High-Energy-Density Material with Magnetically Modulated Ignition.

Preparation of a redox-frustrated high-energy-density energetic material is achieved by gentle protolysis of Mn[N(SiMe3)2]2 with the perchlorate salt of the tetrazolamide [H2NtBuMeTz]ClO4 (Tz = tetrazole), yielding the Mn6N6 hexagonal prismatic cluster, Mn6(µ3-NTztBuMe)6(ClO4)6. Quantum mechanics-based molecular dynamics simulations of the decomposition of this molecule predict that magnetic ordering of the d5 Mn2+ ions influences the pathway and rates of decomposition, suggesting that the initiation of decomposition of the bulk material might be significantly retarded by an applied magnetic field. We report here experimental tests of the prediction showing that the presence of a 0.5 T magnetic field modulates the ignition onset temperature by +10.4 ± 3.9 °C (from 414 ± 4 °C), demonstrating the first example of a magnetically modulated explosive.

Deciphering the Atomistic Mechanism of Si(111)-7 × 7 Surface Reconstruction Using a Machine-Learning Force Field.

While the complex 7 × 7 structure that arises upon annealing the Si(111) surface is well-known, the mechanism underlying this unusual surface reconstruction has remained a mystery. Here, we report molecular dynamics simulations using a machine-learning force field for Si to investigate the Si(111)-7 × 7 surface reconstruction from the melt. We find that there are two possible pathways for the formation of the 7 × 7 structure. The first path arises from the growth of a faulted half domain from the metastable 5 × 5 phase to the final 7 × 7 structure, while the second path involves the direct formation of the 7 × 7 reconstruction. Both pathways involve the creation of dimers and bridged five-membered rings, followed by the formation of additional dimers and the stabilization of the triangular halves of the unit cell. The corner hole is formed from the joining of several five-member rings. The insertion of atoms below the adatoms to form a dumbbell configuration involves extra atom diffusion or rearrangement during the evolution of triangular halves and dimer formation along the unit cell boundary. Our findings may provide insights for manipulating the surface structure by introducing other atomic species.

Growth Mechanism and Kinetics of Diamond in Liquid Gallium from Quantum Mechanics Molecular Dynamics Simulations.

Ruoff and co-workers recently demonstrated low-temperature (1193 K) homoepitaxial diamond growth from liquid gallium solvent. To develop an atomistic mechanism for diamond growth underlying this remarkable demonstration, we carried out density functional theory-based molecular dynamics (DFT-MD) simulations to examine the mechanism of single-crystal diamond growth on various low-index crystallographic diamond surfaces (100), (110), and (111) in liquid Ga with CH4. We find that carbon linear chains form in liquid Ga and then react with the growing diamond surface, leading first to the formation of carbon rings on the surface and then initiation of diamond growth. Our simulations find faster growth on the (110) surface than on the (100) or (111) surfaces, suggesting the (110) surface as a plausible growth surface in liquid Ga. For (110) surface growth, we predict the optimum growth temperature to be ∼1300 K, arising from a balance between the kinetics of forming carbon chains dissolved in Ga and the stability of carbon rings on the growing surface. We find that the rate-determining step for diamond growth is dehydrogenation of the growing hydrogenated (110) surface of diamond. Inspired by the recent experimental studies by Ruoff and co-workers demonstrating that Si accelerates diamond growth in Ga, we show that addition of Si into liquid Ga significantly increases the rate of dehydrogenating the growing surface. Extrapolating from the DFT-MD predicted rates at 2800 to 3500 K, we predict the growth rate at the experimental growth temperature of 1193 K, leading to rates in reasonable agreement with the experiment. These fundamental mechanisms should provide guidance in optimizing low-temperature diamond growth.

Deformation and Failure Mechanisms of Thermoelectric Type-I Clathrate Ba8Au6Ge40.

Type-I clathrate Ba8Au6Ge40, possessing an interesting structure stacked by polyhedrons, is a potential "phonon-glass, electron-crystal" thermoelectric material. However, the mechanical properties of Ba8Au6Ge40 vital for industrial applications have not been clarified. Here, we report the first density functional theory calculations of the intrinsic mechanical properties of thermoelectric clathrate Ba8Au6Ge40. Among the different loading directions, the {110}/⟨001⟩ shearing and ⟨110⟩ tension are the weakest, with strengths of 4.51 and 6.64 GPa, respectively. Under {110}/⟨001⟩ shearing, the Ge-Ge bonds undergo significant stretching and twisting, leading to a severe distortion of the tetrakaidecahedral cage, giving rise to the fast softening of the flank Au-Ge bonds. At a strain of 0.2655, the Au-Ge bonds suddenly break, resulting in the collapse of the cage and the failure of the material. Under a ⟨110⟩ tension, the stretching of the Ge-Ge bonds keeps accelerating the softening of the Au-Ge bonds in the top/bottom hexagons, which releases the stress and disables the structure. The Au-Ge bonds are more rigid, contributing two-thirds of the structural deformation resistance. This work provides a new insight to understand the failure mechanisms of type-I clathrates with varied framework constitutions, which should help inform the design of robust thermoelectric clathrate materials.

Estimating the lower-limit of fracture toughness from ideal-strength calculations.

Fracture mechanics is a fundamental topic to materials science. Fracture toughness, in particular, is a material property of great technological importance for device design. The relatively low fracture toughness of many semiconductor materials, including electronic and energy materials, handicaps their use in applications involving large external stresses. Here, it is shown that quantum-mechanical density functional theory calculations of ideal strength, in conjunction with an integral stress-displacement method, can be used to estimate the fracture energy needed to calculate fracture toughness. Using the fracture energy associated with the weakest crystallographic direction provides an estimation for the lower-limit of the fracture toughness of a material. The lower-limit values are in good agreement with experimental single crystal measurements across several orders-of-magnitude of fracture toughness. Furthermore, the proposed methodology is useful for benchmarking experimental measurements of fracture toughness in polycrystalline materials and can serve as a starting point for the construction of more detailed fracture models and the computational design of new materials and devices.

Reaction Mechanism and Energetics of Decomposition of Tetrakis(1,3-dimethyltetrazol-5-imidoperchloratomanganese(II)) from Quantum-Mechanics-based Reactive Dynamics.

Energetic materials (EMs) are central to construction, space exploration, and defense, but over the past 100 years, their capabilities have improved only minimally as they approach the CHNO energetic ceiling, the maximum energy density possible for EMs based on molecular carbon-hydrogen-nitrogen-oxygen compounds. To breach this ceiling, we experimentally explored redox-frustrated hybrid energetic materials (RFH EMs) in which metal atoms covalently connect a strongly reducing fuel ligand (e.g., tetrazole) to a strong oxidizer (e.g., ClO4). In this Article, we examine the reaction mechanisms involved in the thermal decomposition of an RFH EM, [Mn(Me2TzN)(ClO4]4 (3, Tz = tetrazole). We use quantum-mechanical molecular reaction dynamics simulations to uncover the atomistic reaction mechanisms underlying this decomposition. We discover a novel initiation mechanism involving oxygen atom transfer from perchlorate to manganese, generating energy that promotes the fission of tetrazole into chemically stable species such as diazomethane, diazenes, triazenes, and methyl azides, which further undergo exothermic decomposition to finally form stable N2, H2O, CO, CO2, Mn-based clusters, and additional incompletely combusted products.

Proton transport mechanism and pathways in the superprotonic phase of M3H(AO4)2 solid acids from ab initio molecular dynamics simulations.

The proton transport mechanism in superprotonic phases of solid acids has been a subject of experimental and theoretical studies for a number of years. Despite this, details of the mechanism still need further clarification. In particular in the M3H(AO4)2 family of crystals, where M = NH4, K, Rb, Cs, and A = S, Se, the proton diffusion is mostly considered in the (001) plane, whereas it is relatively high in the [001] direction as well. In this paper, we report the results of our ab initio molecular dynamics simulations of the Cs3H(SeO4)2 superprotonic phase and propose an atomic-level mechanism of proton transport and pathways both in the (001) plane and along the [001] direction. It turned out that structural configurations formed by hydrogen-bonded tetrahedral anions during the proton diffusion are more complicated and diverse than those considered so far in the literature. Our predicted values of the proton conductivity and activation energy agree well with available experimental data. This validates the reliability of the computational results obtained.

Design of a Graphene Nitrene Two-Dimensional Catalyst Heterostructure Providing a Well-Defined Site Accommodating One to Three Metals, with Application to CO2 Reduction Electrocatalysis for the Two-Metal Case.

Recently, the reduction of CO2 to fuels has been the subject of numerous studies, but the selectivity and activity remain inadequate. Progress has been made on single-site two-dimensional catalysts based on graphene coupled to a metal and nitrogen for the CO2 reduction reaction (CO2RR); however, the product is usually CO, and the metal-N environment remains ambiguous. We report a novel two-dimensional graphene nitrene heterostructure (grafiN6) providing well-defined active sites (N6) that can bind one to three metals for the CO2RR. We find that homobimetallic FeFe-grafiN6 could reduce CO2 to CH4 at -0.61 V and to CH3CH2OH at -0.68 V versus reversible hydrogen electrode, with high product selectivity. Moreover, the heteronuclear FeCu-grafiN6 system may be significantly less affected by hydrogen evolution reaction, while maintaining a low limiting potential (-0.68 V) for C1 and C2 mechanisms. Binding metals to one N6 site but not the other could promote efficient electron transport facilitating some reaction steps. This framework for single or multiple metal sites might also provide unique catalytic sites for other catalytic processes.

Characterizing local metallic bonding variation induced by external perturbation.

The subtle variation of metallic bonding, induced by external influence, plays an essential role in determining physical, mechanical, and chemical properties of metals. However, it is extremely difficult to describe this variation because of the delocalization nature of metallic bonding. Here, we utilize the reduced density gradient and topological analysis of electron density to capture the local metallic bonding variations (LMBV) caused by lattice distortion and carrier injection in many face-centered cubic (fcc) metals. We find that the LMBV determines the traits of fcc metals such as strength, malleability, and ductility. Moreover, the fcc metals can become more flexible/stronger with the electron/hole injection, providing an important guidance to tune metals for desired mechanical properties.

Grain Boundaries Softening Thermoelectric Oxide BiCuSeO.

Engineering grain boundaries (GBs) are effective in tuning the thermoelectric (TE) properties of TE materials, but the role of GB on mechanical properties, which is important for their commercial applications, remains unexplored. In this paper, we apply ab initio method to examine the ideal shear strength and failure mechanism of GBs in TE oxide BiCuSeO. We find that the ideal shear strength of the GB is much lower than that of the ideal single crystal. The atomic rearrangements accommodating the lattice and neighbor structure mismatch between different grains leads to the much weaker GB stiffness compared with grains. Failure of the GBs arises from either the distortion of the Cu-Se layers or the relative slip between Bi-O and Cu-Se layers. This work is crucial to illustrate the deformation of GBs, laying the basis for the development and design of mechanically robust polycrystalline TE materials.

Enhanced Strength Through Nanotwinning in the Thermoelectric Semiconductor InSb.

The conversion efficiency (zT) of thermoelectric (TE) materials has been enhanced over the last two decades, but their engineering applications are hindered by the poor mechanical properties, especially the low strength at working conditions. Here we used density functional theory (DFT) to show a strength enhancement in the TE semiconductor InSb arising from the twin boundaries (TBs). This strengthening effect leads to an 11% enhancement of the ideal shear strength in flawless crystalline InSb where this theoretical strength is considered as an upper bound on the attainable strength for a realistic material. DFT calculations reveal that the directional covalent bond rearrangements at the TB accommodating the structural mismatch lead to the anisotropic resistance against the deformation combined with the enhanced TB rigidity. This produces a strong stress response in the nanotwinned InSb. This work provides a fundamental insight for understanding the deformation mechanism of nanotwinned TE semiconductors, which is beneficial for developing reliable TE devices.

Superstrengthening Bi_{2}Te_{3} through Nanotwinning.

Bismuth telluride (Bi_{2}Te_{3}) based thermoelectric (TE) materials have been commercialized successfully as solid-state power generators, but their low mechanical strength suggests that these materials may not be reliable for long-term use in TE devices. Here we use density functional theory to show that the ideal shear strength of Bi_{2}Te_{3} can be significantly enhanced up to 215% by imposing nanoscale twins. We reveal that the origin of the low strength in single crystalline Bi_{2}Te_{3} is the weak van der Waals interaction between the Te1 coupling two Te1─Bi─Te2─Bi─Te1 five-layer quint substructures. However, we demonstrate here a surprising result that forming twin boundaries between the Te1 atoms of adjacent quints greatly strengthens the interaction between them, leading to a tripling of the ideal shear strength in nanotwinned Bi_{2}Te_{3} (0.6 GPa) compared to that in the single crystalline material (0.19 GPa). This grain boundary engineering strategy opens a new pathway for designing robust Bi_{2}Te_{3} TE semiconductors for high-performance TE devices.