Pauling Electronegativities Research Articles

Machine learning‐based crystal structure prediction for high‐entropy oxide ceramics

AbstractPredicting the crystal structure is essential to address the reliance on serendipity for facilitating the discovery and design of high‐performance high‐entropy oxides (HEOs). Here, three classic algorithms‐based machine learning models to predict the crystal structure of HEOs are successfully established and analyzed by combining five metrics, and the XGBoost classifier shows excellent accuracy and robustness with ACC and F1 scores up to 0.977 and 0.975, respectively. SHAP summary plot indicates that the anion‐to‐cation radius ratio (rA/rC) has the greatest impact on crystal structure, followed by difference in Pauling and Mulliken electronegativities (ΔχPauling and ΔχMulliken). It is noteworthy that the rA/rC, ΔχPauling, and ΔχMulliken lower than 0.35, 0.1, and 0.2, respectively, tend to lead to a fluorite crystal structure, whereas rock‐salt and spinel crystal structures are always formed. This work is expected to facilitate the discovery and design of HEOs with tailorable crystal structures and properties.

Is hydrogen electronegativity higher than Pauling’s value? New clues from the 13C and 29Si NMR chemical shifts of [CHF3] and [SiHF3] molecules

Abstract We previously demonstrated that the δ NMR chemical shift of central NMR active atoms (A), in simple halido [AXn] (A=C, Si, Ge, Sn, Pb, Pt; Xn = combination of n halides, n = 4 or 6) derivatives, could be directly related to X radii overall sum, Σ(r L ). Further correlation have also been observed for tetrahedral [AX4] (A=C, Si; X4 = combination of four halides) compounds where the X Pauling electronegativities sum, Σ ( χ L Pau ) , $\Sigma (\chi _L^{{\rm{Pau}}}),$ exceeds a specific value (≈12.4). In this work, we focused on these latter systems considering the H vs. X substitution. The analysis of the literature reported δ(13C) and δ(29Si) NMR chemical shift for the mono hydrogenated derivatives and in particular for [CHF3] and [SiHF3], characterized by the lowest Σ(r L ) and the highest Σ ( χ L Pau ) , $\Sigma (\chi _L^{{\rm{Pau}}}),$ suggests a revised value for the H electronegativity ranking with respect to Pauling’s.

A new equation based on ionization energies and electron affinities of atoms for calculating of group electronegativity

In particular, in organic chemistry, the electronegativity of functional groups are taken into consideration significantly to predict reaction mechanism and to explain inductive effects of functional groups. In the present study, considering the relationship with charge of the electronic energy for atoms, Sanderson’s electronegativity equalization principle and Density Functional Theory (DFT), we have obtained a new equation by which group electronegativity can be calculated from ionization energies (I) and electron affinities (A) of atoms that constitute the group, where the results obtained were compared with equation of Sanderson who proposed that group electronegativity is the geometric mean of atomic electronegativities. For a large number of groups, it was found to be a very close agreement with a standard deviation of 0.12. The advantage of the present equation is that it can be used for ionic groups. In addition, the derived equation can be used to calculate Mulliken and Pauling electronegativities of molecules.

A systematic solid state NMR spectroscopic study of the equiatomic lithium half-Heusler phases LiTX (T = Mg, Zn, Cd; X = P, As, Sb, Bi)

The equiatomic intermetallic lithium compounds Li TX ( T = Mg, Zn, Cd; X = P, As, Sb, Bi) were synthesized by reacting the elements in sealed niobium tubes in muffle furnaces. The compounds crystallize with the cubic MgAgAs type structure, space group F 4 ¯ 3 m . The T and X atoms build up blende-type tetrahedral substructures in which the lithium atoms fill octahedral voids formed by the X atoms. Basic structural characterization of the samples was performed by powder X-ray diffraction. The charge distribution in these intermetallic compounds was investigated using metal ion ( 7 Li, 25 Mg and 113 Cd) as well as pnictogen ( 31 P, 75 As, 121 Sb) static and MAS-NMR. The metal ion chemical shifts are uniformly correlated with the pnictogen Pauling electronegativities, reflecting a systematic trend in the bonding characteristics in these phases. Furthermore, static temperature dependent 7 Li wideline NMR spectra show motional narrowing effects on the NMR timescale, indicating lithium ionic mobility. Activation energies estimated from these data decrease systematically with decreasing difference of the electronegativities between the elements Li and X and between the elements T and X . These result can be rationalized in terms of shallower Coulomb potential wells in those compounds with lower ionic character.

The Adsorption Configuration of Valine on Ge(100)

The Adsorption Configuration of Valine on Ge(100)

Periodic Trends and Index of Boron Lewis Acidity

Lewis acidity is customarily gauged by comparing the relative magnitude of coordinate covalent bonding energies, where the Lewis acid moiety is varied and the Lewis base is kept constant. However, the prediction of Lewis acidity from first principles is sometimes contrary to that suggested by experimental bond energies. Specifically, the order of boron trihalide Lewis acidities predicted from substituent electronegativity arguments is opposite to that inferred by experiment. Contemporary explanations for the divergence between theory, computation, and experiment have led to further consternation. Due to the fundamental importance of understanding the origin of Lewis acidity, we report periodic trends for 21 boron Lewis acids, as well as their coordinate covalent bond strengths with NH(3), utilizing ab initio, density functional theory, and natural bond orbital analysis. Coordinate covalent bond dissociation energy has been determined to be an inadequate index of Lewis acid strength. Instead, acidity is measured in the manner originally intended by Lewis, which is defined by the valence of the acid of interest. Boron Lewis acidity is found to depend upon substituent electronegativity and atomic size, differently than for known Brønsted-Lowry periodic trends. Across the second period, stronger substituent electronegativity correlates (R(2) = 0.94) with increased Lewis acidity. However, across the third period, an equal contribution from substituent electronegativity and atomic radii is correlated (R(2) = 0.98) with Lewis acidity. The data suggest that Lewis acidity depends upon electronegativity solely down group 14, while equal contribution from both substituent electronegativity and atomic size are significant down groups 16 and 17. Originally deduced from Pauling's electronegativities, boron's substituents determine acidity by influencing the population of its valence by withdrawing electron density. However, size effects manifest differently than previously considered, where greater sigma bond (not pi bond) orbital overlap between boron and larger substituents increase the electron density available to boron's valence, thereby decreasing Lewis acidity. The computed electronegativity and size effects of substituents establish unique periodic trends that provide a novel explanation of boron Lewis acidity, consistent with first principle predictions. The findings resolve ambiguities between theory, computation, and experiment and provide a clearer understanding of Lewis acidity.

An Intrinsic Radical Stability Scale from the Perspective of Bond Dissociation Enthalpies: A Companion to Radical Electrophilicities

Bond dissociation enthalpies (BDEs) of a large series of molecules of the type A-B, where a series of radicals A ranging from strongly electrophilic to strongly nucleophilic are coupled with a series of 8 radicals (CH2OH, CH3, NF2, H, OCH3, OH, SH, and F) also ranging from electrophilic to nucleophilic, are computed and analyzed using chemical concepts emerging from density functional theory, more specifically the electrophilicities of the individual radical fragments A and B. It is shown that, when introducing the concept of relative radical electrophilicity, an (approximately) intrinsic radical stability scale can be developed, which is in good agreement with previously proposed stability scales. For 47 radicals, the intrinsic stability was estimated from computed BDEs of their combinations with the strongly nucleophilic hydroxymethyl radical, the neutral hydrogen atom, and the strongly electrophilic fluorine atom. Finally, the introduction of an extra term containing enhanced Pauling electronegativities in the model improves the agreement between the computed BDEs and the ones estimated from the model, resulting in a mean absolute deviation of 16.4 kJ mol(-1). This final model was also tested against 82 experimental values. In this case, a mean absolute deviation of 15.3 kJ mol(-1) was found. The obtained sequences for the radical stabilities are rationalized using computed spin densities for the radical systems.

Development of metal borohydrides for hydrogen storage

A metal borohydride M(BH 4) n is a potential candidate for hydrogen storage materials because of its high gravimetric hydrogen density. The important research issues for M(BH 4) n are to control the thermodynamic stability and to achieve the faster reaction kinetics. To clarify the thermodynamic stability, M(BH 4) n ( M=Mg, Ca∼Mn, Zn, Al, Y, Zr and Hf; n=2–4) were synthesized by mechanical milling and its thermal desorption properties were investigated. The hydrogen desorption temperature T d of M(BH 4) n decreases with increasing Pauling's electronegativities χ P of M. Because Mn, Zn, and Al borohydrides ( χ P⩾1.5) desorb borane, they are too unstable for hydrogen storage applications. The enthalpy changes of desorption reaction Δ H des can be estimated by using our predicted heat of formation of M(BH 4) n Δ H boro and reported data for decomposed products Δ H prod, which are useful indicators for searching M(BH 4) n with appropriate stability for hydrogen storage material. In the latter case, microwave processing was adopted for achieving fast reaction kinetics. Among metal borohydrides, LiBH 4 was rapidly heated above 380 K by microwave irradiation, 13.7 mass% of hydrogen was desorbed by microwave irradiation. The composites of LiBH 4 with B or C desorbed hydrogen within 3 min. Microwave heating aids in realizing faster kinetics of the hydrogen desorption reaction.

Lattice stability of intermediate phases of the Sr–Si system

A computational study of the lattice stability of the intermediate phases of the Sr–Si system is presented. Nine compositions have been considered, investigating 26 different crystal lattices by means of density functional theory calculations and pseudopotentials within the generalized-gradient approximation using the VASP code. The heats of formation of the various polymorphs have been derived for all the investigated compositions and found to be in excellent agreement with the available experimental data in the literature. The Sr 2Si, Sr 5Si 3 and SrSi phases have been predicted to undergo high pressure transitions: the lattice transitions Sr 2Si(oP12→hP6), Sr 5Si 3(tI32-Cr 5B 3→tI32-Mo 5Si 3) and SrSi(oC8→oP8→tP2) have been calculated to occur at 5.5, 19.9, 11.8 and 60 GPa, respectively. Electronic structure of the computed ground states and the predicted four new high pressure polymorphs of Sr 2Si, Sr 5Si 3 and SrSi phases are calculated and discussed in relation with their corresponding crystal structures and heats of formation. The band gap of the semiconducting oP12 Sr 2Si ground state structure has been calculated to be 0.29 eV. The bonding of the Sr–Si phases is found to be mainly ionic, as expected from Pauling's electronegativities, although there is evidence of formation of directional covalent bonds between neighboring Si atoms in the silicon richest phases.

Electronic properties of metal/MgO(001) interfaces

The Schottky barrier heights (SBH) at various metal-MgO interfaces (Mg, Al, Ni, Fe, Pd, and Ag) were studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). In order to determine the Schottky barrier height, the energy distance between the 02s core level of MgO and the MgO valence band maximum (VBM) is determined to be 18.0 ± 0.1 eV at first. The Schottky barrier heights variation with metal Pauling electronegativities is linear fitted with a slope parameter of 1.86 ± 0.15.

Relative Energies of α and β Isomers of Keggin Dodecatungstogallate

The relative energies of beta Keggin heteropolytungstates, X(n+) W12O40(8-n)-, decrease as X(n+) is varied within period 3, from P5+ to Si4+ to Al3+. With heating of alpha-H5Ga3+ W12O40 at 200 degrees C in water, an equilibrated mixture of alpha (T(d); one 183W NMR signal) and beta (C(3v); three signals; 1:2:1 ratio) isomers is obtained. From deltaG(exp) = -RT ln K(beta-->alpha), in which (from 71Ga NMR spectra) K(beta-->alpha) (= [alpha]/[beta]) = 5.0, beta-GaW12O40(5-) is 0.65 kcal mol(-1) higher in energy than alpha-GaW12O40(5-). This finding is evaluated by analysis of the X-ray crystal structure alpha-K2Na3[GaW12O40] x 9.3 H2O [trigonal, space group P3(2)21, a = 18.9201(13) A, b = 18.9201(13) A, c = 12.5108(12) A, Z = 3, T = 100(2)K], comparison of the Shannon and Prewitt radii and Pauling electronegativities of Al3+ and Ga3+, and insight from density functional theory calculations, which predicted Ebeta - Ealpha = 0.32 kcal mol(-1).

Connecting Pauling and Mulliken Electronegativities.

A conversion formula between Pauling and Mulliken electronegativities is derived from a new analytic treatment of the classic valence bond model for a single bond A-B in terms of Mulliken-Jaffe "a and b" parameters for the atomic valence states. The new formula works better than the traditional linear and half-power empirical formulas. The results are consistent with an absolute Pauling scale in which both electronegativities approach zero simultaneously.

Recent advances in photocurrent spectroscopy of passive films

The quantitative application of photocurrent spectroscopy (PCS) for the in-situ determination of the composition of passive films and corrosion layers is reviewed in the light of recent theoretical advances, that have allowed to relate the measured optical gaps to the Pauling electronegativities of the film components. The correlations derived are tested versus recent experimental results regarding mixed oxides, anhydrous passive films on metallic alloys and hydroxide layers. The effect of the eventual long-range disorder into the passive film on the optical band gap values is also discussed. New experimental evidence reported for mixed d,d-metal oxides and passive films on sp,d-metal alloys gives satisfactory agreement with the theoretical expectations. The experimental results evidence the role of the alloying metal in determining the crystalline or disordered structure of passive films grown on Al,d-metal alloys.

Electronic structure of SiO2: Charge redistribution contributions to the dynamic dipoles/effective charges of the infrared active normal modes

This article applies ab initio calculations at the (i) Hartree–Fock self-consistent field single determinant and (ii) configuration interaction multideterminant expansion levels to study the diagonal components of the dynamic dipoles, Δμi/Δqj with i=j, or equivalently infrared effective charges, ei*, associated with asymmetric bond stretching νy, symmetric bond stretching νz, and out-of-plane bond rocking νx, normal mode infrared active vibrations of noncrystalline SiO2. The normal mode dynamic dipoles (hereafter, Δμ/qi) are decomposed into equilibrium charge density (ionic) and orbital variation (charge redistribution) contributions. The calculations are based on small clusters in which Si–O–Si groups are connected through O atoms to embedding Si atom terminators Si* that emulate the connectivity of these Si–O–Si groups to the SiO2 continuous random network. Values of Δμ/Δqi have been determined as a function of the Si–O–Si bond angle α at the bridging O-atom sites, and agree with values obtained from analysis of infrared spectra. Finally, the ab initio calculations are extended to noncrystalline silicon–carbon alloys and silicon nitride, and values of Δμ/Δqi are determined for infrared active vibrations associated with N- and C-atom asymmetric stretching normal mode motions. The normalized equilibrium charge density or ionic contributions of the O, N, and C atoms follow trends expected on the basis of their relative Pauling electronegativities in bonds with Si.

Fermi level pinning by metal Schottky contacts on n type GaAs

The Schottky barrier heights, as determined by current–voltage and capacitance–voltage, of thirty six metals which were fabricated by following the same cleaning procedure and using the same high quality organometallic vapour phase epitaxy (OMVPE) grown (100) n type GaAs material, are presented. It was found that there exists no linear relationship between Schottky barrier height and metal work function as is suggested by the Schottky–Mott theory, if all the above-mentioned metals are taken into account. Similar results were obtained if the metal work function was replaced by the Pauling or Miedema electronegativities. In contrast, if only a selected group of metals is chosen, and more specifically those with high melting points which were deposited by means of an electron gun, a linear relationship does exist between Schottky barrier height and metal work function. From this linear dependency, the density of states was determined to be about 6 × 1013 eV cm−2 and the pinning position of the Fermi level as 0.55 eV.

Summary of Schottky barrier height data on epitaxially grown n- and p-GaAs

The Schottky barrier height values, as determined by the current–voltage and capacitance–voltage techniques, of 43 metals which were fabricated by following the same cleaning procedure and using the same high-quality organometallic vapour phase epitaxially (OMVPE) grown (100) n-type GaAs material and 13 metals on molecular beam epitaxially grown (MBE) p-GaAs, are presented. Of all the metals involved in this study, Ga had the lowest mean Schottky barrier height of about 0.60 eV on n-GaAs and the highest on p-GaAs of 0.83 eV. Cu, Ag, Pt and Sb had the highest barrier heights of about 1 eV on n-GaAs. It was found that there exists no linear relationship between Schottky barrier height and metal work function as is suggested by the Schottky–Mott theory, if all 43 metals are taken into account. Similar results were obtained if the metal work function was replaced by the Pauling or Miedema electronegativities. In contrast with this, if only a selected group of metals is chosen and more specifically those with the higher melting points which were deposited by means of an electron gun, an approximately linear tendency does exist between Schottky barrier height and metal work function. From this linear dependency, the density of states was determined to be about 6×10 13/eV per cm 2 and the average pinning position of the Fermi level as 0.55 eV below the conduction band.

Group electronegativities from electronegativity equilibration Applications to organic thermochemistry

A simple scheme for electronegativity equilibration (as opposed to equalisation) allows the electronegativity of an atom in a molecule to be expressed as a weighted harmonic mean of the prebonded electronegativities of all the atoms. Fractional atomic charges and group electronegativities can be easily calculated. It is proposed that the heats of formation of gaseous organic substances can be estimated using constant bond-energy terms E(C–X) (X = H, Cl, etc.) and E(CC), except that E(C–C) varies according to the difference in electronegativity between the groups linked by the carbon–carbon bond, and E(C–H) is dependent on the hybridisation of the carbon atom. The enthalpies of formation of alkanes, alkenes, alkynes, alkyl radicals and halogenoalkanes can be satisfactorily reproduced using traditional Pauling electronegativities, although the results can be improved by making some adjustments. The charge distributions obtained by electronegativity equilibration help to rationalise the relative stabilities of primary, secondary and tertiary carbocations, as well as the apparent electron-donating function of alkyl groups relative to hydrogen atoms.

A High-Level Computational Study on the Thermochemistry of Vinyl and Formyl Halides: Heats of Formation, Dissociation Energies, and Stabilization Energies

The heats of formation, ΔHf 298, of vinyl and formyl halides calculated using G2 theory are −145.9 (CH2CHF), 21.8 (CH2CHCl), 79.6 (CH2CHBr), 147.1 (CH2CHI), −393.7 (HFCO), −192.7 (HClCO), −133.9 (HBrCO), and −66.2 kJ mol-1 (HICO), which agree with the available experimental data. While the covalent radii of sp3 and sp2 hybridized carbons are 0.762 and 0.735 Å, respectively, at the MP2/6-31G(d) level, the C−X bond lengths in formyl bromide and iodide are 0.016 and 0.045 Å, respectively, longer when compared with those in methyl bromide and iodide. However, the C−F bond in formyl fluoride is 0.040 Å shorter than the C−F bond in methyl fluoride. The C−X bond dissociation energies (DC-X) for vinyl, formyl, and methyl halides show good linear correlations with the Mulliken and Pauling electronegativites of the halogens. The DC-X values for formyl bromide and iodide are smaller than those calculated for methyl bromide and iodide. The stabilization energies (SE) yielding estimates of the stabilization of the CC and CO double bonds by halogens decrease from X = F to X = I and show reasonable linear correlations with the Mulliken and Pauling electronegativities of the halogens. While the DC-X energies for formyl halides are smaller than those for vinyl halides, the SE(formyl) values are larger than the corresponding SE(vinyl). The elimination of HX from formyl halides is exothermic, in contrast to the endothermic elimination of HX from vinyl halides.

FACETING INDUCED BY ULTRATHIN METAL FILMS ON W(111) AND Mo(111): STRUCTURE, REACTIVITY, AND ELECTRONIC PROPERTIES

We have studied ultrathin films of transition and noble metals on Mo(111) and W(111) using Auger spectroscopy, LEED, thermal desorption spectroscopy (TDS) and scanning tunneling microscopy (STM). The atomically rough, open bcc(111) surfaces are morphologically unstable when covered by films ≥ 1 monolayer thick of certain metals, i.e. they form faceted structures. For example, using a UHV STM to study Pd/W(111) , we find that the Pd-covered W(111) surface becomes completely faceted to three-sided {211} pyramids upon annealing, for Pd coverages greater than a critical coverage θc. Formation of pyramidal facets also occurs when W(111) or Mo(111) surfaces are dosed with Pt, Au, Ir, Rh, oxygen or sulfur. In contrast, monolayer films of Ti, Co, Ni, Cu, Ag and Gd do not induce massive reconstruction or faceting on W(111) and Mo(111) surfaces. The faceting appears to be thermodynamically driven but kinetically limited: faceting is caused by an increased anisotropy in surface free energy that occurs for the film-covered surfaces. An interesting correlation has been observed for both substrates: faceting occurs for overlayer elements having Pauling electronegativities greater than 2.0, suggesting that surface electronic effects are controlling the structural instability of both Mo(111) and W(111) . Structure sensitivity in a model catalytic reaction, n-butane hydrogenolysis, is observed over planar and faceted Pt/W(111) . We have also used soft x-ray photoemission spectrosocopy (SXPS) based on synchrotron radiation methods to characterize the bimetallic interface; for Pt, Pd and Au on W(111) , we find that substrate core level shift effects associated with interface formation are substantial, while those associated with faceting are rather subtle.

Morphological instabilities induced by ultrathin films on W(111)

Using ultrahigh vacuum surface science methods, we find that the atomically rough W(111) surface coated with ∼ 1 monolayer of certain metals (Pt,Pd,Au,…) undergoes extensive restructuring to form a highly faceted surface with pyramidal microfacets after annealing to T > 750 K. The dominant facet orientation is W{211}, and facet dimensions range from ∼1–100 nm. Not all metal monolayer films cause faceting of W(111): metals having Pauling electronegativities > 2.0 (Pt,Pd,Ir,Rh,Au) induce faceting of W(111) upon annealing, whereas those with electronegativities less than 2.0 (Ti,Co,Ni,Cu,Ag) do not. Electronic effects appear to be the driving factors in the observed morphological instability of W(111).