Kun Xu

All Publications

• Topotactically transformable antiphase boundaries with enhanced ionic conductivity. Nature communications Xu, K., Hung, S. W., Si, W., Wu, Y., Huo, C., Yu, P., Zhong, X., Zhu, J. 2023; 14 (1): 7382

  Abstract

  Engineering lattice defects have emerged as a promising approach to effectively modulate the functionality of devices. Particularly, antiphase boundaries (APBs) as planar defects have been considered major obstacles to optimizing the ionic conductivity of mixed ionic-electronic conductors (MIECs) in solid oxide fuel applications. Here our study identifies topotactically transformable APBs (tt-APBs) at the atomic level and demonstrates that they exhibit higher ionic conductivity at elevated temperatures as compared to perfect domains. In-situ observation at the atomic scale tracks dynamic oxygen migration across these tt-APBs, where the abundant interstitial sites between tetrahedrons facilitate the ionic migration. Furthermore, annealing in an oxidized atmosphere can lead to the formation of interstitial oxygen at these APBs. These pieces of evidence clearly clarify that the tt-APBs can contribute to oxygen conductivity as anion diffusion channels, while the topotactically non-transformable APBs cannot. The topotactic transformability opens the way of defect engineering strategies for improving ionic transportation in MIECs.

  View details for DOI 10.1038/s41467-023-43086-5

  View details for PubMedID 37968326

  View details for PubMedCentralID PMC10651924

• Revealing the atomic mechanism of diamond-iron interfacial reaction CARBON ENERGY Ku, Y., Xu, K., Yan, L., Zhang, K., Song, D., Li, X., Li, S., Cheng, S., Shan, C. 2023

  View details for DOI 10.1002/cey2.440

  View details for Web of Science ID 001089954000001

• Colossal Ionic Conductivity in Interphase Strain-Engineered Nanocomposite Films. Journal of the American Chemical Society Huo, C., Xu, K., Ma, L., Li, T., Li, H., Yang, X., Kuang, X., Liu, S., Deng, S., Chen, J. 2023

  Abstract

  Owing to their wide application in oxide-based electrochemical and energy devices, ion conductors have attracted considerable attention. However, the ionic conductivity of the developed systems is still too low to satisfy the low-temperature application. In this study, by developing the emergent interphase strain engineering method, we achieve a colossal ionic conductivity in SrZrO3-xMgO nanocomposite films, which is over one order of magnitude higher than that of the currently widely used yttria-stabilized zirconia below 673 K. Atomic-scale electron microscopy studies ascribe this superior ionic conductivity to the periodically well-aligned SrZrO3 and MgO nanopillars that feature coherent interfaces. Wherein, a tensile strain as large as +1.7% is introduced into SrZrO3, expanding the c-lattice and distorting the oxygen octahedra to decrease the oxygen migration energy. Combining with theoretical assessments, we clarify the strain-dependent oxygen migration path and energy and unravel the mechanisms for strain-tuned ionic conductivity. This study provides a new scope for the property improvement of wide-range ion conductors by strain engineering.

  View details for DOI 10.1021/jacs.3c01298

  View details for PubMedID 37327186

• Ultrathin Non-Ising Charged Domain Walls Confined in BiFeO₃ Nanocrystals ADVANCED FUNCTIONAL MATERIALS Liu, L., Xu, K., Li, Q., Huang, Y., Shu, L., Cheng, Y., Zhang, S., Luo, J., Zhu, J., Li, J. 2022; 32 (46)

  View details for DOI 10.1002/adfm.202207730

  View details for Web of Science ID 000851282800001

• Direct investigation of the atomic structure and decreased magnetism of antiphase boundaries in garnet. Nature communications Xu, K., Lin, T., Rao, Y., Wang, Z., Yang, Q., Zhang, H., Zhu, J. 2022; 13 (1): 3206

  Abstract

  The ferrimagnetic insulator iron garnets, tailored artificially with specific compositions, have been widely utilized in magneto-optical (MO) devices. The adjustment on synthesis always induces structural variation, which is underestimated due to the limited knowledge of the local structures. Here, by analyzing the structure and magnetic properties, two different antiphase boundaries (APBs) with individual interfacial structure are investigated in substituted iron garnet film. We reveal that magnetic signals decrease in the regions close to APBs, which implies degraded MO performance. In particular, the segregation of oxygen deficiencies across the APBs directly leads to reduced magnetic elements, further decreases the magnetic moment of Fe and results in a higher absorption coefficient close to the APBs. Furthermore, the formation of APBs can be eliminated by optimizing the growth rate, thus contributing to the enhanced MO performance. These analyses at the atomic scale provide important guidance for optimizing MO functional materials.

  View details for DOI 10.1038/s41467-022-30992-3

  View details for PubMedID 35680884

  View details for PubMedCentralID PMC9184601

• Atomic-scale insights into quantum-order parameters in bismuth-doped iron garnet. Proceedings of the National Academy of Sciences of the United States of America Xu, K., Zhang, L., Godfrey, A., Song, D., Si, W., Zhao, Y., Liu, Y., Rao, Y., Zhang, H., Zhou, H. A., Jiang, W., Wang, W., Cheng, Z., Zhu, J. 2021; 118 (20)

  Abstract

  Bismuth and rare earth elements have been identified as effective substituent elements in the iron garnet structure, allowing an enhancement in magneto-optical response by several orders of magnitude in the visible and near-infrared region. Various mechanisms have been proposed to account for such enhancement, but testing of these ideas is hampered by a lack of suitable experimental data, where information is required not only regarding the lattice sites where substituent atoms are located but also how these atoms affect various order parameters. Here, we show for a Bi-substituted lutetium iron garnet how a suite of advanced electron microscopy techniques, combined with theoretical calculations, can be used to determine the interactions between a range of quantum-order parameters, including lattice, charge, spin, orbital, and crystal field splitting energy. In particular, we determine how the Bi distribution results in lattice distortions that are coupled with changes in electronic structure at certain lattice sites. These results reveal that these lattice distortions result in a decrease in the crystal-field splitting energies at Fe sites and in a lifted orbital degeneracy at octahedral sites, while the antiferromagnetic spin order remains preserved, thereby contributing to enhanced magneto-optical response in bismuth-substituted iron garnet. The combination of subangstrom imaging techniques and atomic-scale spectroscopy opens up possibilities for revealing insights into hidden coupling effects between multiple quantum-order parameters, thereby further guiding research and development for a wide range of complex functional materials.

  View details for DOI 10.1073/pnas.2101106118

  View details for PubMedID 33975955

  View details for PubMedCentralID PMC8157958

• Interfacial oxygen-octahedral-tilting-driven electrically tunable topological Hall effect in ultrathin SrRuO₃ films JOURNAL OF PHYSICS D-APPLIED PHYSICS Gu, Y., Wei, Y., Xu, K., Zhang, H., Wan, F., Li, F., Saleem, M., Chang, C., Sun, J., Song, C., Feng, J., Zhong, X., Liu, W., Zhang, Z., Zhu, J., Pan, F. 2019; 52 (40)

  View details for DOI 10.1088/1361-6463/ab2fe8

  View details for Web of Science ID 000477646100001