Pressure-Stabilized Ir^3– in a Superconducting Potassium Iridide

The first charge-separated iridide (Ir^3–) in an extended solid was identified at elevated pressure when combined with potassium. According to an unbiased structure searching method that combines first-principles calculations with particle swarm optimization algorithms, K₃Ir in the Cu₃Ti-type structure shows a favorable formation enthalpy (Δ<i>H</i>) compared to the elements and is dynamically stable above 10 GPa. This novel semiconductor (<i>E</i>_g ≈ 1.6 eV) has sufficient orbital separation to allow complete charge transfer from K to Ir, and Bader charge analysis supports the formation of a formally anionic Ir^3–. Further, electron doping of K₃Ir through Pt substitution makes the system metallic, and electron–phonon coupling calculations indicate that K₃(Ir_0.875Pt_0.125) falls in the strong-coupling regime, with a predicted superconducting transition temperature (<i>T</i>_c) of ∼27 K at 20 GPa. These results suggest that systems containing elements isoelectronic with classical BCS superconductors such as mercury might have an increased probability of exhibiting a superconducting transition

Validation of Interstitial Iron and Consequences of Nonstoichiometry in Mackinawite (Fe_1+<i>x</i>S)

A theoretical investigation of the relationship between chemical composition and electronic structure was performed on the nonstoichiometric iron sulfide, mackinawite (Fe_1+xS), which is isostructural and isoelectronic with the superconducting Fe_1+<i>x</i>Se and Fe_1+<i>x</i>(Te_1–<i>y</i>Se_<i>y</i>) phases. Even though Fe_1+xS has not been measured for superconductivity, the effects of stoichiometry on transport properties and electronic structure in all of these iron-excess chalcogenide compounds has been largely overlooked. In mackinawite, the amount of Fe that has been reported ranges from a large excess, Fe_1.15S, to nearly stoichiometric, Fe_1.00(7)S. Here, we analyze, for the first time, the electronic structure of Fe_1+<i>x</i>S to justify these nonstoichiometric phases. First principles electronic structure calculations using supercells of Fe_1+<i>x</i>S yield a wide range of energetically favorable compositions (0 < <i>x</i> < 0.30). The incorporation of interstitial Fe atoms originates from a delicate balance between the Madelung energy and the occupation of Fe–S and Fe–Fe antibonding orbitals. A theoretical assessment of various magnetic structures for “FeS” and Fe_1.06S indicate that striped magnetic ordering along [110] is the lowest energy structure and the interstitial Fe affects the values of moments in the square planes as a function of distance. Moreover, the formation of the magnetic moment is dependent on the unit cell volume, thus relating it to composition. Finally, changes in the composition cause a modification of the Fermi surface and ultimately the loss of a nested vector

Predicting the Band Gaps of Inorganic Solids by Machine Learning

A machine-learning model is developed that can accurately predict the band gap of inorganic solids based only on composition. This method uses support vector classification to first separate metals from nonmetals, followed by quantitatively predicting the band gap of the nonmetals using support vector regression. The superb accuracy of the regression model is obtained by using a training set composed entirely of experimentally measured band gaps and utilizing only compositional descriptors. In fact, because of the unique training set of experimental data, the machine learning predicted band gaps are significantly closer to the experimentally reported values than DFT (PBE-level) calculated band gaps. Not only does this resulting tool provide the ability to accurately predict the band gap for any composition but also the versatility and speed of the prediction based only on composition will make this a great resource to screen inorganic phase space and direct the development of functional inorganic materials

Predicting the Band Gaps of Inorganic Solids by Machine Learning

A machine-learning model is developed that can accurately predict the band gap of inorganic solids based only on composition. This method uses support vector classification to first separate metals from nonmetals, followed by quantitatively predicting the band gap of the nonmetals using support vector regression. The superb accuracy of the regression model is obtained by using a training set composed entirely of experimentally measured band gaps and utilizing only compositional descriptors. In fact, because of the unique training set of experimental data, the machine learning predicted band gaps are significantly closer to the experimentally reported values than DFT (PBE-level) calculated band gaps. Not only does this resulting tool provide the ability to accurately predict the band gap for any composition but also the versatility and speed of the prediction based only on composition will make this a great resource to screen inorganic phase space and direct the development of functional inorganic materials

Magnetic Ordering in Tetragonal 3d Metal Arsenides M₂As (M = Cr, Mn, Fe): An Ab Initio Investigation

The electronic and magnetic structures of the tetragonal Cu₂Sb-type 3d metal arsenides (M₂As, M = Cr, Mn, Fe) were examined using density functional theory to identify chemical influences on their respective patterns of magnetic order. Each compound adopts a different antiferromagnetic (AFM) ordering of local moments associated with the 3d metal sites, but every one involves a doubled crystallographic <i>c</i>-axis. These AFM ordering patterns are rationalized by the results of VASP calculations on several magnetically ordered models using <i>a</i> × <i>a</i> × 2<i>c</i> supercell. Effective exchange parameters obtained from SPRKKR calculations indicate that both direct and indirect exchange couplings play essential roles in understanding the different magnetic orderings observed. The nature of nearest-neighbor direct exchange couplings, that is, either ferromagnetic (FM) or AFM, were predicted by analysis of the corresponding crystal orbital Hamilton population (COHP) curves obtained by TB-LMTO calculations. Interestingly, the magnetic structures of Fe₂As and Mn₂As show tetragonal symmetry, but a magnetostrictive tetragonal-to-orthorhombic distortion could occur in Cr₂As through AFM Cr1–Cr2 coupling between symmetry inequivalent Cr atoms along the <i>a</i>-axis, but FM coupling along the <i>b</i>-axis. A LSDA+U approach is required to achieve magnetic moment values for Mn₂As in better agreement with experimental values, although computations always predict the moment at the M1 site to be lower than that at the M2 site. Finally, a rigid-band model applied to the calculated DOS curve of Mn₂As correctly assesses the magnetic ordering patterns in Cr₂As and Fe₂As

Structure Transformation and Cerium-Substituted Optical Response across the Carbonitridosilicate Solid Solution (La_δY_1−δ)₂Si₄N₆C (δ = 0–0.5)

Following an investigation proving La₂Si₄N₆C crystallizes in a monoclinic space group, isostructural to Y₂Si₄N₆C, the reportedly hexagonal (La_0.5Y_0.5)₂Si₄N₆C was reinvestigated to examine the apparent crystal structure change across the solid solution. Initially, calculating the electronic structure and phonon density of states of (La_0.5Y_0.5)₂Si₄N₆C in the <i>P</i>6₃<i>mc</i> space group revealed an imaginary phonon mode, which is indicative of a structural instability. Displacing the atoms along the pathway of the imaginary vibration led to a previously unreported space group for carbonitridosilicates, trigonal <i>P</i>31<i>c</i>. The assignment of the trigonal space group was subsequently confirmed by synthesizing (La_0.5Y_0.5)₂Si₄N₆C using high-temperature, solid state synthesis and analyzing the crystal structure with high-resolution synchrotron X-ray powder diffraction. Preparing the solid solution, (La_δY_1−δ)_1.98Ce_0.02Si₄N₆C (δ = 0–0.5), showed that the crystal structure changes from the monoclinic to the trigonal space group at δ ≈ 0.25. Finally, substituting Ce³⁺ in the crystal structure to investigate the optical response via steady-state luminescent and photoluminescent quantum yield measurements reveals severe luminescent quenching with increasing La³⁺ content, due to a combination of absorption of luminescence by the host structure and thermal quenching. These results display the virtue of combining computational and experimental techniques to solve inorganic crystal structures and assess potential phosphor hosts

Predicting the Band Gaps of Inorganic Solids by Machine Learning

A machine-learning model is developed that can accurately predict the band gap of inorganic solids based only on composition. This method uses support vector classification to first separate metals from nonmetals, followed by quantitatively predicting the band gap of the nonmetals using support vector regression. The superb accuracy of the regression model is obtained by using a training set composed entirely of experimentally measured band gaps and utilizing only compositional descriptors. In fact, because of the unique training set of experimental data, the machine learning predicted band gaps are significantly closer to the experimentally reported values than DFT (PBE-level) calculated band gaps. Not only does this resulting tool provide the ability to accurately predict the band gap for any composition but also the versatility and speed of the prediction based only on composition will make this a great resource to screen inorganic phase space and direct the development of functional inorganic materials

Producing Tunable Broadband Near-Infrared Emission through Co-Substitution in (Ga_1–<i>x</i>Mg_<i>x</i>)(Ga_1–<i>x</i>Ge_<i>x</i>)O₃:Cr³⁺

Broadband near-infrared (NIR) phosphors are in high demand for creating “smart” NIR phosphor-converted light-emitting diode (pc-LED) sources. In this work, a series of Cr3+-substituted NIR-emitting materials with highly efficient, broad, tunable emission spectra are achieved by modifying the simple oxide Ga2O3 using [Mg2+-Ge4+] and [Ga3+-Ga3+] co-unit substitution. The results show that the emission peak can be shifted from 726 to 830 nm while maintaining a constant excitation peak in the blue light region, enabling extensive application. The optical properties stem from changes in the Cr3+ crystal field environment upon substitution. Intriguingly, the temperature-dependent photoluminescence emission peak position shows virtually no change in the [Mg2+-Ge4+] co-substituted materials. This abnormal phenomenon is found to be a comprehensive embodiment of a weakening crystal field environment (red-shift) as the temperature increases and reduced local structure distortion (blue-shift) with increasing temperature. The high quantum yield, NIR emission, and net-zero emission shift as a function of temperature make this phosphor class optimal for device incorporation. As a result, their performance was studied by coating the phosphor on a 450 nm emitting LED chip. The fabricated device demonstrates an excellent NIR output power and NIR photoelectric conversion efficiency. This study provides a series of efficient, tunable, broadband NIR materials for spectroscopy applications and contributes to the basic foundation of Cr3+-activated NIR phosphors

Iodine Anions beyond −1: Formation of Li_<i>n</i>I (<i>n</i> = 2–5) and Its Interaction with Quasiatoms

Novel phases of Li_<i>n</i>I (<i>n</i> = 2, 3, 4, 5) compounds are predicted to form under high pressure using first-principles density functional theory and an unbiased crystal structure search algorithm. All of the phases identified are thermodynamically stable with respect to decomposition into elemental Li and the binary LiI at a relatively low pressure (≈20 GPa). Increasing the pressure to 100 GPa yields the formation of a high pressure electride where electrons occupy interstitial quasiatom (ISQ) orbitals. Under these extreme pressures, the calculated charge on iodine suggests the oxidation state goes beyond the conventional and expected −1 charge for the halogens. This strange oxidative behavior stems from an electron transfer going from the ISQ to I^– and Li⁺ ions as high pressure collapses the void space. The resulting interplay between chemical bonding and the quantum chemical nature of enclosed interstitial space allows this first report of a halogen anion beyond a −1 oxidation state

Identifying a Structural Preference in Reduced Rare-Earth Metal Halides by Combining Experimental and Computational Techniques

The structures of two new cubic {TnLa₃}­Br₃ (Tn = Ru, Ir; <i>I</i>4₁32, <i>Z</i> = 8; Tn = Ru: <i>a</i> = 12.1247(16) Å, <i>V</i> = 1782.4(4) ų; Tn = Ir: <i>a</i> = 12.1738(19) Å, <i>V</i> = 1804.2(5) ų) compounds belonging to a family of reduced rare-earth metal halides were determined by single-crystal X-ray diffraction. Interestingly, the isoelectronic compound {RuLa₃}­I₃ crystallizes in the monoclinic modification of the {TnR₃}­X₃ family, while {IrLa₃}­I₃ was found to be isomorphous with cubic {PtPr₃}­I₃. Using electronic structure calculations, a pseudogap was identified at the Fermi level of {IrLa₃}­Br₃ in the new cubic structure. Additionally, the structure attempts to optimize (chemical) bonding as determined through the crystal orbital Hamilton populations (COHP) curves. The Fermi level of the isostructural {RuLa₃}­Br₃ falls below the pseudogap, yet the cubic structure is still formed. In this context, a close inspection of the distinct bond frequencies reveals the subtleness of the structure determining factors