2 research outputs found
Chessboard/Diamond Nanostructures and the <i>A</i>āsite Deficient, Li<sub>1/2ā3<i>x</i></sub> Nd<sub>1/2+<i>x</i></sub>TiO<sub>3</sub>, Defect Perovskite Solid Solution
The crystal chemical origin of nanoscale chessboard/diamond
ordering
in perovskite-related solid solutions of composition Li<sub>0.5ā3<i>x</i></sub>Nd<sub>0.5+<i>x</i></sub>TiO<sub>3</sub> (LNT, <i>x</i> ā¼ 0.02ā0.12) 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, ā2<i>a</i><sub>p</sub> Ć ā2<i>a</i><sub>p</sub>, as the local LNT
unit cell viewed along the [001] direction (where <i>a</i><sub>p</sub> 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 ā2<i>a</i><sub>p</sub> periodicity. Bright-field STEM image simulations performed
for this new model reproduced the experimentally observed ā2<i>a</i><sub>p</sub> lattice fringes, thus strongly suggesting
that the finer features of the high-resolution (S)ĀTEM 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<sub>0.5</sub>TiO<sub>3</sub> 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 ādifficult-dopantsā)
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 In<sup>3+</sup> (difficult-dopants) and Nb<sup>5+</sup> (easy-dopants)
codoped anatase TiO<sub>2</sub> 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 In<sup>3+</sup>, Nb<sup>5+</sup> codopants, their resultant Ti<sup>3+</sup>, 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 Ī±-PbO<sub>2</sub>-like metastable polymorphic phases and induce the abnormal
trans-regime structural transition of codoped anatase TiO<sub>2</sub> 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