Vacancy Ordering and Lithium Insertion inⅢ_(2)Ⅵ_(3) Nanowires

Superlattice structures resulting from vacancy ordering have been observed in many materials.Here we report vacancy ordering behavior inⅢ_(2)Ⅵ_(3)nanowires.The formation of layer-like structural vacancies has been achieved during the synthesis of In_(2)Se_(3)nanowires through a vapor-transport route.Doping In_(2)Se_(3)nanowires with small amounts of Ga during synthesis can completely change the structural vacancy ordering from a layer-like to a screw-like pattern for(In_(x)Ga_(1-x))_(2)Se_(3)nanowires.Lithium atoms can fill in the layer-like structural vacancies of In_(2)Se_(3)nanowires and generate new types of vacancy and lithium atom ordering superlattices.The screw-patterned vacancies of(In_(x)Ga_(1-x))_(2)Se_(3)nanowires show reversible lithium insertion.Our results contribute to the understanding of structure property correlations ofⅢ_(2)Ⅵ_(3)materials used in lithium ion storage,photovoltaics,and phase change memory.

ORDERING OF VACANCIES IN RAPIDLYQUENCHED TiNi SHAPE MEMORY ALLOY

The modification of the electron diffraction pattern(EDP) of B2 in an as rapidly quenched then short time aged Ti Ni shape memory alloy(SMA) has been studied systematically using TEM. It was found that the diffusion scattering rings in EDP of an as rapidly quenched Ti-Ni SMA is associated with short range order (SRO) of vancancies in parent B2. After aging at 450℃ for 5 min the diffusion scattering rings are replaced by 1/2 {001} and 1 / 4 {012} extra reflections. It indicates that the SRO of vacancies has transformed into long range order (LRO) of vacancies,moreover,the LRO of vacancies exists in microdomains.

Specific Heat Capacity of A2FeCoO6-δ (A = Ca or Sr)

A2FeCoO6-δ (A = Ca or Sr) is synthesized by the solid-state synthesis method and their specific heat capacities are evaluated at 40˚C using a heat flow meter. The effect of the A-cation size on the specific heat capacity of these compounds is observed. The specific heat capacity of Sr2FeCoO6-δ is found to be the highest, and that of Ca2FeCoO6-δ is the lowest while CaSrFeCoO6-δ shows the intermediate value. The specific heat capacity decreases with the decrease of the average A-site ionic radius, demonstrating the relationship between heat capacity and A-site ionic radius. The relationship between specific heat capacity and molar mass is also confirmed as the δ value decreases or molar mass increases from Ca2FeCoO6-δ to CaSrFeCoO6-δ to Sr2FeCoO6-δ.

Insights into the enhanced structure stability and electrochemical performance of Ti^(4+)/F^(-) co-doped P2-Na_(0.67)Ni_(0.33)Mn_(0.67)O_(2) cathodes for sodium ion batteries at high voltage

P2-Na_(0.67)N_(i0.33)Mn_(0.67)O_(2)is considered as a promising cathode material for sodium-ion battery (SIBs)because of its high capacity and discharge potential.However,its practical use is limited by Na^(+)/vacancy ordering and P2-O2 phase transition.Herein,a Ti^(4+)/F^(-) co-doping strategy is developed to address these issues.The optimal P2-Na_(0.67)Ni_(0.33)Mn_(0.37)Ti_(0.3)O_(1.9)F_(0.1) exhibits much enhanced sodium storage performance in the high voltage range of 2.0–4.4 V,including a cycling stability of 77.2%over 300cycles at a rate of 2 C and a high-rate capability of 87.7 m Ah g^(-1) at 6 C.Moreover,the P2-Na_(0.67)Ni_(0.33)Mn_(0.37)Ti_(0.3)O_(1.9)F_(0.1) delivers reversible capacities of 82.7 and 128.1 m Ah g^(-1) at-10 and 50℃ at a rate of 2 C,respectively.The capacity retentions over 200 cycles at-10℃ is 94.2%,implying more opportunity for practical application.In-situ X-ray diffraction analysis reveals that both P2-O2 phase transitions and Na^(+)/vacancy ordering is suppressed by Ti^(4+)/F^(-) co-doping,which resulting in fast Na^(+) diffusion and stable phase structure.The hard carbon//P2-Na_(0.67)Ni_(0.33)Mn_(0.37)Ti_(0.3)O_(1.9)F_(0.1) full cell exhibits a high energy density of 310.2 Wh kg^(-1) and remarkable cyclability with 82.1%retention after 300 cycles at 1 C in the voltage range of 1.5–4.2 V.These results demonstrate that the co-doping Ti^(4+)/F^(-) is a promising strategy to improve the electrochemical properties of P2-Na_(0.67)Ni_(0.33)Mn_(0.67)O_(2),providing a facile tactic to develop high performance cathode materials for SIBs.

MBenes:Two-dimensional transition-metal borides with ordered metal vacancies

2D MBenes have been theoretically predicted to possess unique electronic structures and physicochemical properties,and thus shown great promise in various applications.However,the synthesis of individual single-layer MBene remains a grand challenge due to its orthorhombic structure of MAB phases.Recently,scientists from Link?ping University have fabricated 2D monolayer Mo4/3B2-xTzwith ordered metal vacancies.Their results demonstrated the feasibility of top-down approach by chemical exfoliation of laminated compounds and provided the principle for further preparation of a wealth of MBenes.

Magnetic order driven by orbital ordering in the semiconducting KFe1.5Se2

The two-orbital Hubbard model is studied numerically by using the Hartree-Fock approximation in both real space and momentum space, and the ground-state properties of the alkali metal iron selenide semiconducting KFel.5Se2 are investigated. A rhombus-type Fe vacancy order with stripe- type antiferromagnetic （AFM） order is found, as was observed in neutron scattering experiments [J. Zhao, et al., Phys. Rev. Lett. 109, 267003 （2012）]. Hopping parameters are obtained by fitting the experimentally observed stripe AFM phase in real space. These hopping parameters are then used to study the ground-state properties of the semiconductor in momentum space. It is found to be a strongly correlated system with a large on-site Coulomb repulsion U, similar to the AFM Mort insulator -- the parent compound of copper oxide superconductors. We also find that the electronic occupation numbers and magnetizations in the dxz and dyz orbitals become different simultaneously when U 〉 Uc （～3.4 eV）, indicating orbital ordering. These results imply that the rotational symmetry between the two orbitals is broken by orbital ordering and thus drives the strong anisotropy of the magnetic coupling that has been observed by experiments and that the stripe-type AFM order in this compound may be caused by orbital ordering together with the observed large anisotropy.

Vacancy manipulating of molybdenum carbide MXenes to enhance Faraday reaction for high performance lithium-ion batteries

“Intrinsic”strategies for manipulating the local electronic structure and coordination environment of defect-regulated materials can optimize electrochemical storage performance.Nevertheless,the structure–activity relationship between defects and charge storage is ambiguous,which may be revealed by constructing highly ordered vacancy structures.Herein,we demonstrate molybdenum carbide MXene nanosheets with customized in-plane chemical ordered vacancies(Mo_(1.33)CT_(x)),by utilizing selective etching strategies.Synchrotron-based X-ray characterizations reveal that Mo atoms in Mo1.33CTx show increased average valence of+4.44 compared with the control Mo_(2)CT_(x).Benefited from the introduced atomic active sites and high valence of Mo,Mo_(1.33)CT_(x)achieves an outstanding capacity of 603 mAh·g^(−1)at 0.2 A·g^(−1),superior to most original MXenes.Li+storage kinetics analysis and density functional theory(DFT)simulations show that this optimized performance ensues from the more charge compensation during charge–discharge process,which enhances Faraday reaction compared with pure Mo_(2)CT_(x).This vacancy manipulation provides an efficient way to realize MXene’s potential as promising electrodes.