Circumventing thermodynamics to synthesize highly metastable 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 in the form of nano eggshell particle morphologies. All past reported synthetic attempts have been unsuccessful owing to their metastable nature, i.e., by their thermodynamic instability towards decomposition to their constituent oxides. A new approach was discovered that finally provides an effective solution to surmounting this intractable synthetic barrier and which can be the key to unlocking the door to many other predicted metastable oxides. A low-melting KSn₂Cl₅ salt was utilized to achieve a soft topotactic exchange of Sn(ii) cations into a Ba-containing perovskite, i.e., BaHfO₃ with particle sizes of ∼350 nm, at a low reaction temperature of 200 °C. The resulting particles exhibit nanoshell-over-nanoshell morphologies, i.e., with SnHfO₃ forming as ∼20 nm thick shells over the surfaces of the BaHfO₃ eggshell particles. Formation of the metastable SnHfO₃ is found to be thermodynamically driven by the co-production of the highly stable BaCl₂ and KCl side products. Despite this, total energy calculations show that Sn(ii) distorts from the A-site asymmetrically and randomly and the interdiffusion has a negligible impact on the energy of the system (i.e., layered vs. solid solution). Additionally, nano eggshell particle morphologies of BaHfO₃ were found to yield highly pure SnHfO₃ for the first time, thus circumventing the intrinsic ion-diffusion limits occurring at this low reaction temperature. In summary, these results demonstrate that the metastability of many theoretically predicted Sn(ii)-perovskites can 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.