Switching Lead for Tin in PbHfO₃: Noncubic Structure of SnHfO₃**

Abstract

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.

Unveiling Stability Factors in Sn(II)-Containing Oxides: Discovery of a Polar Tin Titanate and Photocatalytic Activity for Overall Water Splitting

“…Surface corrosion by the SnClF flux is likely, as has been observed in similar reactions. 14 However, there was no detectable inclusion in the crystal structure, consistent with the full structure refinement of the powder XRD data. Contamination by Cl/F anions is not uncommon in halide-based flux reactions and requires follow-up studies of its impact on morphology and properties.…”

“…The higher Cl/F concentrations in the XPS data (∼10 atom %), was also consistent with their location at the particles’ surfaces. Surface corrosion by the SnClF flux is likely, as has been observed in similar reactions . However, there was no detectable inclusion in the crystal structure, consistent with the full structure refinement of the powder XRD data.…”

“…The stereoactive lone pair is formed by the hybridization of the Sn(5p) with the Sn(5s)–O(2p) antibonding orbitals, consistent with the revised lone pair model described earlier . The addition of Sn(II) into the SnLa 4 Ti 4 O 15 structure thus shows a significantly decreased optical band gap as in recent reports of Sn(II)-substituted oxides. ,, …”

“…For example, the recently reported SnHfO 3 decomposes into SnO and HfO 2 beginning at only ∼350 °C under vacuum. 14 However, SLTO retains its crystal structure even after annealing under vacuum at 700 °C. The color change is consistent with the disproportionation under vacuum of Sn(II) → Sn + Sn(IV) at the surfaces and material decomposition.…”

“…Given these challenges, the layered SnTiO 3 with the ilmenite structure type and Sn 2 TiO 4 have been the only phases reported in the Sn­(II)–Ti–O phase diagram. , Both compounds have been discovered only recently, with no successful reports of a perovskite-type STO due to these associated synthetic challenges. Recent reports have demonstrated the powerful utility of soft chemical, ion-exchange methods in the syntheses of new Sn­(II)-containing hafnate and zirconate perovskites (i.e., SnMO 3 ). ,,, The underlying factors governing stability and synthesizability have been hypothesized as follows: (1) kinetic stability is enhanced with (a) increased lattice energies of the substructure (i.e., MO 6 extended network), and (b) the absence of lower energy polymorphs accessible by ion diffusion, and (2) thermodynamic stability is increased with the coordination of the Sn­(II) cation in asymmetric coordination environments. These hypotheses, however, require significantly more investigation in wider theoretical and experimental studies, such as to enable a deeper understanding of the structural factors leading to the stability and synthesizability of Sn­(II)-containing oxides.…”