilization of the First Sn(II)-Perovskite Oxide as a Nanoshell

“…However, there has been a recent renaissance in the use of topotactic ion-exchange reactions in close-packed structures which has led to the discovery of a number of new, metastable functional solids. 27 The success of these reactions often relies upon the maintenance of the underlying close-packed structure during the exchange process, and which can be facilitated via the soft, or low temperature, exchange conditions. Inspired by these prior studies, the synthesis of core-shell BZT-SZT (Ba(Zr0.5Ti0.5)O3-Sn(Zr0.5Ti0.5)O3) compositions was investigated via low-temperature ion-exchange reactions, e.g., Ba(Zr0.5Ti0.5)O3 + 0.2 SnCl2 + 0.2 SnF2 → 0.6 Ba(Zr0.5Ti0.5)O3-0.4 Sn(Zr0.5Ti0.5)O3 + 0.4 BaClF.…”

“…Importantly, this suggests that potential routes to the synthesis of pure SZT could be to decrease the primary BZT particle size to better match the Sn-diffusion length observed via TEM, or alternatively, to improve the A-site cation diffusion such as through chemical doping. 27 A small region containing both Ba and Sn indicates that it is possible for both A-site cations to coexist in a disordered, solid-solution-type fashion and that the observation of two distinct regions is simply a consequence of the reaction kinetics. As seen in the line scan in Figure 2, the Sn-shell also contains both B-site cations, Ti and Zr, as well as O.…”

Synthesis and Kinetic Stabilization of a Theoretically Predicted Sn(II)-Perovskite Oxide as a Nanoshell

“…For this reason, layered or porous structures have historically been considered the most suitable for exhibiting facile ion-exchange owing to their relatively 'open' diffusion pathways. 27 However, there has been a recent renaissance in the use of topotactic ion-exchange reactions in close-packed structures which has led to the discovery of a number of new, metastable functional solids. The success of these reactions often relies upon the maintenance of the underlying close-packed structure during the exchange process, and which can be facilitated via the soft, or low temperature, exchange conditions.…”

Circumventing Thermodynamics to Synthesize Highly Metastable Tin(II) Perovskites: Nano Eggshells of SnHfO3

“…8 Topotactic ion-exchange mediated by a molten salt flux has recently been demonstrated as an effective method for the synthesis of highly metastable oxides. 6,9,10,41 These synthetic challenges are well exemplified in the pursuit of Sn(II)-based perovskite oxides over the past two decades as promising Pb(II)-free piezoelectrics or as semiconducting photocatalysts. For example, Sn(II)-based perovskites (e.g., Sn(Zr1-xTix)O3) have been predicted to have greater electric polarization than Pb(II) in addition to their reduced toxicity.…”

“…Additionally, these works highlighted the critical role of temperature in both the formation and decomposition of the metastable phase via ion-diffusion mechanisms. 9,41 Thus, in an effort to further drive the limits of metastability in Sn(II)-containing perovskites, we herein have employed a multi-pronged 'Chimie Douce' technique specifically in the barium hafnate perovskite system. BaHfO3 was chosen as a precursor for the synthesis of a model SnHfO3 perovskite owing to its high lattice cohesive energy (as exemplified by the high melting point for BaHfO3 of ~2620 ºC).…”