Boron Suboxide and Boron Subphosphide Crystals: Hard Ceramics That Shear without Brittle Failure

Boron suboxide (B₆O), boron carbide (B₄C), and related materials are superhard. However, they exhibit low fracture toughness, which limits their engineering applications. Here we show the shear deformation mechanism of B₆O using density functional theory along the most plausible slip system (01̅11)/<101̅1>. We discovered an unusual phenomenon in which the highly sheared system recovers its original crystal structure, which indicates the possibility of being sheared to a large strain without failure. We also found a similar structural recovery in boron subphosphide (B₁₂P₂) for shearing along the same slip system. In contrast, for components of B₄C, we found brittle failure. These novel deformation mechanisms under high shear deformation conditions suggest that a key element to designing ductile hard materials is to couple the icosahedra via one- or two-atom chains that allow the system to shear by walking the intericosahedral bonds and chain bonds alternately to accommodate large shear without fracturing the icosahedra

Reaction Mechanism for Ammonia Activation in the Selective Ammoxidation of Propene on Bismuth Molybdates

In this paper, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for various pathways of ammonia activation on bismuth molybdates, a process required for ammoxidation of propene to acrylonitrile. Using a Mo₃O₉ cluster to model the bulk surface, we examined the activation of ammonia by both fully oxidized (Mo^IV) and reduced (Mo^IV) molybdenum sites. Our results show that ammonia activation does not take place on the fully oxidized Mo­(VI) sites. Here the net barriers for the first hydrogen transfer (Δ<i>E</i>^‡ = 44.6 kcal/mol, Δ<i>G</i>^‡_673K = 44.2 kcal/mol) and the second hydrogen transfer (Δ<i>E</i>^‡ = 54.5 kcal/mol, Δ<i>G</i>^‡_673K = 51.7 kcal/mol) are prohibitively high for the reaction temperature of 400 °C. Instead, our calculations show that the reduced Mo­(IV) surface sites are far more suitable for this process. Here, the calculated barrier for the first hydrogen transfer from a Mo­(IV)–NH₃ to an adjacent Mo­(VI)O is 18.2 kcal/mol (Δ<i>G</i>^‡_673K = 15.4 kcal/mol). For the second hydrogen transfer step, we explored three pathways, and found that the H transfer from a Mo–NH₂ to an adjacent Mo­(V)–OH to form water is more favorable (Δ<i>E</i>^‡ = 26.2 kcal/mol (Δ<i>G</i>^‡_673K = 24.0 kcal/mol) than transfer to an adjacent Mo­(VI)O or Mo­(V)O group. These studies complement previous studies for activation and reaction of propene on these surfaces, completing the QM study into the fundamental mechanism

Ductility in Crystalline Boron Subphosphide (B₁₂P₂) for Large Strain Indentation

Our studies of brittle fracture in B₄C showed that shear-induced cracking of the (B₁₁C) icosahedra leading to amorphous B₄C regions induced cavitation and failure. This suggested that to obtain hard boron-rich phases that are ductile, we need to replace the CBC chains of B₄C with two-atom chains that can migrate between icosahedra during shear without cracking the icosahedra. We report here a quantum mechanism (QM) simulation showing that under indentation stress conditions, superhard boron subphosphide (B₁₂P₂) displays such a unique deformation mechanism. Thus, stress accumulated as shear increases is released by slip of the icosahedra planes through breaking and then reforming the P–P chain bonds without fracturing the (B₁₂) icosahedra. This icosahedral slip may facilitate formation of mobile dislocation and deformation twinning in B₁₂P₂ under high-stress conditions, leading to high ductility. However, the presence of twin boundaries (TBs) in B₁₂P₂ will weaken the icosahedra along TBs, leading to the fracture of (B₁₂) icosahedra under indentation stress conditions. These results suggest that crystalline B₁₂P₂ is an ideal superhard material to achieve high ductility

First-Principles-Based Dispersion Augmented Density Functional Theory: From Molecules to Crystals

Standard implementations of density functional theory (DFT) describe well strongly bound molecules and solids but fail to describe long-range van der Waals attractions. We propose here first-principles-based augmentation to DFT that leads to the proper long-range 1/<i>R</i>⁶ attraction of the London dispersion while leading to low gradients (small forces) at normal valence distances so that it preserves the accurate geometries and thermochemistry of standard DFT methods. The DFT-low gradient (DFT-<i>l</i>g) formula differs from previous DFT-D methods by using a purely attractive dispersion correction while not affecting valence bond distances. We demonstrate here that the DFT-<i>l</i>g model leads to good descriptions for graphite, benzene, naphthalene, and anthracene crystals, using just three parameters fitted to reproduce the full potential curves of high-level ab initio quantum mechanics [CCSD(T)] on gas-phase benzene dimers. The additional computational costs for this DFT-<i>l</i>g formalism are negligible

Interfacial Thermodynamics of Water and Six Other Liquid Solvents

We examine the thermodynamics of the liquid–vapor interface by direct calculation of the surface entropy, enthalpy, and free energy from extensive molecular dynamics simulations using the two-phase thermodynamics (2PT) method. Results for water, acetonitrile, cyclohexane, dimethyl sulfoxide, hexanol, <i>N</i>-methyl acetamide, and toluene are presented. We validate our approach by predicting the interfacial surface tensions (IFTexcess surface free energy per unit area) in excellent agreement with the mechanical calculations using Kirkwood–Buff theory. Additionally, we evaluate the temperature dependence of the IFT of water as described by the TIP4P/2005, SPC/Ew, TIP3P, and mW classical water models. We find that the TIP4P/2005 and SPC/Ew water models do a reasonable job of describing the interfacial thermodynamics; however, the TIP3P and mW are quite poor. We find that the underprediction of the experimental IFT at 298 K by these water models results from understructured surface molecules whose binding energies are too weak. Finally, we performed depth profiles of the interfacial thermodynamics which revealed long tails that extend far into what would be considered bulk from standard Gibbs theory. In fact, we find a nonmonotonic interfacial free energy profile for water, a unique feature that could have important consequences for the absorption of ions and other small molecules

Origin of the Pseudogap in High-Temperature Cuprate Superconductors

Cuprate high-temperature superconductors exhibit a pseudogap in the normal state that decreases monotonically with increasing hole doping and closes at <i>x</i> ≈ 0.19 holes per planar CuO₂ while the superconducting doping range is 0.05 < <i>x</i> < 0.27 with optimal <i>T</i>_c at <i>x</i> ≈ 0.16. Using ab initio quantum calculations at the level that leads to accurate band gaps, we found that four-Cu-site plaquettes are created in the vicinity of dopants. At <i>x</i> ≈ 0.05, the plaquettes percolate, so that the Cu d<i>_x</i>_²<i>_y</i>_²/O p_σ orbitals inside the plaquettes now form a band of states along the percolating swath. This leads to metallic conductivity and, below <i>T</i>_c, to superconductivity. Plaquettes disconnected from the percolating swath are found to have degenerate states at the Fermi level that split and lead to the pseudogap. The pseudogap can be calculated by simply counting the spatial distribution of isolated plaquettes, leading to an excellent fit to experiment. This provides strong evidence in favor of inhomogeneous plaquettes in cuprates

In Silico Design of Highly Selective Mo-V-Te-Nb‑O Mixed Metal Oxide Catalysts for Ammoxidation and Oxidative Dehydrogenation of Propane and Ethane

We used density functional theory quantum mechanics with periodic boundary conditions to determine the atomistic mechanism underlying catalytic activation of propane by the M1 phase of Mo-V-Nb-Te-O mixed metal oxides. We find that propane is activated by TeO through our recently established reduction-coupled oxo activation mechanism. More importantly, we find that the C–H activation activity of TeO is controlled by the distribution of nearby V atoms, leading to a range of activation barriers from 34 to 23 kcal/mol. On the basis of the new insight into this mechanism, we propose a synthesis strategy that we expect to form a much more selective single-phase Mo-V-Nb-Te-O catalyst

Universal Properties of Cuprate Superconductors: <i>T</i>_c Phase Diagram, Room-Temperature Thermopower, Neutron Spin Resonance, and STM Incommensurability Explained in Terms of Chiral Plaquette Pairing

We report that four properties of cuprates and their evolution with doping are consequences of simply counting four-site plaquettes arising from doping, (1) the universal <i>T</i>_c phase diagram (superconductivity between ∼0.05 and ∼0.27 doping per CuO₂ plane and optimal <i>T</i>_c at ∼0.16), (2) the universal doping dependence of the room-temperature thermopower, (3) the superconducting neutron spin resonance peak (the “41 meV peak”), and (4) the dispersionless scanning tunneling conductance incommensurability. Properties (1), (3), and (4) are explained with no adjustable parameters, and (2) is explained with exactly one. The successful quantitative interpretation of four very distinct aspects of cuprate phenomenology by a simple counting rule provides strong evidence for four-site plaquette percolation in these materials. This suggests that inhomogeneity, percolation, and plaquettes play an essential role in cuprates. This geometric analysis may provide a useful guide to search for new compositions and structures with improved superconducting properties

The Critical Role of Phosphate in Vanadium Phosphate Oxide for the Catalytic Activation and Functionalization of <i>n</i>‑Butane to Maleic Anhydride

We used density functional theory to study the mechanism of <i>n</i>-butane oxidation to maleic anhydride on the vanadium phosphorus oxide (VPO) surface. We found that O(1)P on the V^VOPO₄ surface is the active center for initiating the VPO chemistry through extraction of H from alkane C–H bonds. This contrasts sharply with previous suggestions that the active center is either the V–O bonds or else a chemisorbed O₂ on the (V^IVO)₂P₂O₇ surface. The ability of O(1)P to cleave alkane C–H bonds is due to its strong basicity coupled with large reduction potentials of nearby V^V ions. We examined several pathways for the subsequent functionalization of <i>n</i>-butane to maleic anhydride and found that the overall barrier does not exceed 21.7 kcal/mol

First-Principles Study of the Role of Interconversion Between NO₂, N₂O₄, <i>cis-</i>ONO-NO₂, and <i>trans</i>-ONO-NO₂ in Chemical Processes

Experimental results, such as NO₂ hydrolysis and the hypergolicity of hydrazine/nitrogen tetroxide pair, have been interpreted in terms of NO₂ dimers. Such interpretations are complicated by the possibility of several forms for the dimer: symmetric N₂O₄, <i>cis</i>-ONO-NO₂, and <i>trans</i>-ONO-NO₂. Quantum mechanical (QM) studies of these systems are complicated by the large resonance energy in NO₂ which changes differently for each dimer and changes dramatically as bonds are formed and broken. As a result, none of the standard methods for QM are uniformly reliable. We report here studies of these systems using density functional theory (B3LYP) and several ab initio methods (MP2, CCSD­(T), and GVB-RCI). At RCCSD­(T)/CBS level, the enthalpic barrier to form <i>cis</i>-ONO-NO₂ is 1.9 kcal/mol, whereas the enthalpic barrier to form <i>trans</i>-ONO-NO₂ is 13.2 kcal/mol, in agreement with the GVB-RCI result. However, to form symmetric N₂O₄, RCCSD­(T) gives an unphysical barrier due to the wrong asymptotic behavior of its reference function at the dissociation limit, whereas GVB-RCI shows no barrier for such a recombination. The difference of barrier heights in these three recombination reactions can be rationalized in terms of the amount of B₂ excitation involved in the bond formation process. We find that the enthalpic barrier for N₂O₄ isomerizing to <i>trans</i>-ONO-NO₂ is 43.9 kcal/mol, ruling out the possibility of such an isomerization playing a significant role in gas-phase hydrolysis of NO₂. A much more favored path is to form <i>cis</i>-ONO-NO₂ first then convert to <i>trans</i>-ONO-NO₂ with a 2.4 kcal/mol enthalpic barrier. We also propose that the isotopic oxygen exchange in NO₂ gas is possibly via the formation of <i>trans</i>-ONO-NO₂ followed by ON⁺ migration