Optical control of 4f orbital state in rare-earth metals.

A change of orbital state alters the coupling between ions and their surroundings drastically. Orbital excitations are hence key to understand and control interaction of ions. Rare-earth elements with strong magneto-crystalline anisotropy (MCA) are important ingredients for magnetic devices. Thus, control of their localized 4f magnetic moments and anisotropy is one major challenge in ultrafast spin physics. With time-resolved x-ray absorption and resonant inelastic scattering experiments, we show for Tb metal that 4f-electronic excitations out of the ground-state multiplet occur after optical pumping. These excitations are driven by inelastic 5d-4f-electron scattering, altering the 4f-orbital state and consequently the MCA with important implications for magnetization dynamics in 4f-metals and more general for the excitation of localized electronic states in correlated materials.

Photoinduced Ultrafast Phase Transition in Bilayer CrI3.

In two-dimensional magnets, the ultrafast photoexcited method represents a low-power and high-speed method of switching magnetic states. Bilayer CrI3 (BLC) is an ideal platform for studying ultrafast photoinduced magnetic phase transitions due to its stacking-dependent magnetic properties. Here, by using time-dependent density functional theory, we explore the photoexcitation phase transition in BLC from the R- to M-stacked phase. This process is found to be induced by electron-phonon interactions. The activated Ag and Bg phonon modes in the xy direction drive the horizontal relative displacements between the layers. The activated Ag mode in the z direction leads to a transition potential reduction. Furthermore, this phase transition can invert the sign of the interlayer spin interaction, indicating a photoinduced transition from ferromagnet to antiferromagnet. This investigation has profound implications for magnetic phase engineering strategies.

Hydrogen in actinides: electronic and lattice properties.

Hydrides of actinides, their magnetic, electronic, transport, and thermodynamic properties are discussed within a general framework of H impact on bonding, characterized by volume expansion, affecting mainly the 5fstates, and a charge transfer towards H, which influences mostly the 6dand 7sstates. These general mechanisms have diverse impact on individual actinides, depending on the degree of localization of their 5fstates. Hydrogenation of uranium yields UH2and UH3, binary hydrides that are strongly magnetic due to the 5fband narrowing and reduction of the 5f-6dhybridization. Pu hydrides become magnetic as well, mainly as a result of the stabilization of the magnetic 5f5state and elimination of the admixture of the non-magnetic 5f6component.Ab-initiocomputational analyses, which for example suggest that the ferromagnetism ofß-UH3is rather intricate involving two non-collinear sublattices, are corroborated by spectroscopic studies of sputter-deposited thin films, yielding a clean surface and offering a variability of compositions. It is found that valence-band photoelectron spectra cannot be compared directly with the 5fnground-state density of states. Being affected by electron correlations in the excited final states, they rather reflect the atomic 5fn-1multiplets. Similar tendencies can be identified also in hydrides of binary and ternary intermetallic compounds. H absorption can be used as a tool for fine tuning of electronic structure around a quantum critical point. A new direction is represented by actinide polyhydrides with a potential for high-temperature superconductivity.

Itinerant-localized dichotomy in magnetic anisotropic properties of U-based ferromagnets.

The electronic structure, spin and orbital magnetic moments, and the magnetic anisotropy energy in selected U-based compounds are investigated making use of the correlated band theory. First, we demonstrate that the LSDA+U approach with exact atomic limit implemented as a combination of the relativistic density functional theory with the Anderson impurity model provides a good quantitative description for UGa[Formula: see text]. Further, the method is applied to UFe[Formula: see text] and UFe[Formula: see text]Si[Formula: see text] ferromagnets. The calculated positive uniaxial magnetic anisotropy together with negative enthalpy of formation for UFe[Formula: see text]Si[Formula: see text] make it as a candidate for the magnetically hard materials. Our studies suggest a viable route for further development of the rare-earth-lean permanent magnets by replacing a part of U atoms by some rare-earth like Sm in UFe[Formula: see text]Si[Formula: see text].

Lattice dynamics in CePd2Al2 and LaPd2Al2.

The interaction between phonons and 4f electrons, which is forming a new quantum state (quasi-bound state) beyond Born-Oppenheimer approximation, is very prominent and lattice dynamics plays here a key role. There is only a small number of compounds in which the experimental observation suggest such a scenario. One of these compounds is CePd2Al2. Here the study of phonon dispersion curves of (Ce,La)Pd2Al2 at 1.5, 7.5, 80 and 300 K is presented. The inelastic X-ray scattering technique was used for mapping the phonon modes at X and Z points as well as in Λ and Δ directions, where the symmetry analysis of phonon modes was performed. The measured spectra are compared with the theoretical calculation, showing very good agreement. The measurements were performed in several Brillouin zones allowing the reconstruction of phonon dispersion curves. The results are discussed with respect to the magneto-elastic interaction and are compared with other cerium compounds. The phonon mode symmetry A1g was found to be unaffected by the interaction, which is in contrast to previous assumptions.

Electrochemical Pourbaix diagrams of Mg-Zn alloys from first-principles calculations and experimental thermodynamic data.

Mg-Zn alloys have attracted much attention as biodegradable alloys owing to their superior mechanical properties and excellent biocompatibility. However, their corrosion/degradation behaviour has become a major issue for various biomedical applications. To understand their corrosion behaviours in aqueous environments, the first-principles informed Pourbaix diagrams, that is, electrochemical phase diagrams with respect to electrode potential and solution pH, were constructed for Mg-Zn alloys and compared with experimental observations. It was found that for Mg-rich alloys, the MgZn phase has a higher potential than the Mg matrix and may act as a cathode, resulting in galvanic corrosion, while for Zn-rich alloys, the phase Mg2Zn11 corrodes first. In Zn-rich alloys, Mg(OH)2 preferably precipitates under alkaline conditions, thus hindering the increase in pH and preventing the release of dissolved ZnO22- ions. In a Cl-containing solution, the soluble ZnCl2 eases the corrosion of the Zn matrix by decreasing the corrosion potential. These results are supported by various experimental observations; thus, they provide an in-depth understanding of the degradation behaviour of various Mg-Zn alloys as well as a feasible pathway in the design of biocompatible Mg-Zn alloys with first-principles informed Pourbaix diagrams.

Rational Design of Highly Stable and Active MXene-Based Bifunctional ORR/OER Double-Atom Catalysts.

Designing highly active and bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts has attracted great interest toward metal-air batteries. Herein, an efficient solution to the search for MXene-based bifunctional catalysts is proposed by introducing non-noble metals such as Fe/Co/Ni at the surfaces. These results indicate that the ultrahigh activities in Ni1/Ni2- and Fe1/Ni2-modified MXene-based double-atom catalysts (DACs) for bifunctional ORR/OER are better than those of well-known unifunctional catalysts with low overpotentials, such as Pt(111) for the ORR and IrO2 (110) for the OER. Strain can profoundly regulate the catalytic activities of MXene-based DACs, providing a novel pathway for tunable catalytic behavior in flexible MXenes. An electrochemical model, based on density functional theory and theoretical polarization curves, is proposed to reveal the underlying mechanisms, in agreement with experimental results. Electronic structure analyses indicate that the excellent catalytic activities in the MXene-based DACs are attributed to the electron-capturing capability and synergistic interactions between Fe/Co/Ni adsorbents and MXene substrate. These findings not only reveal promising candidates for MXene-based bifunctional ORR/OER catalysts but also provide new theoretical insights into rationally designing noble-metal-free bifunctional DACs.

MAELASviewer: An Online Tool to Visualize Magnetostriction.

The design of new materials for technological applications is increasingly being assisted by online computational tools that facilitate the study of their properties. In this work, based on modern web application frameworks, the online app MAELASviewer has been developed to visualize and analyze magnetostriction via a user-friendly interactive graphical interface. The features and technical details of this new tool are described in detail. Among other applications, it could potentially be used for the design of magnetostrictive materials for sensors and actuators.

Defect-Enhanced CO2 Reduction Catalytic Performance in O-Terminated MXenes.

Electrochemical carbon dioxide reduction reaction (CO2 RR) represents a promising way to generate fuels and chemical feedstock sustainably. Recently, studies have shown that two-dimensional metal carbides and nitrides (MXenes) can be promising CO2 RR electrocatalysts due to the alternating -C and -H coordination with intermediates that decouples scaling relations seen on transition metal catalysts. However, further by tuning the electronic and surface structure of MXenes it should still be possible to reach higher turnover number and selectivities. To this end, defect engineering of MXenes for electrochemical CO2 RR has not been investigated to date. In this work, first-principles modelling simulations are employed to systematically investigate CO2 RR on M2 XO2 -type MXenes with transition metal and carbon/nitrogen vacancies. We found that the -C-coordinated intermediates take the form of fragments (e. g., *COOH, *CHO) whereas the -H-coordinated intermediates form a complete molecule (e. g., *HCOOH, *H2 CO). Interestingly, the fragment-type intermediates become more strongly bound when transition-metal vacancies are present on most MXenes, while the molecule-type intermediates are largely unaffected, allowing the CO2 RR overpotential to be tuned. The most promising defective MXene is Hf2 NO2 containing Hf vacancies, with a low overpotential of 0.45 V. More importantly, through electronic structure analysis it could be observed that the Fermi level of the MXene changes significantly in the presence of vacancies, indicating that the Fermi level shift can be used as an ideal descriptor to rapidly predict the catalytic performance of defective MXenes. Such an evaluation strategy is applicable to other catalysts beyond MXenes, which could enhance high throughput screening efforts for accelerated catalyst discovery.

Building Fast Diffusion Channel by Constructing Metal Sulfide/Metal Selenide Heterostructures for High-Performance Sodium Ion Batteries Anode.

Heterostructure engineering is one of the most promising modification strategies toward improving sluggish kinetics for the anode of sodium ion batteries (SIBs). Herein, we report a systemic investigation on the different types of heterostructure interfaces' effects of discharging products (Na2O, Na2S, Na2Se) on the rate performance. First-principle calculations reveal that the Na2S/Na2Se interface possesses the lowest diffusion energy barrier (0.39 eV) of Na among three kinds of interface structures (Na2O/Na2S, Na2O/Na2Se, and Na2S/Na2Se) due to its smallest recorded interface deformation, similar electronegativity, and lattice constant. The experimental evidence confirms that the metal sulfide/metal selenide (SnS/SnSe2) hierarchical anode exhibits outstanding rate performance, where the normalized capacity at 10 A g-1 compared to 0.1 A g-1 is 45.6%. The proposed design strategy in this work is helpful to design high rate performance anodes for advanced battery systems.

Suppressed phase transition of a Rb/K incorporated inorganic perovskite with a water-repelling surface.

Inorganic cesium lead halide (CsPbI3) is a promising candidate for next-generation photovoltaic devices, but photoactive α-phase CsPbI3 can rapidly transform to non-photoactive yellow δ-CsPbI3 in a humid atmosphere. Here, we report that partial substitution of cesium by the potassium or rubidium element can effectively improve the phase stability against moisture by forming a water-repelling surface layer with Rb/K segregation. Using density functional theory, we found that the water-induced polarization, which triggers the PbI62- octahedron distortion and accelerates the phase transition, can be effectively alleviated by incorporating Rb/K elements. Further exploration of transition states suggests that Rb/K doped surface layers result in a higher activation barrier for water penetration. The electronic structure analysis further reveals that the barrier enhancement originates from the absence of the participation of inner 5p electrons in Rb/K-H2O binding, which induces a much lower energy barrier in pristine CsPbI3. Based on these improvements, the doped perovskites remained in the major α-phase after direct exposure to ambient air (RH ∼ 30%) for 5 hours, while pristine CsPbI3 showed an irreversible degradation. With the clarified mechanism of enhanced phase stability of Rb/K incorporation, we suggest such a doping method as a promising strategy to be widely applied in the field of photovoltaic devices.

Ultrastrong π-Bonded Interface as Ductile Plastic Flow Channel in Nanostructured Diamond.

A combinational effect of nanostructured crystallites and π-bonded interfaces is much attractive in solving the conflict between strength/hardness and toughness to design extrinsically superhard materials with enhanced fracture toughness and/or other properties such as tunable electronic properties. In the present work, taking the experimentally observed π-bonded interfaces in nanostructured diamond as the prototype, we theoretically investigated their stabilities, electronic structures, and mechanical strengths with special consideration of the size effect of nanocrystallites or nanolayers. It is unprecedentedly found that the π-bonded interfaces exhibit tunable electronic semiconducting properties, superior fracture toughness, and anomalously large creep-like plasticity at the cost of minor losses in strength/hardness; such unique combination is uncovered to be attributed to the ductile bridging effect of the sp2 bonds across the π-bonded interface that dominates the localized plastic flow channel. As the length scale of nanocrystallites/nanolayers is lower than a critical value, however, the first failure occurring inside nanocrystallites/nanolayers features softening and embrittling. These findings not only provide a novel insight into the unique strengthening and toughening origin observed in ultrahard nanostructured diamonds consisting of nanotwins, nanocomposites, and nanocrystallites but also highlight a unique pathway by combining the nanostructured crystallites and the strongly bonded interface to design the novel superhard materials with superior toughness.

Rational Design of Flexible Two-Dimensional MXenes with Multiple Functionalities.

In the past decade, two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides (MXenes) have attracted attention and interest from the scientific community due to their superior mechanical strength and flexibility, physical/chemical properties, and multiple exciting functionalities. Among these materials, the ingenious and effective combination of the mechanical and functional properties of MXenes provides a promising opportunity for designing flexible and wearable devices. This review summarizes the recent research progress in the structural stabilities, mechanical strength and deformation mechanism, strain-tunable energy storages, and catalytic and thermoelectric properties along with certain strain modifications and strain-controllable electronic/topological properties of MXenes from a combined theoretical and experimental perspective and illustrates their electronic origins. Taking the design principles as a focus, the theoretical predictions provide guidance, while the experimental work gives a thorough validation, thus setting the foundation for the current scientific achievements, challenges, and prospects in the field of MXenes.

A generalized solid strengthening rule for biocompatible Zn-based alloys, a comparison with Mg-based alloys.

Solid solution strengthening has been widely used in designing various high-performance biocompatible Mg-based alloys, but its transferability to other biocompatible metals such as Zn-based alloys is questionable or nearly absent. In the present study, an ab initio informed Peierls-Nabarro model and Leyson et al.'s strengthening model are used for a systematic investigation on solute strengthening in Zn-based alloys, which is compared with the widely studied Mg-based alloys. Although an inverse relationship was revealed between volume misfit εb and chemical misfit εSFE for both Zn-based and Mg-based alloys, most solutes would however result in positive εb and negative εSFE for Zn-based alloys, differing from Mg-based alloys. With εb and εSFE as two key descriptors, a generalized scaling diagram is finally drawn for a fast evaluation of solid solution strengthening in Zn-based alloys, indicating that the alkaline-earth and rare earth elements are better strengtheners for Zn-based alloys, which provides a general rule in designing novel biocompatible materials.

Catalytic Effect on CO2 Electroreduction by Hydroxyl-Terminated Two-Dimensional MXenes.

Electrocatalysis represents a promising method to generate renewable fuels and chemical feedstock from the carbon dioxide reduction reaction (CO2RR). However, traditional electrocatalysts based on transition metals are not efficient enough because of the high overpotential and slow turnover. MXenes, a family of two-dimensional metal carbides and nitrides, have been predicted to be effective in catalyzing CO2RR, but a systematic investigation into their catalytic performance is lacking, especially on hydroxyl (-OH)-terminated MXenes relevant in aqueous reaction conditions. In this work, we utilized first-principles simulations to systematically screen and explore the properties of MXenes in catalyzing CO2RR to CH4 from both aspects of thermodynamics and kinetics. Sc2C(OH)2 was found to be the most promising catalyst with the least negative limiting potential of -0.53 V vs RHE. This was achieved through an alternative reaction pathway, where the adsorbed species are stabilized by capturing H atoms from the MXene's OH termination group. New scaling relations, based on the shared H interaction between intermediates and MXenes, were established. Bader charge analyses reveal that catalysts with less electron migration in the *(H)COOH → *CO elementary step exhibit better CO2RR performance. This study provides new insights regarding the effect of surface functionalization on the catalytic performance of MXenes to guide future materials design.

Elaboration of Aggregated Polysulfide Phases: From Molecules to Large Clusters and Solid Phases.

With the increasing strategies aimed at repressing shuttle problems in the lithium-sulfur battery, dissolved contents of polysulfides are significantly reduced. Except for solid-state Li2S2 and Li2S, aggregated phases of polysulfides remain unexplored, especially in well confined cathode material systems. Here, we report a series of nanosize polysulfide clusters and solid phases from an atomic perspective. The calculated phase diagram and formation energy evolution process demonstrate their stabilities and cohesive tendency. It is interesting to find that Li2S6 can stay in the solid state and contains short S3 chains, further leading to the unique stability and dense structure. Simulated electronic properties indicate reduced band gaps when polysulfides are aggregated, especially for solid phase Li2S6 with a band gap as low as 0.47 eV. Their dissolution behavior and conversion process are also investigated, which provides a more realistic model and gives further suggestions on the future design of the lithium-sulfur battery.

Designing ultrastrong 5d transition metal diborides with excellent stability for harsh service environments.

Much effort was devoted towards the rational design of ultrastrong transition metal borides (TMBs) with remarkable mechanical properties and excellent stabilities, owing to promising applications in machining, drilling tools and protective coatings for the aerospace industry. Although an enormous number of investigations have been performed on these TMBs under normal conditions, studies on the stability and mechanical strength in harsh high-pressure environments, which are critical for safe service behavior and a realistic understanding of stabilities and strengthening mechanisms, are yet nearly absent. In this work, taking 5d TMB2 (TM = Hf, Ta, W, Re, Os, Ir and Pt) as an illustration, we performed comprehensive high-throughput first-principles screening for thermodynamically stable and metastable structures under various pressures. Four experimentally observed structures are found to be thermodynamically feasible for most 5d TMB2 (TM = Hf, Ta, W, Re, Os and Ir) at 0 and 100 GPa. By exploiting orbital-decomposed electronic structures, we reveal that the pressure-induced stabilization and phase transitions of 5d TMB2 can be rationalized by the splitting of bonding and antibonding states around the Fermi level. Further investigations on the pressure-induced strengthening indicate that 5d TMB2 in the hP6[194] structure exhibit a profound strengthening effect under high pressure, which can be rationalized by the proposed strengthening factor η, but η fails in the oP6[59] structure due to the changed instability modes at different pressures. These findings suggest the necessity to explore the plasticity parameters for a realistic understanding of pressure-induced strengthening in TMBs, providing a strong argument for rules based on bond parameters at equilibrium in designing strong solids.

Unique Double-Interstitialcy Mechanism and Interfacial Storage Mechanism in the Graphene/Metal Oxide as the Anode for Sodium-Ion Batteries.

Graphene/metal oxides (G/MO) composite materials have attracted much attention as the anode of sodium ion batteries (SIBs), because of the high theoretical capacity. However, most metal oxides operate based on the conversion mechanism and the alloying mechanism has changed to Na2O after the first cycle. The influence of G/Na2O (G/N) on the subsequent sodiation process has never been clearly elucidated. In this work, we report a systematic investigation on the G/N interface from both aspects of theoretical simulation and experiment characterization. By applied first-principles simulations, we find that the sluggish kinetics in the G/MO materials is mainly caused by the high diffusion barrier (0.51 eV) inside the Na2O bulk, while the G/N interface shows a much faster transport kinetics (0.25 eV) via unique double-interstitialcy mechanism. G/N interface possesses an interfacial storage of Na atom through the charge separation mechanism. The experimental evidence confirms that high interfacial ratio structure of G/N greatly improves the rate performance and endows G/MO materials the interfacial storage. Furthermore, the experimental investigation finds that the high interfacial ratio structure of G/N also benefits from the reversible reaction between SnO2 and Sn during cycling. Lastly, the effects of (N, O, S) doping in graphene systems at the G/N interface were also explored. This work provides a fundamental comprehension on the G/MO interface structure during the sodiation process, which is helpful to design energy storage materials with high rate performance and large capacity.

Mechanical Properties of the Quasi-One-Dimensional Antiferromagnet Cu(en)(H2O)2SO4.

The density functional theory was used to calculate elastic tensor for the quasi-one-dimensional antiferromagnetic compound Cu(en)(H2O)2SO4, with en = C2H8N2. The bulk, shear, Young moduli, Poisson ratio, and elastic anisotropy were determined for the polycrystalline Cu(en)(H2O)2SO4 by employing the Hill averaging of the single crystal elastic constants. They were compared to the respective quantities for other low-dimensional magnetic systems. The Cu(en)(H2O)2SO4 compound is predicted to be soft and ductile. We have also estimated the Debye temperature for the Cu(en)(H2O)2SO4 single crystal by using the sound velocities obtained from the calculated elastic constants.

Crystal Structure and Magnetic Properties of Uranium Hydride UH2 Stabilized as a Thin Film.

A new type of uranium binary hydride, UH2, with the CaF2 crystal structure, was synthesized in a thin-film form using reactive sputter deposition at low temperatures. The material has a grain size of 50-100 nm. The lattice parameter a = (535.98 ± 0.14 pm) is close to that in known Np (534.3 pm) and Pu (535.9 pm) iso-types. UH2 is a metallic ferromagnet with the Curie temperature TC ≈ 120 K. A very wide hysteresis loop indicates strong magnetocrystalline anisotropy. X-ray photoelectron spectroscopy reveals similarities with electronic structure of UH3, which is also ferromagnet with higher TC = 165 K.