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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., PbHfO₃in space groupPbam, 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 withPbamsymmetry, as confirmed by X‐ray and electron diffraction, elemental analysis, and¹¹⁹Sn Mössbauer spectroscopy. In situ diffraction shows SnHfO₃also 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.

 

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 ns²stereoactive lone pairs (e. g., Sn(II)) or unpaired nd^xelectrons (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 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. 

This Perspective addresses the current state-of-the-art with the development of multinary oxides—a family of compounds that has long interested Prof. John B. Goodenough. Specifically, here we focus on their use as photoelectrodes for solar fuels generation. Using optical data and assuming an idealized 100% incident photon-to-electron conversion efficiency, it is possible to project the maximum short circuit photocurrent efficiency to be expected for a given oxide semiconductor. The performance gap between this theoretical value and that realized experimentally, is shown to be sizable for all but a couple of candidates. The technical issues underlying this gap and strategies for closing it are presented below. 

Surface functionalized barium titanate (BaTiO 3 ) nanocrystals have been explored for highly tunable chemical and electronic properties, potentially of use in ceramic-polymer composites for flexible ferroelectric device applications, directed synthesis of ferroelectric thin films or other nano-architectures, and other potential applications. The detailed temperature dependent local structure evolution of BaTiO 3 nanocubes capped with nonpolar oleic acid (OA) and polar tetrafluoroborate (BF 4 − ) ligands are investigated using in situ synchrotron X-ray diffraction and pair distribution function (PDF) analysis, in conjunction with piezoresponse force microscopy (PFM) and 137 Ba nuclear magnetic resonance (NMR) spectroscopy measurements. Diffraction analysis reveals that nanocubes capped by polar BF 4 − ligands undergo sharper ferroelectric to paraelectric phase transitions than nanocubes capped with nonpolar OA ligands, with the smallest ∼12 nm nanocubes displaying no transition. Local non-centrosymmetric symmetry is observed by PDF analysis and confirmed by NMR, persisting across the phase transition temperature. Local distortion analysis, manifested in tetragonality ( c / a ) and Ti off-centering ( z Ti ) parameters, reveals distinct temperature and length-scale dependencies with particle size and capping group. Ferroelectric order is increased by polar BF 4 − ligands, which is corroborated by an enhancement of PFM response. 

Noncentrosymmetric hybrid framework (HF) materials are an important system in discovering new practical second-order nonlinear optical materials. We calculated the second harmonic generation (SHG) response of a noncentrosymmetric (NCS) organic–inorganic HF compound, CuMoO3(p2c) (p2c = pyrazine-2-carboxylate) to find that it exhibits the largest SHG response among all known NCS HF materials with one-dimensional helical chains. Further atom response theory analysis revealed that the metal atoms Cu and Mo contribute much more strongly than do nonmetal atoms in determining the strength of the SHG response, which is a novel example in nonlinear optical materials known to date.