Vacancy-Induced Niobate Perovskite-Tungsten Bronze Composite for Synergetic Tuning of Ferroelectricity and Band Gaps

Niobate perovskites
like the (K,Na)­NbO3 (KNN) family
are among the most important lead-free ferroelectrics. Ba and Ni have
been co-doped into KNN to induce Ni2+-oxygen vacancy defect
dipoles to significantly reduce the band gap while maintaining the
ferroelectricity. This opens doors to novel optoferroelectric applications
such as multisensors and photocatalysts. However, obtaining a single
phase of the above co-doped KNN is difficult due to the sensitive
stoichiometry on phase formation and arduous nickel diffusion into
the KNN unit cells during synthesis. This paper reports an alternative
approach to simultaneously tune the band gap and ferroelectricity.
A-site vacancies are intentionally introduced into the mixtures of
starting reactants. The homogeneously distributed vacancies trigger
self-assembly of a niobate perovskite and niobate tungsten bronze
phase and thus form a composite. The interface between the two phases,
which mimics a heterojunction, rather than any individual phase, is
proven to be responsible for the resultant narrow band gap and strong
ferroelectricity. Hypotheses are proposed based on the results of
ferroelectric, photoconductivity, and density functional theory-based
studies to explain the mechanism. This paper offers an additional
option to engineer polarizations and band structures in complex photoferroelectric
oxides, especially alkaline niobates.