A synthetic route has been discovered to thermodynamically unstable, i.e., metastable, Sn(II)−perovskite oxides that have been highly sought after as lead-free dielectrics and small bandgap semiconductors. A highly facile exchange of Sn(II) is found by using a low melting SnCl 2 /SnF 2 peritectic flux, yielding mixed A-site (Ba 1−x Sn x )ZrO 3 and mixed A-and B-site (Ba 1−x Sn x )(Zr 1−y Ti y )O 3 solid solutions that exhibit a very high metastability, with up to 60% Sn(II) cations and a calculated reaction energy for decomposition of up to −0.3 eV atom −1 . Kinetic stabilization of the higher Sn(II) concentrations is achieved by the high cohesive energy of the perovskite compositions containing Zr(IV) and mixed Zr(IV)/Ti(IV) cations. Significantly red-shifted bandgaps are found with increasing Sn(II) substitution, enabling the optical absorption edge to be broadly tuned from ∼3.90 to ∼1.95 eV. Percolation pathways are calculated to occur for BSZT compositions with >12.5% Sn(II) and >25% Ti(IV) cations. High photocatalytic rates are found for molecular oxygen production for compositions which exceed the percolation thresholds, wherein extended diffusion pathways should "open up" across the structure and the charge carriers become delocalized rather than trapped. These results establish the critical importance of synthetically accessing metastable semiconductors for the discovery of advanced optical and photocatalytic properties.
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.
A systematic study of (1−x)Pb(Fe0.5Nb0.5)O3–xBiFeO3 (x = 0–0.5) was performed in order to investigate the strengthening of the relaxor properties when adding BiFeO3 into Pb(Fe0.5Nb0.5)O3 and forming a solid solution.
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