Interchain-expanded extra-large-pore zeolites.

Stable aluminosilicate zeolites with extra-large pores that are open through rings of more than 12 tetrahedra could be used to process molecules larger than those currently manageable in zeolite materials. However, until very recently1-3, they proved elusive. In analogy to the interlayer expansion of layered zeolite precursors4,5, we report a strategy that yields thermally and hydrothermally stable silicates by expansion of a one-dimensional silicate chain with an intercalated silylating agent that separates and connects the chains. As a result, zeolites with extra-large pores delimited by 20, 16 and 16 Si tetrahedra along the three crystallographic directions are obtained. The as-made interchain-expanded zeolite contains dangling Si-CH3 groups that, by calcination, connect to each other, resulting in a true, fully connected (except possible defects) three-dimensional zeolite framework with a very low density. Additionally, it features triple four-ring units not seen before in any type of zeolite. The silicate expansion-condensation approach we report may be amenable to further extra-large-pore zeolite formation. Ti can be introduced in this zeolite, leading to a catalyst that is active in liquid-phase alkene oxidations involving bulky molecules, which shows promise in the industrially relevant clean production of propylene oxide using cumene hydroperoxide as an oxidant.

Regulating the Potential of Anion Redox to Reduce the Voltage Hysteresis of Li-Rich Cathode Materials.

Layered Li-rich oxides (LROs) that exhibit anionic and cationic redox are extensively studied due to their high energy storage capacities. However, voltage hysteresis, which reduces the energy conversion efficiency of the battery, is a critical limitation in the commercial application of LROs. Herein, using two Li2RuO3 (LRO) model materials with C2/c and P21/m symmetries, we explored the relationship between voltage hysteresis and the electronic structure of Li2RuO3 by neutron diffraction, in situ X-ray powder diffraction, X-ray absorption spectroscopy, macro magnetic study, and electron paramagnetic resonance (EPR) spectroscopy. The charge-transfer band gap of the LRO cathode material with isolated eg electron filling decreases, reducing the oxidation potential of anion redox and thus displaying a reduced voltage hysteresis. We further synthesized Mn-based Li-rich cathode materials with practical significance and different electron spin states. Low-spin Li1.15Ni0.377Mn0.473O2 with isolated eg electron filling exhibited a reduced voltage hysteresis and high energy conversion efficiency. We rationalized this finding via density functional theory calculations. This discovery should provide critical guidance in designing and preparing high-energy layered Li-rich cathode materials for use in next-generation high-energy-density Li-ion batteries based on anion redox activity.

A Distinct Spin Structure and Giant Baromagnetic Effect in MnNiGe Compounds with Fe-Doping.

Spin structure of a magnetic system results from the competition of various exchange couplings. Pressure-driven spin structure evolution, through altering interatomic distance, and hence, electronic structure produces baromagnetic effect (BME), which has potential applications in sensor/actuator field. Here, we report a new spin structure(CyS-AFMb) with antiferromagnetic(AFM) nature in Fe-doped Mn0.87Fe0.13NiGe. Neutron powder diffraction (NPD) under in situ hydrostatic pressure and magnetic field was conducted to reveal the spin configuration and its instabilities. We discovered that a pressure higher than 4 kbar can induce abnormal change of Mn(Fe)-Mn(Fe) distances and transform the CyS-AFMb into a conical spiral ferromagnetic(FM) configuration(45°-CoS-FMa) with easily magnetized but shortened magnetic moment by as much as 22%. The observed BME far exceeds previous reports. Our first-principles calculations provide theoretical supports for the enhanced BME. The compressed lattice by pressure favors the 45°-CoS-FMa and significantly broadened 3d bandwidth of Mn(Fe) atoms, which leads to the shortened magnetic moment and evolution of spin structure.

NaOH-Intercalated Iron Chalcogenides (Na1-xOH)Fe1-yX (X = Se, S): Ion-Exchange Synthesis and Physical Properties.

The discovery of the (Li1-xFexOH)FeSe superconductor has aroused significant interest in metal hydroxide-intercalated iron chalcogenides. However, all efforts made to intercalate NaOH between FeSe and FeS layers have failed so far. Here we report two NaOH-intercalated iron chalcogenides (Na1-xOH)Fe1-yX (X = Se, S) that were synthesized by a low-temperature hydrothermal ion-exchange method. Their crystal structures were solved through single-crystal X-ray diffraction and refined against powder X-ray and neutron diffraction data. Different from the (Li1-xFexOH)FeX superconductors that crystallize in a tetragonal space group P4/nmm with Z = 2, (Na1-xOH)Fe1-yX belong to an orthorhombic space group Cmma with Z = 4. The structural solution also reveals that there are vacancies in both Na and Fe sites and there are not iron ions in the (Na1-xOH) layer. This is probably why both Fe(II) and Fe(III) species exist in the title compounds, as detected by X-ray photoelectron spectroscopy. Based on magnetization and electrical resistivity measurements, the two compounds were found to be paramagnetic semiconductors. The absence of superconductivity should be closely related to the iron vacancies in the Fe1-yX layer. Theoretical calculations suggest that inducing superconductivity in (Na1-xOH)Fe1-ySe is promising due to the similarity of the electronic structures between stoichiometric (NaOH)FeSe and the (Li1-xFexOH)FeSe superconductor.

Atomic reconstruction for realizing stable solar-driven reversible hydrogen storage of magnesium hydride.

Reversible solid-state hydrogen storage of magnesium hydride, traditionally driven by external heating, is constrained by massive energy input and low systematic energy density. Herein, a single phase of Mg2Ni(Cu) alloy is designed via atomic reconstruction to achieve the ideal integration of photothermal and catalytic effects for stable solar-driven hydrogen storage of MgH2. With the intra/inter-band transitions of Mg2Ni(Cu) and its hydrogenated state, over 85% absorption in the entire spectrum is achieved, resulting in the temperature up to 261.8 °C under 2.6 W cm-2. Moreover, the hydrogen storage reaction of Mg2Ni(Cu) is thermodynamically and kinetically favored, and the imbalanced distribution of the light-induced hot electrons within CuNi and Mg2Ni(Cu) facilitates the weakening of Mg-H bonds of MgH2, enhancing the "hydrogen pump" effect of Mg2Ni(Cu)/Mg2Ni(Cu)H4. The reversible generation of Mg2Ni(Cu) upon repeated dehydrogenation process enables the continuous integration of photothermal and catalytic roles stably, ensuring the direct action of localized heat on the catalytic sites without any heat loss, thereby achieving a 6.1 wt.% H2 reversible capacity with 95% retention under 3.5 W cm-2.

Thermally Induced Oxygen Vacancies and High Oxide Ion Conduction in K2ZnV2O7 with a Melilite-Related Structure.

Donor-doped melilite materials with interstitial oxygen defects in the structure are good oxide ion conductors with negligible electronic conduction and show great potential in the ceramic electrolyte of intermediate-temperature solid oxide fuel cells (IT-SOFC). However, the parent melilite-structured materials with stoichiometric oxygen are usually insulators. Herein, we reported high and pure oxide ion conduction in the parent K2ZnV2O7 material with a melilite-related structure, e.g., ∼1.14 × 10-3 S/cm at 600 °C, which is comparable to that of the state-of-the-art yttrial-stabilized ZrO2 applied in practical fuel cells. Neutron diffraction data revealed the interesting thermally induced formation of oxygen vacancies at elevated temperatures, which triggered the transformation of the material from electronically conducting to purely and highly oxide ion-conducting. The VO4 tetrahedron with non-bridging terminal oxygen in K2ZnV2O7 was proved to be the key structural factor for transporting oxygen vacancies. The molecular dynamic simulation based on the interatomic potential approach revealed that long-range oxide ion diffusion was achieved by breaking and re-forming the 5-fold MO4 (M = Zn and V) tetrahedral rings. These findings enriched our knowledge of melilite and melilite-related materials, and creating oxygen vacancies in a melilite-related material may be a new strategy for developing novel oxide ion conductors.

Compressive-Strain-Facilitated Fast Oxygen Migration with Reversible Topotactic Transformation in La0.5Sr0.5CoOx via All-Solid-State Electrolyte Gating.

Modifying the crystal structure and corresponding functional properties of complex oxides by regulating their oxygen content has promising applications in energy conversion and chemical looping, where controlling oxygen migration plays an important role. Therefore, finding an efficacious and feasible method to facilitate oxygen migration has become a critical requirement for practical applications. Here, we report a compressive-strain-facilitated oxygen migration with reversible topotactic phase transformation (RTPT) in La0.5Sr0.5CoOx films based on all-solid-state electrolyte gating modulation. With the lattice strain changing from tensile to compressive strain, significant reductions in modulation duration (∼72%) and threshold voltage (∼70%) for the RTPT were observed, indicating great promotion of RTPT by compressive strain. Density functional theory calculations verify that such compressive-strain-facilitated efficient RTPT comes from significant reduction of the oxygen migration barrier in compressive-strained films. Further, ac-STEM, EELS, and sXAS investigations reveal that varying strain from tensile to compressive enhances the Co 3d band filling, thereby suppressing the Co-O hybrid bond in oxygen vacancy channels, elucidating the micro-origin of such compressive-strain-facilitated oxygen migration. Our work suggests that controlling electronic orbital occupation of Co ions in oxygen vacancy channels may help facilitate oxygen migration, providing valuable insights and practical guidance for achieving highly efficient oxygen-migration-related chemical looping and energy conversion with complex oxides.

Enhanced Performance of ΔTad upon Frequent Alternating Magnetic Fields in FeRh Alloys by Introducing Second Phases.

The cyclability and frequency dependence of the adiabatic temperature change (ΔTad) under an alternating magnetic field (AMF) are significantly important from the viewpoint of refrigeration application. Our studies demonstrated, by direct measurements, that the cyclability and low-magnetic-field performance of ΔTad in FeRh alloys can be largely enhanced by introducing second phases. The ΔTad under a 1.8 T, 0.13 Hz AMF is reduced by 14%, which is much better than that (40-50%) of monophase FeRh previously reported. More importantly, the introduction of second phases enables the antiferromagnetic-ferromagnetic phase transition to be driven by a lower magnetic field. Thus, ΔTad is significantly enhanced under a 0.62 T, 1 Hz AMF, and its value is 70% larger than that of monophase FeRh previously reported. Although frequency dependence of ΔTad occurs, the specific cooling power largely increases by 11 times from 0.17 to 1.9 W/g, as the frequency increases from 1 to 18.4 Hz under an AMF of 0.62 T. Our analysis of the phase transition dynamics based on magnetic relaxation measurements indicates that the activation energy barrier is lowered owing to the existence of second phases in FeRh alloys, which should be responsible for the reduction of the driving field. This work provides an effective way to enhance the cyclability and low-magnetic-field performance of ΔTad under an AMF in FeRh alloys by introducing second phases.

Negative Thermal Expansion in the Materials With Giant Magnetocaloric Effect.

Negative thermal expansion (NTE) behaviors in the materials with giant magnetocaloric effects (MCE) have been reviewed. Attentions are mainly focused on the hexagonal Ni2In-type MM'X compounds. Other MCE materials, such as La(Fe,Si)13, RCo2, and antiperovskite compounds are also simply introduced. The novel MCE and phase-transition-type NTE materials have similar physics origin though the applications are distinct. Spin-lattice coupling plays a key role for the both effect of NTE and giant MCE. Most of the giant MCE materials show abnormal lattice expansion owing to magnetic interactions, which provides a natural platform for exploring NTE materials. We anticipate that the present review can help finding more ways to regulate phase transition and dig novel NTE materials.