Ordered and twinned structure in hexagonal-based potassium tungsten bronze nanosheets

Na+ Migration Mediated Phase Transitions Induced by Electric Field in the Framework Structured Tungsten Bronze

Framework structured tungsten bronzes serve as promising candidates for electrode materials in sodium-ion batteries (SIBs). However, the tungsten bronze framework structure changes drastically as mediated by the sodium ion concentration at high temperatures. While the three-dimensional ion channels facilitate fast ion storage and transport capabilities, the structural instability induced by Na⁺ migration is a big concern regarding the battery performance and safety, which unfortunately remains elusive. Here, we show the real-time experimental evidence of the phase transitions in framework structured Na_0.36WO_3.14 (triclinic phase) by applying different external voltages. The Na⁺-rich (Na_0.48WO₃, tetragonal phase) or -deficient (Na_xWO₃, x < 0.36, hexagonal phase) phase nucleates under the positive or negative bias, respectively. Combined with the theoretical calculations, the atomistic phase transition mechanisms associated with the Na⁺ migration are directly uncovered. Our work sheds light on the phase instability in sodium tungsten bronzes and paves the way for designing advanced SIBs with high-stability.

Cation Defect Mediated Phase Transition in Potassium Tungsten Bronze

Applying in situ transmission electron microscopy, the phase instability in potassium tungsten bronze (K_xWO₃, 0.18 < x < 0.57) induced by heating was investigated. The atomistic phase transition pathway of monoclinic K_0.20WO₃ → hexagonal K_mWO₃ (0.18 < m < 0.20) → cubic WO₃ induced by cationic defects (K and W vacancies) was directly revealed. Unexpectedly, a K⁺-rich tetragonal K_nWO₃ (0.40 < n < 0.57) phase would nucleate as well, which may result from the blockage of K⁺ diffusion at the grain boundaries. Our results point out the critical role of the cationic defects in mediating the crystal structures in K_xWO₃, which provide reference to rational structural design for extensive high-temperature applications.

Rational design of alkali-resistant catalysts for selective NO reduction with NH3

A rationale was proposed for designing alkali-resistant SCR catalysts, which have common characteristics of separate active sites and alkali-trapping sites.

Orientation domains in a monoclinic Mg–Al–O phase

Two types of Mg–Al–O structures were successfully synthesized under high temperature (above 1173 K). Transmission electron microscopy and group theory analysis reveal the existence of cubic MgAl2O4 and an unreported monoclinic MgAl x O y phase with four domain variants. The structural relationship between these two phases is discussed in detail. The results shed light on the structural investigation of Mg–Al–O oxides, which are important mineral components of the Earth's lower mantle as well as substrates for the epitaxial growth of semiconductor films. Monoclinic MgAl x O y nanowires with domain boundaries may also provide a possible high-strength candidate for industrial applications.

Atomic-Scale Study of Cation Ordering in Potassium Tungsten Bronze Nanosheets

It has long been accepted that the formation of superlattices in hexagonal-based potassium tungsten bronzes is attributed to K vacancies only, together with small displacements of W cations. Here, the superlattices within potassium tungsten bronze nanosheets both structurally and spectroscopically at atomic resolution using comprehensive transmission electron microscopy techniques are studied. The multidimensional chemical analyses are realized by energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy, the atomic-scale structures are characterized using aberration-corrected scanning transmission electron microscopy with high-angle annular-dark-field detector. The observed superstructures are mainly attributed to small amount of W vacancies within single atomic layer, which would recover to more uniform distributions of W vacancies with lower concentrations at higher temperature. The band regions of different orientation from the matrix tend to regulate the superstructures to be pinned along the same direction, forming domains of highly ordered structures. The characterization of cation ordering and recovery processes of nanostructures from chemical and structural point of view at atomic resolution enables rational design of optoelectronic devices with controlled physical properties.