Surface- and Strain-Mediated Reversible Phase Transformation in Quantum-Confined ZnO Nanowires.

The phase stability of ZnO in a quantum-confinement size regime (sub-2-nm) remains fiercely debated. Applying in situ (scanning) transmission electron microscopy, we present the atomistic view of the phase transitions from the original wurtzite structure to an intermediate body-centered tetragonal and h-MgO structure under tensile strain in quantum-confined ZnO nanowires. Strikingly, such structural transitions are reversible after releasing the stress. Further theoretical calculations mirror the transition pathway and provide basic insight into the overall landscape regarding surface- and strain-dependent phase transition behavior. Our results provide the critical piece to solve the puzzle in phase stability of ZnO, which may prove essential for advancing a variety of nanotechnologies, e.g., quantum-dot light-emitting devices.

Atomistic Observation of Desodiation-Induced Phase Transition in Sodium Tungsten Bronze.

The phase instability in layered-structure Na0.5WO3.25 induced by the extraction of Na ions was investigated by applying transmission electron microscopy. Real-time atomic-scale observation reveals the phase transition pathway: Na0.5WO3.25 (triclinic) → NaxWO3 (cubic) → WO3 (monoclinic) with specific orientation relationships. The dynamic evolution of Na0.5WO3.25/NaxWO3 phase boundaries shows that Na0.5WO3.25 will cleave along the (100)T and (010)T and recrystallize as (101)C and (010)C of NaxWO3, respectively. The phase transition pathway can be well-explained according to the structural characteristics in the three phases. By better understanding of the phase instability induced by the extraction of Na ions in possible layered-structure cathode materials, this work provides a reference for the design of sophisticated strategies toward high-performance Na-ion batteries.

Microstructure of long period stacking ordered phase in an as-cast Mg-Ni-Zn-Dy alloy.

An as-cast Mg-5Ni-4Zn-1Dy (wt.%) alloy was prepared by traditional ingot metallurgy. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to study the microstructural characteristics of 14H LPSO phase in the as-cast Mg-5Ni-4Zn-1Dy (wt.%) alloy. Selected-area electron diffraction, convergent-beam electron diffraction, energy dispersive spectrum and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atomic-resolution energy dispersive spectrum (EDS) mapping were used to study the microscopic characteristics of the 14H LPSO phase. The unit cell of 14H LPSO phase has two ABCA-type building blocks and the Zn, Ni and Dy elements were enriched on the ABCA-type stacking sequence, especially on the B and C layers. Space group of the 14H LPSO phase was determined to be P63mc and atomic structural model was constructed.

Transmission electron microscopy investigations of the microstructures in rapidly solidified Mg-Sn ribbons.

Ribbons of an Mg-9.76 wt.%Sn alloy have been fabricated using rapid solidification technology. X-ray diffraction and transmission electron microscopy confirm that the rapidly solidified ribbons consist of α-Mg, ß-Mg2Sn and ß″-Mg3Sn phases. The α-Mg grains are refined in the ribbons. The predominant fraction of the ß-Mg2Sn phase distributes at the grain boundaries of the α-Mg grains. The minor fraction of the ß-Mg2Sn phase reveals a spherical morphology with a typical grain size of 170 nm. The orientation relationship between the ß″-Mg3Sn particles and α -Mg matrix is identified in the ribbons and an atomic structure model of the ß″-Mg3Sn phase is proposed.