Exploratory synthesis in new chemical spaces is the essence of solid-state chemistry. However, uncharted chemical spaces can be difficult to navigate, especially when materials synthesis is ...challenging. Nitrides represent one such space, where stringent synthesis constraints have limited the exploration of this important class of functional materials. Here, we employ a suite of computational materials discovery and informatics tools to construct a large stability map of the inorganic ternary metal nitrides. Our map clusters the ternary nitrides into chemical families with distinct stability and metastability, and highlights hundreds of promising new ternary nitride spaces for experimental investigation-from which we experimentally realized seven new Zn- and Mg-based ternary nitrides. By extracting the mixed metallicity, ionicity and covalency of solid-state bonding from the density functional theory (DFT)-computed electron density, we reveal the complex interplay between chemistry, composition and electronic structure in governing large-scale stability trends in ternary nitride materials.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The Gibbs energy, G, determines the equilibrium conditions of chemical reactions and materials stability. Despite this fundamental and ubiquitous role, G has been tabulated for only a small fraction ...of known inorganic compounds, impeding a comprehensive perspective on the effects of temperature and composition on materials stability and synthesizability. Here, we use the SISSO (sure independence screening and sparsifying operator) approach to identify a simple and accurate descriptor to predict G for stoichiometric inorganic compounds with ~50 meV atom
(~1 kcal mol
) resolution, and with minimal computational cost, for temperatures ranging from 300-1800 K. We then apply this descriptor to ~30,000 known materials curated from the Inorganic Crystal Structure Database (ICSD). Using the resulting predicted thermochemical data, we generate thousands of temperature-dependent phase diagrams to provide insights into the effects of temperature and composition on materials synthesizability and stability and to establish the temperature-dependent scale of metastability for inorganic compounds.
We employ quantum chemical calculations to investigate the mechanism of homogeneous CO2 reduction by pyridine (Py) in the Py/p-GaP system. We find that CO2 reduction by Py commences with PyCOOH0 ...formation where: (a) protonated Py (PyH+) is reduced to PyH0, (b) PyH0 then reduces CO2 by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO2, and finally (c) proton transfer (PT) from PyH0 to CO2 produces PyCOOH0. The predicted enthalpic barrier for this proton-coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH0 to CO2. However, when PT is mediated by one to three water molecules acting as a proton relay, the barrier decreases to 29.5, 20.4, and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6–16.5 kcal/mol, depending on the number and configuration of the solvating waters. This agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pK a for PyH0 of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH0 formation, confirming PyH0’s pK a as irrelevant for predicting PT from PyH0 to CO2. Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO2, Py, and Py·CO2 of 47.4, 37.9, and 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO– stabilizes the anionic radicals CO2 – and Py– to facilitate low barrier ET. As the reduction of CO2 proceeds through ET and then PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO2 reduction by stabilizing the PCET TS and the PyCOOH0 product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid and formaldehyde en route to formation of CH3OH through a series of one-electron reductions analogous to the PCET reduction of CO2 examined here, where the electrode only acts to reduce PyH+ to PyH0.
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IJS, KILJ, NUK, PNG, UL, UM
We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduction of CO2 to methanol involving homogeneous catalytic steps. The first phase of the catalytic cycle ...involves generation of the key catalytic agent, 1,2-dihydropyridine (PyH 2 ). First, pyridine (Py) undergoes a H+ transfer (PT) to form pyridinium (PyH+), followed by an e– transfer (ET) to produce pyridinium radical (PyH0). Examples of systems to effect this ET to populate PyH+’s LUMO (E 0 calc ∼ −1.3 V vs SCE) to form the solution phase PyH0 via highly reducing electrons include the photoelectrochemical p-GaP system (E CBM ∼ −1.5 V vs SCE at pH 5) and the photochemical Ru(phen)32+/ascorbate system. We predict that PyH0 undergoes further PT–ET steps to form the key closed-shell, dearomatized (PyH 2 ) species (with the PT capable of being assisted by a negatively biased cathode). Our proposed sequential PT–ET–PT–ET mechanism for transforming Py into PyH 2 is analogous to that described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH 2 is a potent recyclable organo-hydride donor that mimics important aspects of the role of NADPH in the formation of C–H bonds in the photosynthetic CO2 reduction process. In particular, in the second phase of the catalytic cycle, which involves three separate reduction steps, we predict that the PyH 2 /Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely, formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first of these reduction steps, the homogeneous reduction of CO2, are sequential in nature (in which the formate to formic acid protonation can be assisted by a negatively biased cathode). In contrast, these transfers are coupled in each of the two subsequent homogeneous hydride and proton transfer steps to reduce formic acid and formaldehyde.
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Inorganic nitrides with wurtzite crystal structures are well-known semiconductors used in optical and electronic devices. In contrast, rocksalt-structured nitrides are known for their superconducting ...and refractory properties. Breaking this dichotomy, herewe report ternary nitride semiconductors with rocksalt crystal structures, remarkable electronic properties, and the general chemical formula MgₓTM
1−xN (TM = Ti, Zr, Hf, Nb). Our experiments show that these materials form over a broad metal composition range, and that Mg-rich compositions are nondegenerate semiconductors with visible-range optical absorption onsets (1.8 to 2.1 eV) and up to 100 cm² V−1·s−1 electron mobility for MgZrN₂ grown on MgO substrates. Complementary ab initio calculations reveal that these materials have disorder-tunable optical absorption, large dielectric constants, and electronic bandgaps that are relatively insensitive to disorder. These ternary MgₓTM
1−xN semiconductors are also structurally compatible both with binary TMN superconductors and main-group nitride semiconductors along certain crystallographic orientations. Overall, these results highlight MgₓTM
1−xN as a class of materials combining the semiconducting properties of main-group wurtzite nitrides and rocksalt structure of superconducting transition-metal nitrides.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Reactions of HF with uncoated and Al and Zn oxide-coated surfaces of LiCoO2 cathodes were studied using density functional theory. Cathode degradation caused by reaction of HF with the hydroxylated ...(101̅4) LiCoO2 surface is dominated by formation of H2O and a LiF precipitate via a barrierless reaction that is exothermic by 1.53 eV. We present a detailed mechanism where HF reacts at the alumina coating to create a partially fluorinated alumina surface rather than forming AlF3 and H2O and thus alumina films reduce cathode degradation by scavenging HF and avoiding H2O formation. In contrast, we find that HF etches monolayer zinc oxide coatings, which thus fail to prevent capacity fading. However, thicker zinc oxide films mitigate capacity loss by reacting with HF to form a partially fluorinated zinc oxide surface. Metal oxide coatings that react with HF to form hydroxyl groups over H2O, like the alumina monolayer, will significantly reduce cathode degradation.
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Compared to oxides, the nitrides are relatively unexplored, making them a promising chemical space for novel materials discovery. Of particular interest are nitrogen-rich nitrides, which often ...possess useful semiconducting properties for electronic and optoelectronic applications. However, such nitrogen-rich compounds are generally metastable, and the lack of a guiding theory for their synthesis has limited their exploration. Here, we review the remarkable metastability of observed nitrides, and examine the thermodynamics of how reactive nitrogen precursors can stabilize metastable nitrogen-rich compositions during materials synthesis. We map these thermodynamic strategies onto a predictive computational search, training a data-mined ionic substitution algorithm specifically for nitride discovery, which we combine with grand-canonical DFT-SCAN phase stability calculations to compute stabilizing nitrogen chemical potentials. We identify several new nitrogen-rich binary nitrides for experimental investigation, notably the transition metal nitrides Mn3N4, Cr3N4, V3N4, and Nb3N5, the main group nitride SbN, and the pernitrides FeN2, CrN2, and Cu2N2. By formulating rational thermodynamic routes to metastable compounds, we expand the search space for functional technological materials beyond equilibrium phases and compositions.
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Electrochemical supercapacitors utilizing α-MnO2 offer the possibility of both high power density and high energy density. Unfortunately, the mechanism of electrochemical charge storage in α-MnO2 and ...the effect of operating conditions on the charge storage mechanism are generally not well-understood. Here, we present the first detailed charge storage mechanism of α-MnO2 and explain the capacity differences between α- and β-MnO2 using a combined theoretical electrochemical and band structure analysis. We identify the importance of the band gap, work function, the point of zero charge, and the tunnel sizes of the electrode material, as well as the pH and stability window of the electrolyte in determining the viability of a given electrode material. The high capacity of α-MnO2 results from cation induced charge-switching states in the band gap that overlap with the scanned potential allowed by the electrolyte. The charge-switching states originate from interstitial and substitutional cations (H+, Li+, Na+, and K+) incorporated into the material. Interstitial cations are found to induce charge-switching states by stabilizing Mn-O antibonding orbitals from the conduction band. Substitutional cations interact with O2p dangling bonds that are destabilized from the valence band by Mn vacancies to induce charge-switching states. We calculate the equilibrium electrochemical potentials at which these states are reduced and predict the effect of the electrochemical operating conditions on their contribution to charge storage. The mechanism and theoretical approach we report is general and can be used to computationally screen new materials for improved charge storage via ion incorporation.
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All-inorganic halide double perovskites have emerged as a promising class of materials that are potentially more stable and less toxic than lead-containing hybrid organic–inorganic perovskite ...optoelectronic materials. In this work, 311 cesium chloride double perovskites (Cs2 BB′Cl6) were selected from a set of 903 compounds as likely being stable on the basis of a statistically learned tolerance factor (τ) for perovskite stability. First-principles calculations on these 311 double perovskites were then performed to assess their stability and identify candidates with band gaps appropriate for optoelectronic applications. We predict that 261 of the 311 Cs2 BB′Cl6 compounds are likely synthesizable on the basis of a thermodynamic analysis of their decomposition to competing compounds (decomposition enthalpy <0.05 eV/atom). Of these 261 likely synthesizable compounds, 47 contain no toxic elements and have direct or nearly direct (within 100 meV) band gaps between 1 and 3 eV, as computed with hybrid density functional theory (HSE06). Within this set, we identify the triple-alkali perovskites Cs2Alk+TM3+Cl6, where Alk is a group 1 alkali cation and TM is a transition-metal cation, as a class of Cs2 BB′Cl6 double perovskites with remarkable optical properties, including large and tunable exciton binding energies as computed by the GW-Bethe–Salpeter equation (GW-BSE) method. We attribute the unusual electronic structure of these compounds to the mixing of the Alk-Cl and TM-Cl sublattices, leading to materials with small band gaps, large exciton binding energies, and absorption spectra that are strongly influenced by the identity of the transition metal. The role of the double-perovskite structure in enabling these unique properties is probed through an analysis of the electronic structures and chemical bonding of these compounds in comparison with other transition-metal and alkali transition-metal halides.
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Oxygen vacancies (V O) in oxides are extensively used to manipulate vital material properties. Although methods to predict defect formation energies have advanced significantly, an understanding of ...the intrinsic material properties that govern defect energetics lags. We use first-principles calculations to study the connection between intrinsic (bulk) material properties and the energy to form a single, charge neutral oxygen vacancy (E V). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (ΔH f), (ii) the midgap energy relative to the O 2p band center (E O 2p + (1/2)E g), and (iii) atomic electronegativities reproduces calculated E V within ∼0.2 eV. This result provides both valuable insights into the key properties influencing E V and a direct method to predict E V. We then predict the E V of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials.
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