A correlated electron diffraction, in situ neutron diffraction and dielectric properties investigation of poled (1-x)Bi0.5Na0.5TiO3-xBaTiO3 ceramics

A correlated electron diffraction, temperature-dependent in situ neutron diffraction, and temperature-dependent dielectric properties investigation of poled (1-x)Bi0.5Na0.5TiO3-xBaTiO3 (BNTBT100x) (x ¼ 0.04, 0.07, and 0.12) samples has been carried out. The results show that the depolarization temperature, Td, of the rhombohedral BNTBT 4 sample is associated with the disappearance of G 6 1=2 [111]*p satellite reflections and aaa- octahedral tilting while that of the BNTBT 12 sample is associated with a metrically tetragonal to metrically cubic or pseudo-cubic symmetry. In the case of the poled BNTBT 7 sample in the MPB region, the dielectric properties show a quite distinct two stage transition from a room temperature clearly metrically tetragonal phase again to a metrically cubic or pseudo-cubic symmetry above 150 C. There is no apparent change in its average structure in vicinity of Td in BNTBT 7. Electron diffraction shows the presence of considerable octahedral tilt twin disorder in all three samples.J.W., Y.L., R.L.W., Q.L., and Y.P.G. appreciate the support of the Australian Research Council (ARC) in the form of a Discovery Grant. Y.L. also appreciates support from the ARC Future Fellowships program

In-situ neutron diffraction study of Pb(In₁/₂Nb₁/₂)O₃-Pb(Mg⅓Nb⅔)O₃-PbTiO₃ single crystals under uniaxial mechanical stress

In this paper, we report the phase transition behavior of ternary relaxor ferroelectric single crystals of 0.25Pb(In1/2Nb1/2)O₃-0.44Pb(Mg1/3Nb2/3)O₃-0.31PbTiO₃ subject to a uniaxial mechanical stress up to 400 MPa. The resultant in situneutron diffraction data are interpreted in terms of the polarization rotation theory and provide direct structural evidence for the stress-induced polarization rotation pathway deduced from studies of macroscopic physical properties under stress. It is suggested that an intermediate, metastable orthorhombic phase is induced above a critical pressure of ∼75 MPa. This critical stress level appears to be unaffected by sample poling although the ground states (at zero stress) for the poled and unpoled crystals are different. The critical stress level, however, does decrease with increasing temperature. The elastic behavior of the intermediate phases is also studied based on a calculation of the associated lattice strains.Q.L., Y.L., and R.L.W. acknowledge financial support from the Australian Research Council (ARC) in the form of an ARC Discovery Grant. Y.L. also acknowledges support from the ARC Future Fellowships program. The authors also thank the Australian Institute of Nuclear Science and Engineering (ANSIE) for financial support to access the national neutron facilities at ANSTO

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

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