(276e) Controlling Structural Transformations in Sr(Ti,Nb)O3 Nanocrystals Using Defect Chemistry for Next Generation Ferroelectric Devices

Among various perovskite oxides, strontium titanate (SrTiO₃, STO) is known for its interesting electronic properties which arise due to phase transitions induced via external stresses (temperature, pressure, and doping). Recently, the family of strontium niobate (Sr_1-xNb_1-yO_3+δ, SNO) metal oxides, which are isostructural to STO, have attracted interest for their plasmonic and low-loss response making them promising materials for optoelectronic devices due to their unique optical, magnetic, and electronic properties. Upon Ca doping the SNO lattice, local dipoles are induced by the off-center Ca ions which results in the observed macroscopic ferroelectricity. The current project proposes manipulating the crystal symmetry/structure of a Sr(Ti,Nb)O₃ (STNO) host nanocrystal via doping (Ca) to obtain novel ferroelectric responses. STNO nanoparticles were synthesized via a two-step co-precipitation/pressure-controlled molten salt technique. This low-pressure synthetic route limits available oxygen during the crystallization process allowing for systematic control of oxygen vacancies and dopant incorporation. The nanoparticle crystal structure was modified by Ca doping and the effect of this symmetry transformation on the A-site environment is systematically studied using Eu³⁺ as a photoluminescent probe. Preliminary XRD measurements affirm the presence of two successive structural transitions with increasing Ca doping, i.e. from cubic (Pm-3m) to tetragonal (P4mm) symmetry at 10 mol% Ca and to orthorhombic (Pnma) symmetry at 80 mol% Ca concentration. Interestingly, these structural transformations are observed at lower Ca doping concentrations than the thin film analogue due to the presence of ferroelectric distortions, which result in symmetry breakdown. Additionally, the local environment surrounding the Ca atom, probed via X-ray absorption spectroscopy and Photoluminescence spectroscopy, indicates that the reconstruction of the infused lattice results in reduction of A-site symmetry. Furthermore, X-ray photoelectron and Photoluminescence spectroscopy studies reveal that the presence of oxygen vacancies also break the symmetry around the A-site. Overall, these local/bulk structural and optoelectronic characterization results demonstrate that the composition-induced site asymmetry in low dimensional STNO nanoparticles can induce ferroelectric polarization, which is imperative for the design of next-generation ferroelectric devices.