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Shear-Assisted Formation of Cation-Disordered Rocksalt NaMO 2 (M = Fe or Mn)
Herein we report the synthesis of cation-disordered rocksalt NaMO 2 (M = 3d transition metal), i.e., NaFeO 2–δ (δ ~ 0.18), for the first time via a martensitic-like phase transformation from β-NaFeO 2 . This disordered rocksalt NaFeO 2–δ does not exhibit long-range cation ordering, as evidenced by synchrotron X-ray diffraction, neutron diffraction, and transmission electron microscopy analyses. Pair distribution function analysis revealed that although the cation sublattice mostly preserved the rocksalt symmetry, O atoms were significantly displaced to accommodate the different Na + –O, Fe 2+ –O, and Fe 3+ –O bond lengths (~2.4, 2.1, and 2 Å, respectively). In addition, we confirmed the presence of O vacancies and partial Fe reduction from 3+ to 2+ in NaFeO 2–δ based on 57 Fe Mössbauer and X-ray absorption spectroscopy analyses. Controlled experiments revealed that three factors are essential for the successful synthesis of the rocksalt structure: (1) an appropriate precursor structure, (2) the application of a large stress by high-energy milling, and (3) the creation of O vacancies via carbon reduction. Density functional theory calculations demonstrated that these factors substantially affect the energetics of the minimum-energy reaction path of the phase transformation, making the highly metastable disordered rocksalt phase accessible. Our work suggests the effectiveness of mechanical milling to access metastable phases that are structurally related to the precursors via a shearing transformation. Utilizing this mechanism, we were able to predict and synthesize another cation-disordered rocksalt structure, NaMnO 2 , and explain the syntheses of several other related materials.
Direct Mechanochemical Synthesis, Phase Stability, and Electrochemical Performance of α-NaFeO 2
To better understand polymorph control in transition metal oxides, the mechanochemical synthesis of NaFeO 2 was explored. In this paper, we report the direct synthesis of α-NaFeO 2 through a mechanochemical process. By milling Na 2 O 2 and γ-Fe 2 O 3 for 5 h, α-NaFeO 2 was prepared without high-temperature annealing needed in other synthesis methods. While investigating the mechanochemical synthesis, it was observed that changing the starting precursors and mass of precursors affects the resulting NaFeO 2 structure. Density functional theory calculations on the phase stability of NaFeO 2 phases show that the α phase is stabilized over the β phase in oxidizing environments, which is provided by the oxygen-rich reaction between Na 2 O 2 and Fe 2 O 3 . This provides a possible route to understanding polymorph control in NaFeO 2 . Annealing the as-milled α-NaFeO 2 at 700 °C has resulted in increased crystallinity and structural changes that improved electrochemical performance in terms of capacity over the as-milled sample.
Density Functional Theory Study of Local Environment Effects on Oxygen Vacancy Properties in Magnetite
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LaMnO 3 Dopants for Efficient Thermochemical Water Splitting Identified by Density Functional Theory Calculations
While ceria is the standard material for two-step water splitting, perovskites are emerging as viable alternatives. In this work, based on the orthorhombic LaMnO 3 supercell, we substitute Li Na K Rb Mg Ca Sr Ba on the A-sites (La sites) and Al Ga In Mg Zn on the B-sites (Mn sites) at a concentration of 37.5%. The range of temperature and oxygen partial pressure at which each composition is stable is predicted. For compositions that are stable in relevant temperature and pressure ranges, the oxygen vacancy formation energies are determined for all of the oxygen vacancy site positions available in the computational supercell. Mg, Ca, Sr, and Ba A-site-substituted LaMnO 3 and Al and In B-site-substituted LaMnO 3 meet these two criteria for candidates in solar-thermal water splitting applications. Finally, oxygen vacancy formation energy can also be controlled by adjusting the doping strategy.
Activating Nitrogen for Electrochemical Ammonia Synthesis via an Electrified Transition-Metal Dichalcogenide Catalyst
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Stacking Faults and Topological Properties in MnBi 2 Te 4 : Reconciling Gapped and Gapless States
Despite theoretical predictions of a gapped surface state for the magnetic topological insulator MnBi 2 Te 4 (MBT), there has been a series of experimental evidence pointing toward gapless states. Here, we theoretically explore how stacking faults could influence the topological characteristics of MBT. We envisage a scenario that a stacking fault exists at the surface of MBT, causing the uppermost layer to deviate from the ground state and its interlayer separation to be expanded. This stacking fault with modulated interlayer couplings hosts a nearly gapless state within the topmost layer due to charge redistribution as the outermost layer recedes. Furthermore, we find evidence of spin-momentum locking and preservation of weak band inversion in the gapless surface state, suggesting the nontrivial topological surface states in the presence of the stacking fault. In conclusion, our findings provide a plausible elucidation to the long-standing conundrum of reconciling the observation of gapped and gapless states on MBT surfaces.
In-Situ Observation of the Continuous Phase Transition in Determining the High Thermoelectric Performance of Polycrystalline Sn 0.98 Se
Here, we report a comprehensive in-situ phase-change study on polycrystalline Sn 0.98 Se via high-temperature X-ray diffraction and in-situ high-voltage transmission electron microscopy from room temperature to 843 K. The results clearly demonstrate a continuous phase transition from Pnma to Cmcm starting from 573 to 843 K, rather than a sudden transition at 800 K. We also find that the thermal-conductivity rise at high temperature after the phase transition, as commonly seen in pristine SnSe, does not occur in Sn 0.98 Se, leading to a high thermoelectric figure of merit. Density functional theory calculations reveal the origin to be the suppression of bipolar thermal conduction in the Cmcm phase of Sn 0.98 Se due to the enlarged bandgap. This work fills the gap of in-situ characterization on polycrystalline Sn 0.98 Se and provides new insights into the outstanding thermoelectric performance of polycrystalline Sn 0.98 Se.
A Two-Dimensional MoS 2 Catalysis Transistor by Solid-State Ion Gating Manipulation and Adjustment (SIGMA)
A variety of methods including tuning chemical compositions, structures, crystallinity, defects and strain, and electrochemical intercalation have been demonstrated to enhance the catalytic activity. However, none of these tuning methods provide direct dynamical control during catalytic reactions. Here we propose a new method to tune the activity of catalysts through solid-state ion gating manipulation and adjustment (SIGMA) using a catalysis transistor. SIGMA can electrostatically dope the surface of catalysts with a high electron concentration over 5 × 10 13 cm –2 and thus modulate both the chemical potential of the reaction intermediates and their electrical conductivity. The hydrogen evolution reaction (HER) on both pristine and defective MoS 2 were investigated as model reactions. Furthermore, our theoretical and experimental results show that the overpotential at 10 mA/cm 2 and Tafel slope can be in situ, continuously, dynamically, and reversibly tuned over 100 mV and around 100 mV/dec, respectively.
Pseudosymmetry in Tetragonal Perovskite SrIrO 3 Synthesized under High Pressure
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Graphene Activated Magnesium Diboride for Moderate Pressure and Temperature Hydrogenation to Magnesium Borohydride
The hydrogenation conditions of magnesium diboride (MgB2) to magnesium borohydride (Mg(BH4)2) can be significantly enhanced through the discovery of improved modifiers. This study demonstrates that the modification of MgB2 by mechanical milling with graphene nanoplatelets significantly reduces the hydrogenation conditions of MgB2 from 900 bar and 400 degrees C for pure MgB2 to 400 bar and 300 degrees C while achieving 77% conversion to Mg(BH4)2. The introduction of the graphene additives coupled with milling leads to a reduction of the temperature and pressure required for bulk hydrogenation by 100 degrees C and 500 bar, respectively, from that of pure MgB2. The identification of graphene additives that drastically improve the hydrogenation conditions of MgB2 represents an important step toward improving hydrogen uptake kinetics to Mg(BH4)2.
CeTi 2 O 6 —A Promising Oxide for Solar Thermochemical Hydrogen Production
A large entropy of reduction is crucial in achieving materials capable of high-efficiency solar thermochemical hydrogen (STCH) production through two-step thermochemical water splitting cycles. We have recently demonstrated that the onsite electronic entropy of reduction attains an extreme value of 4.26 kB at 1500 K in Ce 4+ → Ce 3+ redox reactions, which explains the high performance and uniqueness of CeO 2 as an archetypal STCH material. However, ceria requires high temperatures (T > 1500 °C) to achieve a reasonable reduction extent because of its large reduction enthalpy, which is a major obstacle in practical applications. Therefore, new materials with a large entropy of reduction and lower reduction enthalpy are required. Here, we perform a systematic screening to search for Ce 4+ -based oxides which possess thermodynamics superior to CeO 2 for STCH production. We first search the Inorganic Crystal Structure Database (ICSD) and literature for Ce 4+ -based oxides and subsequently use density functional theory to compute their reduction enthalpies (i.e., oxygen vacancy formation energies). Here, we find that CeTi 2 O 6 with the brannerite structure is the most promising candidate for STCH because it possesses three essential characteristics of an STCH material: (i) a smaller reduction enthalpy compared to ceria yet large enough to split water, (ii) a high thermal stability, as reported experimentally, and (iii) a large entropy of reduction associated with Ce 4+ → Ce 3+ redox. Our proposed design strategy suggests that further exploration of Ce 4+ oxides for STCH production is warranted.
Simultaneous Successive Twinning Captured by Atomic Electron Tomography
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Telecom-Wavelength Single-Photon Emitters in Multilayer InSe
The development of robust and efficient single-photon emitters (SPEs) at telecom wavelengths is critical for advancements in quantum information science. Two-dimensional (2D) materials have recently emerged as promising sources for SPEs, owing to their high photon extraction efficiency, facile coupling to external fields, and seamless integration into photonic circuits. In this study, we demonstrate the creation of SPEs emitting in the 1000–1550 nm near-infrared range by coupling 2D indium selenide (InSe) with strain-inducing nanopillar arrays. The emission wavelength exhibits a strong dependence on the number of layers. Hanbury Brown and Twiss experiments conducted at 10 K reveal clear photon antibunching, confirming the single-photon nature of the emissions. In conclusion, density-functional-theory calculations and scanning-tunneling-microscopy analyses provide insights into the electronic structures and defect states, elucidating the origins of the SPEs.
Stacking Faults Assist Lithium-Ion Conduction in a Halide-Based Superionic Conductor
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Tracking oxygen vacancy migration in memristor devices using operando hard X-ray photoelectron spectroscopy
Fabrication of ultrathin (sub-2 nm) oxide semiconductor memristors poses the fundamental challenge of achieving oxide growth with atomic precision in terms of electronic structure and defect formation. Recently, ultrathin memristors consisting of bilayers of mixed Al 2 O 3 and MgO atomic layers were fabricated using an in vacuo atomic layer deposition process. This approach offers a unique platform for precise atomic control of oxygen vacancies in the device in which the vacancies are introduced by placing MgO atomic layers between pristine Al 2 O 3 layers. In this work, we present a systematic operando Hard X-ray PhotoElectron Spectroscopy (HAXPES) study of the switching of such memristors, combined with complementary current-voltage and capacitance-voltage (C-V) measurements. We used a memristor stack of Al 2 O 3 /MgO atomic layers, with the MgO-containing oxide deposited on the bottom Al metal electrode and a pure Al 2 O 3 layer below the top Pd electrode. HAXPES analysis shows a substantial change in the chemical shift of the Aluminum oxide when switching between the ”OFF” and ”ON” states indicative of a redistribution of oxygen vacancies in the device active layer. Interestingly, subsequent switching to the OFF state shows hysteretic behavior indicating the retention of some oxygen vacancies in the top Al 2 O 3 layer. This vacancy retention can be correlated with the stochastic behavior of the switching voltage observed in these devices. C–V measurements show a clear frequency-dependent response in the OFF state, consistent with enhanced polarization and vacancy trapping at low frequencies.
Recent progress of MnBi 2 Te 4 epitaxial thin films as a platform for realising the quantum anomalous Hall effect
Since the first realisation of the quantum anomalous Hall effect (QAHE) in a dilute magnetic-doped topological insulator thin film in 2013, the quantisation temperature has been limited to less than 1 K due to magnetic disorder in dilute magnetic systems. With magnetic moments ordered into the crystal lattice, the intrinsic magnetic topological insulator MnBi 2 Te 4 has the potential to eliminate or significantly reduce magnetic disorder and improve the quantisation temperature. Surprisingly, to date, the QAHE has yet to be observed in molecular beam epitaxy (MBE)-grown MnBi 2 Te 4 thin films at zero magnetic field, and what leads to the difficulty in quantisation is still an active research area. Although bulk MnBi 2 Te 4 and exfoliated flakes have been well studied, revealing both the QAHE and axion insulator phases, experimental progress on MBE thin films has been slower. Furthermore, understanding how the breakdown of the QAHE occurs in MnBi 2 Te 4 thin films and finding solutions that will enable mass-produced millimetre-size QAHE devices operating at elevated temperatures are required. In this mini-review, we will summarise recent studies on the electronic and magnetic properties of MBE MnBi 2 Te 4 thin films and discuss mechanisms that could explain the failure of the QAHE from the aspects of defects, electronic structure, magnetic order, and consequences of their delicate interplay. Finally, we propose several strategies for realising the QAHE at elevated temperatures in MnBi 2 Te 4 thin films.