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.
Sn(II)-based perovskite oxides, being the subject of longstanding theoretical interest for the past two decades, have been synthesized for the first time in the form of nano eggshell particles. All...
Sn(II)-based perovskite oxides, being the subject of longstanding theoretical interest for the past two decades, have been synthesized for the first time with a nanoshell approach. All past reported synthetic attempts had been rendered impotent by the extremely high metastabilities, i.e., thermodynamic instability. Herein, a soft topotactic exchange of Sn(II) cations into Ba-containing perovskites is demonstrated to successfully yield ~20 nm thick shells of Sn(II) perovskites, i.e. SnHfO3. Additionally, highly pure SnHfO3 was obtained for the first time as nano-eggshell morphologies that circumvent the intrinsic ion-diffusion limits occurring at a low reaction temperature of 200 oC. In summary, the high metastability of the Sn(II) perovskites is shown to be overcome by leveraging the high cohesive energies of the reactants, the exothermic formation of a stable salt side product, and a shortened diffusion pathway for the Sn(II) cations. The new approach finally provides an effective solution to surmounting highly intractable synthetic barriers, and which can be the key to unlocking the door to many other new metastable oxides.
The removal of lead from commercialized perovskite-oxide-based piezoceramics has been a recent major topic in materials research owing to legislation in many countries. In this regard, Sn(II)-perovskite oxides have garnered keen interest due to their predicted large spontaneous electric polarizations and isoelectronic nature for substitution of Pb(II) cations. However, they have not been considered synthesizable owing to their high metastability. Herein, the perovskite lead hafnate, i.e., PbHfO3 in space group Pbam, is shown to react with SnClF at a low temperature of 300 °C, and resulting in the first complete Sn(II)-for-Pb(II) substitution, i.e. SnHfO3. During this topotactic transformation, a high purity and crystallinity is conserved with Pbam symmetry, as confirmed by X-ray and electron diffraction, elemental analysis, and 119Sn Mössbauer spectroscopy. In situ diffraction shows SnHfO3 also possesses reversible phase transformations and is potentially polar between ~130-200 °C. This so-called ‘de-leadification’ is thus shown to represent a highly useful strategy to fully remove lead from perovskite-oxide-based piezoceramics, and opening the door to new explorations of polar and antipolar Sn(II)-oxide materials.
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