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

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.