Colossal Dielectric Behavior of Ga+Nb Co-Doped Rutile TiO₂

Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO₂, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO₂, i.e., (Ga_0.5Nb_0.5)_<i>x</i>Ti_1–<i>x</i>O₂, where Ga³⁺ is from the same group as In³⁺ but with a much smaller ionic radius. Colossal permittivity of up to 10⁴–10⁵ with an acceptably low dielectric loss (tan δ = 0.05–0.1) over broad frequency/temperature ranges is obtained at <i>x</i> = 0.5% after systematic synthesis optimizations. Systematic structural, defect, and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (∼10–40 K), polaronlike electron hopping/transport at higher temperatures, and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent observed CP. We believe that this work provides comprehensive guidance for the design of new CP materials

Superstructure of Mullite-type KAl₉O₁₄

Large whiskers of a new KAl₉O₁₄ polymorph with mullite-type structure were synthesized. The chemical composition of the crystals was confirmed by energy-dispersive X-ray spectroscopy, and the structure was determined using single-crystal X-ray diffraction. Nanosized twin domains and one-dimensional diffuse scattering were observed utilizing transmission electron microscopy. The compound crystallizes in space group <i>P</i>2₁/<i>n</i> (<i>a</i> = 8.1880(8), <i>b</i> = 7.6760(7), <i>c</i> = 8.7944(9) Å, β = 110.570(8)°, <i>V</i> = 517.50(9) ų, <i>Z</i> = 2). Crystals of KAl₉O₁₄ exhibit a mullite-type structure with linear edge-sharing AlO₆ octahedral chains connected with groups of two AlO₄ tetrahedra and one AlO₅ trigonal bipyramid. Additionally, disproportionation of KAl₉O₁₄ into K β-alumina and corundum was observed using in situ high-temperature optical microscopy and Raman spectroscopy

Chessboard/Diamond Nanostructures and the <i>A</i>‑site Deficient, Li_{1/2–3<i>x</i>} Nd_1/2+<i>x</i>TiO₃, Defect Perovskite Solid Solution

The crystal chemical origin of nanoscale chessboard/diamond ordering in perovskite-related solid solutions of composition Li_{0.5–3<i>x</i>}Nd_0.5+<i>x</i>TiO₃ (LNT, <i>x</i> ∼ 0.02–0.12) is investigated. Experimental and simulated scanning transmission electron microscopy (STEM) images are found to be consistent with the compositional modulation model proposed by previous authors. However, these earlier models do not satisfactorily explain other features observed in high-resolution STEM and TEM images, such as the two-dimensional {100} lattice fringes with the same periodicity, √2<i>a</i>_p × √2<i>a</i>_p, as the local LNT unit cell viewed along the [001] direction (where <i>a</i>_p is the parent perovskite unit cell parameter). Based on bond valence sum calculations, we propose a new set of crystal structures for LNT in which Li ions are primarily bonded to only two O ions, and order one-dimensionally with √2<i>a</i>_p periodicity. Bright-field STEM image simulations performed for this new model reproduced the experimentally observed √2<i>a</i>_p lattice fringes, thus strongly suggesting that the finer features of the high-resolution (S)­TEM images are the result of Li ion ordering and associated local structural relaxation. In this new model, the LNT chessboard supercell then results from the ordered combinations of two sublattices: the Li ion sublattice and its translational variants on the one hand, and the Nd_0.5TiO₃ sublattice and its oxygen octahedral tilt twin variants on the other. Dielectric measurements indicate the presence of long-lived polar clusters that are easily activated under an applied electric field. This suggests that these clusters consist of spatially correlated Li ions

Design Synthesis of Nitrogen-Doped TiO₂@Carbon Nanosheets toward Selective Nitroaromatics Reduction under Mild Conditions

The development of a facile, low-cost, and eco-friendly approach to the synthesis of aromatic amines remains a great scientific challenge. TiO₂, as a low-cost and earth abundant metal oxide, is usually not active for thermo-catalyzed nitro reduction. Herein, we report a composite nanosheet catalyst, composed of nitrogen-doped TiO₂ and carbon (N-TiO₂@C), which exhibits highly efficient, thermo-catalytic performance for selective nitroaromatic reduction at room temperature. The N-TiO₂@C nanosheet catalyst is synthesized via a facile approach where C₃N₄ nanosheets are utilized not only as a structure-directing agent to control the shape, size, and crystal phase of TiO₂ but also as a source of nitrogen for doping into both TiO₂ and carbon nanosheets. Furthermore, the origin of the superior performance of the N-TiO₂@C nanosheet composite catalyst, along with a possible nitroaromatic reduction mechanism, has also been explored

Above-Band Gap Photoinduced Stabilization of Engineered Ferroelectric Domains

The effect of above-band gap photons on the domains of the BiFeO₃ (BFO) thin film was investigated via piezoresponse force microscopy and Kelvin probe force microscopy. It is found that under above-band gap illumination, the relaxation time of the polarization state was significantly extended, while the effective polarizing voltage for the pristine domains was reduced. We propose that this photoinduced domain stabilization can be attributed to the interaction between photogenerated surface charges and domains. Importantly, a similar phenomenon is observed in other ferroelectric (FE) materials with an internal electric field once they are illuminated by above-band gap light, indicating that this photoinduced stabilization is potentially universal rather than specific to BFO. Thus, this study will not only contribute to the knowledge of photovoltaic (PV) phenomena but also provide a new route to promote the stability of PV and FE materials

Bimetallic Ions Codoped Nanocrystals: Doping Mechanism, Defect Formation, and Associated Structural Transition

Ionic codoping offers a powerful approach for modifying material properties by extending the selection of potential dopant ions. However, it has been a major challenge to introduce certain ions that have hitherto proved difficult to use as dopants (called “difficult-dopants”) into crystal structures at high concentrations, especially through wet chemical synthesis. Furthermore, the lack of a fundamental understanding of how codopants are incorporated into host materials, which types of defect structures they form in the equilibrium state, and what roles they play in material performance, has seriously hindered the rational design and development of promising codoped materials. Here we take In³⁺ (difficult-dopants) and Nb⁵⁺ (easy-dopants) codoped anatase TiO₂ nanocrystals as an example and investigate the doping mechanism of these two different types of metal ions, the defect formation, and their associated impacts on high-pressure induced structural transition behaviors. It is experimentally demonstrated that the dual mechanisms of nucleation and diffusion doping are responsible for the synergic incorporation of these two dopants and theoretically evidenced that the defect structures created by the introduced In³⁺, Nb⁵⁺ codopants, their resultant Ti³⁺, and oxygen vacancies are locally composed of both defect clusters and equivalent defect pairs. These formed local defect structures then act as nucleation centers of baddeleyite- and α-PbO₂-like metastable polymorphic phases and induce the abnormal trans-regime structural transition of codoped anatase TiO₂ nanocrystals under high pressure. This work thus suggests an effective strategy to design and synthesize codoped nanocrystals with highly concentrated difficult-dopants. It also unveils the significance of local defect structures on material properties