High-Temperature Mechanical and Dynamical Properties of γ-(U,Zr) Alloys.

High-temperature body-centered cubic (BCC) γ-U is effectively stablized by γ-(U,Zr) alloys that also make it feasible to use it as a nuclear fuel. However, relatively little research has focused on γ-(U,Zr) alloys due to their instability at room temperature. The effect of Zr composition on its mechanical properties is not clear yet. Herein, we perform molecular dynamics simulations to investigate the mechanical and dynamical stabilities of γ-(U,Zr) alloys under high temperatures, and we calculate the corresponding lattice constants, various elastic moduli, Vickers hardness, Debye temperature, and dynamical structure factor. The results showed that γ-U, ß-Zr, and γ-(U,Zr) are all mechanically and dynamically stable at 1200 K, which is in good agreement with the previously reported high-temperature phase diagram of U-Zr alloys. We found that the alloying treatment on γ-U with Zr can effectively improve its mechanical strength and melting points, such as Vickers hardness and Debye temperature, making it more suitable for nuclear reactors. Furthermore, the Zr concentrations in γ-(U,Zr) alloys have an excellent effect on these properties. In addition, the dynamical structure factor reveals that γ-U shows different structural features after alloying with Zr. The present simulation data and insights could be significant for understanding the structures and properties of UZr alloy under high temperatures.

Localizing Oxygen Lattice Evolutions Eliminates Oxygen Release and Voltage Decay in All-Mn-Based Li-Rich Cathodes.

All-Mn-based Li-rich cathodes Li2 MnO3 have attracted extensive attention because of their cost advantage and ultrahigh theoretical capacity. However, the unstable anionic redox reaction (ARR), which involves irreversible oxygen releases, causes declines in cycling capacity and intercalation potential, thus hindering their practical applications. Here, it is proposed that introducing stacking-fault defects into the Li2 MnO3 can localize oxygen lattice evolutions and stabilize the ARR, eliminating oxygen releases. The thus-made cathode has a highly reversible capacity (320 mA h g-1 ) and achieves excellent cycling stability. After 100 cycles, the capacity retention rate is 86% and the voltage decay is practically eliminated at 0.19 mV per cycle. Attributing to the stable ARR, samples show reduced stress-strain and phase transitions. Neutron pair distribution function (nPDF) measurements indicate that there is a structure response of localized oxygen lattice distortion to the ARR and the average oxygen lattice framework is well-preserved which is a prerequisite for the high cycle reversibility.

Theoretical prediction of superconductivity in two-dimensional MXenes of molybdenum carbides.

Theoretically and experimentally, MXenes consisting of Mo and C have aroused much interest due to superconductivity in their films and even monolayer forms. Here, based on first-principles calculations, we systematically calculate the electronic structure, phonon dispersion, and electron-phonon coupling (EPC) of monolayer Mo2C (both T- and H-phases), Mo3C2, and Mo3C3. The results show that H-MoxCy (x = 2 or 3, y = 1-3) always have lower total energies than their corresponding T phase and other configurations. All these two-dimensional (2D) molybdenum carbides are metals and some of them display weak phonon-mediated superconductivity at different superconducting transition temperatures (Tc). The Mo 4d-orbitals play a critical role in their electronic properties and the Mo atomic vibrations play a dominant role in their low-frequency phonons, EPC, and superconductivity. By comparison, we find that increasing the Mo content can enhance the EPC and Tc. Besides, we further explore the impact of strain engineering on their superconducting related physical quantities. With increasing biaxial stretching, the phonon dispersions are gradually softened to form some soft modes, which can trigger some peaks of α2F(ω) in the low-frequency region and evidently increase the EPC λ. The Tc of H-Mo2C can be increased up to 11.79 K. Upon further biaxial stretching, charge density waves may appear in T-Mo2C, H-Mo3C2, and H-Mo3C3.

Point Defects Stability, Hydrogen Diffusion, Electronic Structure, and Mechanical Properties of Defected Equiatomic γ(U,Zr) from First-Principles.

At present, many experimental fast reactors have adopted alloy nuclear fuels, for example, U-Zr alloy fuels. During the neutron irradiation process, vacancies and hydrogen (H) impurity atoms can both exist in U-Zr alloy fuels. Here, first-principles density functional theory (DFT) is employed to study the behaviors of vacancies and H atoms in disordered-γ(U,Zr) as well as their impacts on the electronic structure and mechanical properties. The formation energy of vacancies and hydrogen solution energy are calculated. The effect of vacancies on the migration barrier of hydrogen atoms is revealed. The effect of vacancies and hydrogen atom on densities of states and elastic constants are also presented. The results illustrate that U vacancy is easier to be formed than Zr vacancy. The H interstitial prefers the tetrahedral site. Besides, U vacancy shows H-trap ability and can raise the H migration barrier. Almost all the defects lead to decreases in electrical conductivity and bulk modulus. It is also found that the main effect of defects is on the U-5f orbitals. This work provides a theoretical understanding of the effect of defects on the electronic and mechanical properties of U-Zr alloys, which is an essential step toward tailoring their performance.

Ultralow thermal conductivity and anisotropic thermoelectric performance in layered materials LaMOCh (M = Cu, Ag; Ch = S, Se).

In layered materials with the stacking axis perpendicular to the basal plane, anharmonicity strongly affects phonon propagation due to weak interlayer coupling, which is helpful to reduce the lattice thermal conductivity and improve the thermoelectric (TE) performance significantly. By combining first-principles calculations and the Boltzmann transport equation, we systematically analyzed and evaluated the lattice thermal conductivity and TE properties of LaMOCh (M = Cu, Ag; Ch = S, Se). The results indicate that these layered materials exhibit ultralow lattice thermal conductivities of 0.24-0.37 W m-1 K-1 along the interlayer direction at room temperature. The low lattice thermal conductivities have been analyzed from some inherent phonon properties, such as low acoustic phonon group velocity, large Grüneisen parameters, and a short phonon relaxation time. Originating from their natural layered crystal structure, the thermal and electronic transports (i.e., thermal conductivity, Seebeck coefficient, and electrical conductivity) are both highly anisotropic between their intralayer and interlayer directions. Finally, we obtained ZT values of 1.17 and 1.26 at 900 K along the interlayer direction for n-type LaCuOSe and LaAgOSe, respectively. Generally, LaMOSe exhibit larger anisotropy than LaMOS, in both n- and p-types of doping. Our findings of low thermal conductivities and large anisotropic TE performances of these layered systems should stimulate much attention in BiCuOSe and alike layered TE families.

Superconductivity and topological states in hexagonal TaC and NbC.

Materials with superconductivity and a nontrivial band structure near the Fermi level are promising candidates in realizing topological superconductivity. Using first-principles calculations, we systematically investigated the stability, mechanical properties, superconductivity, electronic structures, and topological states of hexagonal TaC and NbC. The results show that they are stable and have excellent mechanical properties. We predicted that these two carbides are strong electron-phonon coupling superconductors with superconducting transition temperatures of 14.8 and 17.1 K, respectively. Strong coupling is mainly dominated by in-plane Ta/Nb atomic vibrations and in-plane Ta/Nb-dxy/dx2-y2 electronic orbitals. The electronic structure calculations demonstrate that a nodal line and a triply degenerate point coexist when not including the spin-orbit coupling (SOC) effect. After including the SOC effect, the nodal line is gapped. The complicated surface states are also calculated and need further experiments to verify. The present results indicate that the hexagonal TaC and NbC are potential candidates as topological superconductors, and pave the way towards exploring the superconductivity and topological materials in condensed matter systems.

Two Dimensional MOene: From Superconductors to Direct Semiconductors and Weyl Fermions.

The number of semiconducting MXenes with direct band gaps is extremely low; thus, it is highly desirable to broaden the MXene family beyond carbides and nitrides to expand the palette of desired chemical and physical properties. Here, we theoretically report the existence of the single-layer (SL) dititanium oxide 2H-Ti2O MOene (MXene-like 2D transition oxides), showing an Ising superconducting feature. Moreover, SL halogenated 2H- and 1T-Ti2O monolayers display tunable semiconducting features and strong light-harvesting ability. In addition, the external strains can induce Weyl fermions via quantum phase transition in 2H-Ti2OF2 and Ti2OCl2 monolayers. Specifically, 2H- and 1T-Ti2OF2 are direct semiconductors with band gaps of 0.82 and 1.18 eV, respectively. Furthermore, the carrier lifetimes of SL 2H- and 1T-Ti2OF2 are evaluated to be 0.39 and 2.8 ns, respectively. This study extends emerging phenomena in a rich family of 2D MXene-like MOene materials, which provides a novel platform for next-generation optoelectronic and photovoltaic fields.

Strain-Enhanced Thermoelectric Performance in GeS2 Monolayer.

Strain engineering has attracted extensive attention as a valid method to tune the physical and chemical properties of two-dimensional (2D) materials. Here, based on first-principles calculations and by solving the semi-classical Boltzmann transport equation, we reveal that the tensile strain can efficiently enhance the thermoelectric properties of the GeS2 monolayer. It is highlighted that the GeS2 monolayer has a suitable band gap of 1.50 eV to overcome the bipolar conduction effects in materials and can even maintain high stability under a 6% tensile strain. Interestingly, the band degeneracy in the GeS2 monolayer can be effectually regulated through strain, thus improving the power factor. Moreover, the lattice thermal conductivity can be reduced from 3.89 to 0.48 W/mK at room temperature under 6% strain. More importantly, the optimal ZT value for the GeS2 monolayer under 6% strain can reach 0.74 at room temperature and 0.92 at 700 K, which is twice its strain-free form. Our findings provide an exciting insight into regulating the thermoelectric performance of the GeS2 monolayer by strain engineering.

A family of superconducting boron crystals made of stacked bilayer borophenes.

Monolayer borophenes tend to be easily oxidized, while thicker borophenes have stronger antioxidation properties. Herein, we proposed four novel metallic boron crystals by stacking the experimentally synthesized borophenes, and one of the crystals has been reported in our previous experiments. Bilayer units tend to act as blocks for crystals as determined by bonding analyses. Their kinetic, thermodynamic and mechanical stabilities are confirmed by our calculated phonon spectra, molecular dynamics and elastic constants. Our proposed allotropes are more stable than the boron α-Ga phase below 1000 K at ambient pressure. Some of them become more stable than the α-rh or γ-B28 phases at appropriate external pressure. More importantly, our calculations show that three of the proposed crystals are phonon-mediated superconductors with critical temperatures of about 5-10 K, higher than those of most superconducting elemental solids, in contrast to typical boron crystals with significant band gaps. Our study indicates a novel preparation method for metallic and superconducting boron crystals dispensing with high pressure.

Monolayer SnI2: An Excellent p-Type Thermoelectric Material with Ultralow Lattice Thermal Conductivity.

Using density functional theory and semiclassical Boltzmann transport equation, the lattice thermal conductivity and electronic transport performance of monolayer SnI2 were systematically investigated. The results show that its room temperature lattice thermal conductivities along the zigzag and armchair directions are as low as 0.33 and 0.19 W/mK, respectively. This is attributed to the strong anharmonicity, softened acoustic modes, and weak bonding interactions. Such values of the lattice thermal conductivity are lower than those of other famous two-dimensional thermoelectric materials such as MoO3, SnSe, and KAgSe. The two quasi-degenerate band valleys for the valence band maximum make it a p-type thermoelectric material. Due to its ultralow lattice thermal conductivities, coupled with an ultrahigh Seebeck coefficient, monolayer SnI2 possesses an ultrahigh figure of merits at 800 K, approaching 4.01 and 3.34 along the armchair and zigzag directions, respectively. The results indicate that monolayer SnI2 is a promising low-dimensional thermoelectric system, and would stimulate further theoretical and experimental investigations of metal halides as thermoelectric materials.

Superconductivity in Mo-P compounds under pressure and in double-Weyl semimetal Hex-MoP2.

Based on first-principles calculations, we predict five global stable molybdenum phosphorus compounds in the pressure range of 0-300 GPa. All of them display superconductivity with different transition temperatures. Meanwhile, we find that a metastable crystal hex-MoP2, crystallized in a noncentrosymmetric structure, is a double-Weyl semimetal and the Weyl point is in the H-K path. The long Fermi arcs and the topological surface states, which can be observed by angle-resolved photoemission spectroscopy, emerge at the (100) surface below the Fermi level. Furthermore, we find that the superconductivity in hex-MoP2 can be enhanced by carrier doping. Due to the breaking of inversion symmetry, the unconventional spin-triplet pairing coexists with spin-singlet pairing in channel . Based on our theoretical model, there are the superconducting band gaps in both pairings. Our work provides a new platform of hex-MoP2 for studying both topological double-Weyl semimetal and superconductivity.

Low thermal conductivity and high performance anisotropic thermoelectric properties of XSe (X = Cu, Ag, Au) monolayers.

Combining density functional theory (DFT) and semi-classic Boltzmann transport theory, we report the thermoelectric (TE) performance of a family of two-dimensional (2D) group IB-selenides XSe (X = Cu, Ag, Au). The results show that these monolayers exhibit small and anisotropic phonon velocities (0.98-3.84 km s-1), large Grüneisen parameters (up to 100), and drastic phonon scattering between the optical and acoustic phonons. These intrinsic properties originate from strong phonon anharmonicity and suppress the heat transport capacity, resulting in low lattice thermal conductivities (12.54 and 1.22 W m-1 K-1) along the x- and y-directions for a CuSe monolayer. Among our studied monolayers, the 2D CuSe monolayer possesses the most remarkable TE performance with ultrahigh ZT (3.26) for n-type doping along the y-direction at 300 K. CuSe monolayer can achieve higher thermoelectric conversion efficiency at a lower synthetic preparation cost than the expensive AgSe and AuSe monolayers, and our work provides a theoretical basis for paving the way for further experimental studies.

Surface passivation induced a significant enhancement of superconductivity in layered two-dimensional MSi2N4 (M = Ta and Nb) materials.

Two-dimensional (2D) transition metal di-nitrides (TMN2) have been arousing great interest for their unique mechanic, electronic, optoelectronic, and magnetic properties. The recent successful growth of monolayer MSi2N4 (M = Mo and W) further motivates us to explore new physics and unusual properties behind this family. By using first-principles calculations and Bardeen-Cooper-Schrieffer theory, we predicted the existence of the superconductivity in single-layer (SL) 1T- and 1H-TaN2 with superconducting transition temperatures (Tc) of ∼0.86 and 1.3 K. Specifically, the Tc could be greatly enhanced to ∼24.6 K by passivating the TaN2 monolayer with Si-N bilayers. Furthermore, the superconductivity could be increased to ∼30.4 K via substituting lighter Nb for Ta. This enhancement of superconductivity mainly stems from the softer vibration modes consisting of in-plane Ta/Nb vibrations mixed with Si-xy vibrations. The superconductivity can be further tuned by applying external strains and carrier doping. This enhancement strategy of surface passivation and light atom substitution would suggest a new platform for 2D superconductors and provide an instructive pathway for next-generation nanoelectronics.

Octahedral rotations trigger electronic and magnetic transitions in strontium manganate under volume expansion.

The perovskite structure of manganate yields a series of intriguing physical properties. Based on the results of first-principles calculations, strontium manganate appears to undergo a magnetic phase transition and a metal-insulator transition-from antiferromagnetic insulator to ferromagnetic metal and then to ferromagnetic insulator-under isotropic volume expansion combined with oxygen octahedral distortions. Interestingly, the results show that increasing the Mn-O bond length and adding rotation of the oxygen octahedra can soften the breathing distortion and account for the insulator phase. We further build a simple model to explain such transitions. Due to electron transfer and the favoring of a hole state of ligandporbitals, the electron state transfer from2(t2g3)to2(eg1+t2g2)and then tot2g3eg1+(t2g3L̲1). Such rearrangement of charges is responsible for the transitions of its magnetic order and electronic structure. Furthermore, we calculate spin susceptibility under the bare conditions and random phase approximation. The magnetic order of the intermediate metal state of itinerant electrons behaves as a ferromagnetic.

Quadruple-layer group-IV tellurides: low thermal conductivity and high performance two-dimensional thermoelectric materials.

Through first-principles calculations, we report the thermoelectric properties of two-dimensional (2D) hexagonal group-IV tellurides XTe (X = Ge, Sn and Pb), with quadruple layers (QL) in the Te-X-X-Te stacking sequence, as promising candidates for mid-temperature thermoelectric (TE) materials. The results show that 2D PbTe exhibits a high Seebeck coefficient (∼1996 µV K-1) and a high power factor (6.10 × 1011 W K-2 m-1 s-1) at 700 K. The lattice thermal conductivities of QL GeTe, SnTe and PbTe are calculated to be 2.29, 0.29 and 0.15 W m-1 K-1 at 700 K, respectively. Using our calculated transport parameters, large values of the thermoelectric figure of merit (ZT) of 0.67, 1.90, and 2.44 can be obtained at 700 K under n-type doping for 2D GeTe, SnTe, and PbTe, respectively. Among the three compounds, 2D PbTe exhibits low average values of sound velocity (0.42 km s-1), large Grüneisen parameters (∼2.03), and strong phonon scattering. Thus, 2D PbTe shows remarkable mid-temperature TE performance with a high ZT value under both p-type (2.39) and n-type (2.44) doping. The present results may motivate further experimental efforts to verify our predictions.

Prediction of superconductivity in bilayer borophenes.

Borophenes and related two-dimensional materials have exhibited many exotic properties, especially for superconductivity, although the superconductivity of single-layer borophene is suppressed by the strains or doping from its substrates. Intriguingly, bilayer (BL) borophenes can be stabilized by appropriate pillar density and hexagonal holes density, rather than being supported by Ag(111) or Cu(111) substrates. Thus, we studied the two most stable structures, namely BL-B8 and BL-B30, stabilized by the above-mentioned two methods. Within density functional theory and Bardeen-Cooper-Schrieffer theory framework, their stability, electron structures, and phonon properties, as well as possible superconductivity are systematically scrutinized. The metallic BL-B8 and BL-B30 exhibit intrinsic superconducting features with superconductivity transition temperatures (T c) of 11.9 and 4.9 K, respectively. The low frequency (below 400 cm-1) consisting of out-of-plane vibrations of boron atoms plays crucial rule in their superconductivity. In particular, a Kohn anomaly appears at the Γ point in BL-B8, leading to substantial electron-phonon coupling. Here, our findings will provide instructive clues for experimentally determining the superconductivity of borophene and will broaden the two-dimensional superconductor family.

Single-Layer Dititanium Oxide Ti2O MOene: Multifunctional Promises for Electride, Anode Materials, and Superconductor.

Using the first-principles calculations, we report the existence of the single-layer (SL) dititanium oxide Ti2O (labeled as MOene) that constructs a novel family of MXene based on transition-metal oxides. This MOene material strongly contrasts the conventional ones consisting of transition-metal carbides and/or nitrides. SL Ti2O has high thermal and dynamical stabilities because of the strong Ti-O ionic bonding interactions. Moreover, this material is an intrinsic electride and exhibits extremely low diffusion barriers of ∼12.0 and 6.3 meV for Li and Na diffusion, respectively. When applied as anode materials in lithium-ion batteries and sodium-ion batteries, it possesses a high energy storage capacity (960.23 mAhg-1), surpassing the traditional MXenes-based anodes. The superb electrochemical performance stems from the existing anionic electron on Ti2O surface. Astonishingly, SL Ti2O is also determined to be a superconductor with a superconducting transition temperature (Tc) of ∼9.8 K, which originates from the soft-mode of the first acoustic phonon branch and enhanced electron-phonon coupling in the low-frequency region. Furthermore, this soft-mode behaves much softer upon applying a compressive strain of 2%, leading to a higher Tc of 11.9 K. Our finding broadens the family of MXenes and could facilitate more experimental efforts toward future nanodevices.

Effects of Monovacancy and Divacancies on Hydrogen Solubility, Trapping and Diffusion Behaviors in fcc-Pd by First Principles.

The hydrogen blistering phenomenon is one of the key issues for the target station of the accelerator-based neutron source. In the present study, the effect of monovacancies and divacancies defects on the solution, clustering and diffusion behaviors of H impurity in fcc-Pd were studied through first principles calculations. Our calculations prove that vacancies behave as an effective sink for H impurities. We found that, although the H-trap efficiency of the larger vacancy defect was reduced, its H-trap ability strengthened. There is a short-ranged area around the vacancy defects in which H impurities tend to diffuse to vacancy defects, gather and form hydrogen bubbles. Therefore, the characteristic of large vacancy defects formation in materials should be considered when screening anti-blistering materials for neutron-producing targets or when designing radiation resistant composite materials.

Inelastic Electron Tunneling in 2H-Ta_{x}Nb_{1-x}Se_{2} Evidenced by Scanning Tunneling Spectroscopy.

We report a detailed study of tunneling spectra measured on 2H-Ta_{x}Nb_{1-x}Se_{2} (x=0∼0.1) single crystals using a low-temperature scanning tunneling microscope. The prominent gaplike feature, which has not been understood for a long time, was found to be accompanied by some "in-gap" fine structures. By investigating the second-derivative spectra and their temperature and magnetic field dependencies, we were able to prove that inelastic electron tunneling is the origin of these features and obtain the Eliashberg function of 2H-Ta_{x}Nb_{1-x}Se_{2} at an atomic scale, providing a potential way to study the local Eliashberg function and the phonon spectra of the related transition-metal dichalcogenides.

Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb.

Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound's intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.