Electronegativity Difference Research Articles

Electronegativity Difference as a Descriptor for the Oxidation-Inhibiting Effect of the Alloying Element during the Early Stages of Titanium Oxidation.

Degradation of the mechanical properties of α-titanium, which is used to manufacture parts of jet engines, due to high-temperature oxidation is detrimental for the engine components. Therefore, to overcome this problem there are ongoing endeavors to develop novel oxidation-resistant titanium alloys and improve the properties of the existing ones. In an effort to understand the effect of alloying on oxidation of the α-Ti(0001) surface and to identify descriptors for rational design of oxidation-resistant alloys, in this work, using density functional theory-based calculations, we studied oxygen sorption and surface to subsurface diffusion on pure and alloyed α-Ti(0001) surfaces. Zr, Hf, Nb, and Mo from the d block and Al, Ga, Si, and Ge from the p block were used as alloying elements. We find that the alloying elements prefer to segregate on the surface compared to the subsurface layers. Our calculations show that the diffusion barrier correlates with the difference in the electronegativity between the alloying element and Ti. Elements which are more electropositive than Ti are found to hinder the oxygen dissolution in Ti and vice versa. We propose that the electronegativity difference can act as a good descriptor for choosing alloying elements. Our results are in reasonably good agreement with experimental reports on the growth of oxide layers on these alloyed Ti surfaces.

On cooling rates dependence of microstructure and mechanical properties of refractory high-entropy alloys HfTaTiZr and HfNbTiZr

Structural stability of high entropy alloys (HEAs) at intermediate temperatures during which the configurational entropy loses its dominance is crucial. In this work, we have scrutinized the effects of cooling rates on the phase stability of the HfTaTiZr(Ta1) and HfNbTiZr (Nb1) refractory HEAs (RHEAs). Ta1 exhibited strong microstructure and tensile property dependence on the cooling route, but not Nb1. Ta1 retained the BCC lattice under water quenching but experienced complex phase decomposition upon air and furnace cooling. The different dependence on cooling rates in these RHEAs arises from three main reasons: (i) the wider metastable temperature range in Ta1, (ii) the larger mobility difference between Ta and other constituents, and (iii) the larger electronegativity difference of Nb1. The current work not only sheds new insights into the understanding of the thermal and mechanical stability of HEAs but also provides a new paradigm for the optimization of their overall properties.

Significantly suppressed thermal transport by doping In and Al atoms in gallium nitride.

Thermal transport plays a key role in the working stability of gallium nitride (GaN) based optoelectronic devices, where doping has been widely employed for practical applications. However, it remains unclear how doping affects thermal transport. In this study, based on first-principles calculations, we studied the doping effect on the thermal transport properties of GaN by substituting Ga with In/Al atoms. The thermal conductivities at 300 K along the in-plane(out-of-plane) directions of In- and Al-doped GaN are calculated to be 7.3(8.62) and 12.45(11.80) W m-1 K-1, respectively, which are more than one order of magnitude lower compared to that of GaN [242(239) W m-1 K-1]. From the analysis of phonon transport properties, we find that the low phonon group velocity and small phonon relaxation time dominate the degenerated thermal conductivity, which originated from the strong phonon anharmonicity of In/Al-doped GaN. Furthermore, by examining the crystal structure and electronic properties, the lowered thermal conductivity is revealed lying in the strong polarization of In-N and Al-N bonds, which is due to the large difference in electronegativity of In/Al and N atoms. The results achieved in this study have guiding significance to the thermal transport design of GaN-based optoelectronic devices.

Methanol sensing characteristics in undoped and (Zn, Mn) doped Fe(phen)2(NCS)2 spin-crossover thin films

The methanol sensing properties of spin-crossover (SCO) complex Fe(phen)2(NCS)2 thin, as well as those of thin films doped with Zn or Mn are demonstrated here. The synchrotron x-ray diffraction (SXRD) data clearly confirms the crystal structure of the SCO complex. The microstructure studied by transmission electron microscopy (TEM) clearly shows the formation of cuboid-shaped growth of nanocrystals and is similar for all the samples. The optical photographs of the samples show an evident change in color from reddish-purple color to pink upon exposure to methanol vapor. The optical absorption band (∼540 nm) in the presence of methanol of the SCO films is shifted to a longer wavelength with exposure time. The methanol sensitivity of SCO films is further investigated using electrical resistance measurements, which reveal that the response differs between Zn and Mn doped films and undoped ones. The difference in electronegativity of metals plausibly alters the polaron hopping transport in the SCO films and explains the observed variation of resistance with methanol exposure. The data suggest that using spin-crossover materials may be pretty important in designing smart sensors in the future.

A piezoelectric quantum spin Hall insulator VCClBr monolayer with a pure out-of-plane piezoelectric response.

The combination of piezoelectricity with a nontrivial topological insulating phase in two-dimensional (2D) systems, namely piezoelectric quantum spin Hall insulators (PQSHI), is intriguing for exploring novel topological states toward the development of high-speed and dissipationless electronic devices. In this work, we predict a PQSHI Janus monolayer VCClBr constructed from VCCl2, which is dynamically, mechanically and thermally stable. In the absence of spin orbital coupling (SOC), VCClBr is a narrow gap semiconductor with a gap value of 57 meV, which is different from Dirac semimetal VCCl2. The gap of VCClBr is due to a built-in electric field caused by asymmetrical upper and lower atomic layers, which is further confirmed by the external-electric-field induced gap in VCCl2. When including SOC, the gap of VCClBr is increased to 76 meV, which is larger than the thermal energy of room temperature (25 meV). The VCClBr is a 2D topological insulator (TI), which is confirmed by Z2 topological invariant and nontrivial one-dimensional edge states. It is proved that the nontrivial topological properties of VCClBr are robust against strain (biaxial and uniaxial cases) and external electric fields. Due to broken horizontal mirror symmetry, only an out-of-plane piezoelectric response can be observed, when a biaxial or uniaxial in-plane strain is applied. The predicted piezoelectric strain coefficients d31 and d32 are -0.425 pm V-1 and -0.219 pm V-1, respectively, and they are higher than or compared with those of many 2D materials. Finally, Janus monolayer VCFBr and VCFCl (dynamically unstable) are also constructed, and they are still PQSHIs. Moreover, the d31 and d32 of VCFBr and VCFCl are higher than those of VCClBr, and the d31 (absolute value) of VCFBr is larger than one. According to out-of-plane piezoelectric coefficients of VCXY (X ≠ Y = F, Cl and Br), CrX1.5Y1.5 (X = F, Cl and Br; Y = I) and NiXY (X ≠ Y = Cl, Br and I), it is concluded that the size of the out-of-plane piezoelectric coefficient has a positive relation with the electronegativity difference of X and Y atoms. Our studies enrich the diversity of Janus 2D materials, and open a new avenue in the search for PQSHI with a large out-of-plane piezoelectric response, which provides a potential platform in nanoelectronics.

Eco-friendly cooling materials with synergistic behavior of electromechanical and electrocaloric effects based on constructing B-site defect field

Nowadays, the materials with synergistic electromechanical (EM) and electrocaloric (EC) effects have gained significant inspiration and expectation for potential self-actuating cooling technology. Such a technique relies on high-performance materials that have both of two effects. Due to environmental consideration, lead-free Bi0.5Na0.5TiO3 (BNT) system was selected to construct the B-site local random field, which successfully achieved high mechanical and electrical performance in a large temperature span, to cope with the challenges of narrow operating temperature region, high driving electric field and poor performance. The relaxor evolution is induced by the difference in valence state, electronegativity and ion radius between substituted and original B-site ions. Going further, it is found that the B-site defects and oxygen vacancies produced by the process are crucial to the reversible polarization response. Phenomenological theory reveals that the essence of reducing the driving electric field in the optimized system lies in lowering the potential barrier from relaxor to ferroelectric phase transition. To a certain extent, our work should therefore motivate the exploration and development of environmental-friendly smart cooling materials.

Enhanced out-of-plane piezoelectricity of group-III(A) Janus hydrofluoride monolayers

Two-dimensional (2D) piezoelectric materials utilizing the unique electronic characteristic were widely needed for microelectronic sensors and energy conversion devices. However, the currently reported 2D piezoelectric films face thorny challenges in practical application due to lack of the mechanical durability or the strong vertical piezoelectric effect. Herein, based on first-principles calculations, we theoretically simulate the Janus semiconductor nanosheets, namely, $\mathrm{F}\text{\ensuremath{-}}M\text{\ensuremath{-}}H$ ($M=\mathrm{B}$, Al, and Ga) monolayers. The result shows that they have admirable dynamical and thermal stabilities to achieve strong mechanical durability and possess the desired vertical piezoelectric effect by applying uniaxial strain in the $z$-axis direction vertical to the nanosheet surface. For the three materials, F-B-H has the strongest stability and the most significant vertical piezoelectric effect. The out-of-plane piezoelectric stress coefficient ${e}_{31}$ of F-B-H is higher than other reported 2D piezoelectric SnOSe films by two orders of magnitude. Furthermore, we found an interesting relationship among these three nanosheets: the higher the electronegativity difference ratio, the higher the piezoelectric stress coefficient. Finally, this work not only deepens the understanding of 2D piezoelectric materials, but also provides a platform for fabricating cutting-edge piezoelectric equipment such as wearable electronic, medical blood pressure detectors and robotic bionic skin tactile sensors.

Boron Nitride Nanotube-Based Separator for High-Performance Lithium-Sulfur Batteries.

To prevent global warming, ESS development is in progress along with the development of electric vehicles and renewable energy. However, the state-of-the-art technology, i.e., lithium-ion batteries, has reached its limitation, and thus the need for high-performance batteries with improved energy and power density is increasing. Lithium-sulfur batteries (LSBs) are attracting enormous attention because of their high theoretical energy density. However, there are technical barriers to its commercialization such as the formation of dendrites on the anode and the shuttle effect of the cathode. To resolve these issues, a boron nitride nanotube (BNNT)-based separator is developed. The BNNT is physically purified so that the purified BNNT (p−BNNT) has a homogeneous pore structure because of random stacking and partial charge on the surface due to the difference of electronegativity between B and N. Compared to the conventional polypropylene (PP) separator, the p−BNNT loaded PP separator prevents the dendrite formation on the Li metal anode, facilitates the ion transfer through the separator, and alleviates the shuttle effect at the cathode. With these effects, the p−BNNT loaded PP separators enable the LSB cells to achieve a specific capacity of 1429 mAh/g, and long-term stability over 200 cycles.

Structure and microwave dielectric properties of (Zn1/3Nb2/3)4+ co-substituted MgTiO3 ceramic

In this work, MgTi1-x(Zn1/3Nb2/3)xO3 (0 ≤ x ≤ 0.3) ceramics were synthesized by a solid-state reaction method. MgTiO3 is identified as main phase, while MgTi2O5 is detected as secondary phase at 0 ≤ x ≤ 0.3, in association with a minor Mg2TiO4 phase at x = 0. The (Zn1/3Nb2/3)4+ co-substituted ions are responsible for the change of bond length and the TiO6 octahedral distortion. The quality factor (Q × f) depends on the electronegativity difference (Δe) and average covalency. The dielectric constant (εr) is related to molecular polarizability (αtheo) and tolerance factor (t). Besides, the temperature coefficient of resonance frequency (τf) strongly correlates with TiO6 octahedral distortion and the secondary phase. In comparison to pure MgTiO3, desirable microwave dielectric properties of εr = 17.59, Q × f = 211 600 GHz, and τf = −50 ppm/°C were obtained when x = 0.12 sintered at 1250 °C. The MgTi0.88(Zn1/3Nb2/3)0.12O3 ceramic is a candidate material for the wireless communication applications.

Correlating electronegativity in bimetallic oxygen carriers for chemical looping combustion

Metallic tailoring of oxygen carrier plays a crucial role within chemical looping combustion (CLC) applications, which can be correlated to the redox reaction enhancement and sustainability. Such attributes arise from the electric valence of the material constituents, which can bring about superior oxygen vacancy and structural stability. In the concurrent study, the role of electronegativity of the bimetallic carriers upon CLC performance was scrutinized based on various bimetallic metal oxides composition. Two metallic systems were examined in the current study: Iron based oxides and copper-based oxides, where the secondary metallic element within each of the examined oxide carrier is composed of one of the 5 metallic elements, predefined for the purpose of electronegativity variation. The secondary metallic element include: Calcium, Lanthanum, Magnesium, Manganese, and Chromium. Several gas byproducts were monitored during the course of the CLC reaction, via mass spectrometer, in order to quantify the variations incurred as a result of different metallic composition inclusion. Additionally, electron microscopic techniques, such as the SEM, were utilized in the study, so as to visualize the structural alteration of sintered materials. The experimental results indicate a higher reactivity with the fuel at a higher electronegativity difference within the bimetallic structure, in the case of iron-based oxide system. Also, copper-based oxide system was associated with higher carbon dioxide production with higher selectivity, relative to iron-based oxide systems.

The high piezoelectricity, flexibility and electronic properties of new Janus ZnXY2 (X = Ge, Sn, Si and Y = S, Se, Te) monolayers: A first-principles research

Due to the broken of inversion and mirror symmetry, Janus structures are believed to have abundant properties and can be applied in two-dimensional (2D) materials, but the research of 2D Janus materials is insufficient. In this work, Janus materials ZnXY2 (X = Ge, Sn, Si and Y = S, Se, Te) monolayers are designed referring to group-III monochalcogenides. The electronic, mechanical and piezoelectric properties and stabilities of ZnXY2 monolayers are investigated by the first-principles calculations. The electronic property shows that ZnXY2 monolayers are semiconductors with wide direct band-gaps and large effective masses difference between holes and electrons. The mechanical property shows that ZnXY2 monolayers possess low Young’s modulus, bending modulus and ductile properties, which is beneficial for applications in flexible nanodevices. The piezoelectric property shows that, compared with conventional Janus group-III monochalcogenides and bulk piezoelectric materials, ZnXY2 monolayers exhibit higher in-plane and comparable out-of-plane piezoelectricity. The further analyzations indicate that the distances and electronegativity differences between atoms are two important influence factors to piezoelectricity. In summary, the flexibility, piezoelectricity, direct band-gaps and large effective mass difference make ZnXY2 monolayers promising candidates for optoelectronics, photocatalysis, flexible nanodevices and electromechanical systems.

Blue and black phosphorene on metal substrates: a density functional theory study

We have performed density functional theory calculations to study blue phosphorene and black phosphorene on metal substrates. The substrates considered are the (111) and (110) surfaces of Al, Cu, Ag, Ir, Pd, Pt and Au and the (0001) and (100) surfaces of Zr and Sc. The formation energy E F is negative (energetically favorable) for all 36 combinations of overlayer and substrate. By comparing values of ΔΩ, the change in free energy per unit area, as well as the overlayer-substrate binding energy E b, we identify that Ag(111), Al(110), Cu(111), Cu(110) and possibly Au(110) may be especially suitable substrates for the synthesis and subsequent exfoliation of blue phosphorene, and the Ag(110) and Al(111) substrates for the synthesis of black phosphorene. However, these conclusions are drawn assuming the source of P atoms is bulk phosphorus, and can alter upon changing synthesis conditions (chemical potential of phosphorus). Thus, when the source of phosphorus atoms is P4, blue phosphorene is favored only over Pt(111). We find that for all combinations of overlayer and substrate, the charge transfer per bond can be captured by the universal descriptor , where Δχ and are, respectively, the differences in electronegativity and atomic size between phosphorus and the substrate metal.

Role of electronegativity to induce magnetism in ZnX1−x Y x (X = S/Se/Te and Y = Li/Be/B) chalcogenides

Ab-initio calculations were performed to investigate the structural, electronic, and magnetic characteristics of the selected first row (Y = Li, Be, and B) doped zinc-blende ZnX (X = S, Se, and Te) chalcogenides. Firstly, the structural stability of the doped materials is analyzed by computing the formation energies, which substantially depends on the dopant atomic numbers and Y-doped ZnSe systems are energetically more stable. It is established that when the electronegativity of the dopant is less than that of the host atom, magnetism is induced. Our results revealed that selected intrinsically non-magnetic dopants (Y = Li, Be, and B) induce magnetic characteristics in all the studied ZnX chalcogenides structures except the B-doped ZnTe system due to a very small electronegativity difference between B and Te ions. The most striking feature of the present study is that Be-doped ZnX materials display the half-metallic ferromagnetism, and Be 2p non-degenerate orbitals are playing a major role in inducing magnetism and metallicity. Hence, the present work proposed that doping engineering with suitable impurity elements having electronegativity larger than that of the host atom could be an effective way to tune the physical properties of chalcogenides for their technological potential applications in advanced-spin-based devices.

Why the Second Bond Exceptionally Effective on pKa Values of -ynoic (-C≡C-) Compounds? Chemical Education Perspective

Sudden decrease of nearly more than two orders of magnitude of pKa units of ynoic series of compounds is explained based on the electronegativity differences of sp, sp2 and sp3 carbons and on the subsequent stability of the corresponding anions. Both resonance and inductive effects operate in tandem to stabilize the ynoic anions.

Ensemble-based machine learning models for phase prediction in high entropy alloys

High entropy alloys (HEAs), which are multi-component alloys having constituent elements in equi-atomic or near equi-atomic ratios, are receiving immense attention owing to their remarkable mechanical and physical properties. These unusual properties depend on one or more of the phases that these alloy systems constitute, namely solid solution (SS), intermetallic compound (IM), and amorphous (AM) phases. Therefore, phase prediction is crucial in selecting appropriate elements that lead to the formation of a HEA with desirable properties. In this work, machine learning (ML) models are used on a set of design parameters using multi-classification for HEA phase prediction. The ML models comprised ensemble-based (Random Forest and Stacked ensemble) and support vector machine (SVM) methods. To predict solid solution phase formation, researchers used atomic size difference and the parameter Ω=TmΔSmixΔHmix where Tm is average melting point, ΔSmix is entropy of mixing and ΔHmix is mixing enthalpy. The features used in this study are Ω, atomic size difference, average electron concentration, electronegativity difference, average melting point, mean atomic radius, mixing enthalpy, the entropy of mixing, and bulk modulus. The training dataset comprising of 601 as cast alloys is used with cross-validation for test data, and the phases SS, IM, AM, SS + IM, and IM + AM are predicted. The phase prediction accuracies are calculated using one-vs-rest, and precision-recall curves are plotted to determine the model performance. For phases AM, SS, and IM, the stacked ensemble displayed better accuracies when compared to SVM and Random Forest. The findings indicate that the stacked ensemble, which combines weak learners and meta-models, provides accuracy comparable to the neural network model accuracy reported in the literature. These findings provide insights to researchers and practitioners in selecting features and ML models while designing HEAs.

Nanocrystalline single-phase high-entropy alloy synthesized by using intermetallic compound type (TiZrHf)-(NiCuCo) high-entropy metallic glass precursor

‘AB’ intermetallic compound (IC) type (TiZrHf)-(NiCuCo) high-entropy metallic glass precursor (HE-MGP) has been designed under considering physical and thermodynamic relations between constitutive elements, such as atomic size difference, electronegativity difference, mixing enthalpy and entropy, and phase stability, for development of nanocrystalline single-phase high-entropy alloy. The IC type HE-MGP exhibits high thermal stability and polymorphic crystallization behavior. From the systematic investigations on crystallization kinetics and microstructural evolution, it was found that the HE-MGP crystallized into nanocrystalline single-phase high-entropy alloy with B2 structure by polymorphic crystallization, and nucleation and growth rates for crystallization processing strongly depend on the annealing temperature. Our approach and discovery provide an effective way to develop the nanocrystalline single-phase high-entropy alloys by designing IC type HE-MGP.

Cover Feature: The Chemical Bond: When Atom Size Instead of Electronegativity Difference Determines Trend in Bond Strength (Chem. Eur. J. 63/2021)

Atom size, not electronegativity difference, determines C−X bond strength, which weakens from X=F to I. In their Communication (DOI: 10.1002/chem.202103544), M. Bickelhaupt et al. reveal that, as the atom size increases, it is the increase in steric Pauli repulsion that weakens the C−X bond, and not the weakening of the orbital interaction (which, in fact, strengthens it!), as is traditionally considered in the electronegativity model. With state-of-the-art DFT calculations, they provide compelling evidence to challenge the electronegativity model.

Phase prediction and new parametric approach for multi-component alloys with using deep learning API: Keras

In this work phase prediction was estimated by deep learning algorithm using Keras. 400 alloys were randomly selected from the literature. The valence electron concentration (VEC), atomic size difference (δ), electronegativity difference (∆χ) and κ values are calculated for each alloy and a dataset has been created. Here κ value was defined as a new parametric value to be used in phase estimation. The κ value defined in this study will be an important tool in phase estimations. 320 multi-components’ data was used to train and 80 multi-components’ data was used as validation data. Here three different adaptive optimizers were used: Adam, Adagrad and RMSprop. Most important key feature of adaptive optimizers is that do not require a tuning of the learning rate. The aim of the machine-learning is to develop the capability of the models’ generalisation. Estimation of the models must be succeeded in the data that never seen before. Critical point for machine-learning model is overfitting. In this study; simple hold-out validation was used to avoid overfitting. 320 data used for training and 80 data used for validation. A second check was applied using dropout regularization techniques to avoid overfitting. Adam, Adagrad and RMSprop estimation achieved 90%, 66% and 88% respectively. After all these processes, the physical parameters of the 58 multi-component alloy were entered, which are not in 400 data, and predicted that it had an amorphous or crystalline structure. The κ value defined in this study has shown that it is a simple and easily calculable limit value in phase estimation for multi-component alloys.

Enhanced electrochemical advanced oxidation on boride activated carbon: The influences of boron groups

Exploring an efficient electro-catalyst is fundamentally important to enhance degradation of persistent organic pollutants during electrochemical advanced oxidation process. In this work, boride activated carbon (BAC) containing different boron group content was prepared as low-cost and efficient catalysts. BAC possessed a more positive onset potential of oxygen reduction reaction (0.458 V vs. RHE) and a faster rate of H2O2 generation (365 mg/L pH=3), compared with activated carbon (0.425 V vs. RHE, 138 mg/L pH=3). DFT calculations and XPS results revealed that -BC3 and -BC2O active groups enhanced oxygen adsorption, and small electronegativity difference between B and O facilitated the desorption of •OOH. The rate of in-situ phenol degradation (0.193 min−1) increased by an order of magnitude as compared to activated carbon (0.018 min−1), because BAC promoted the in-situ conversion of H2O2 to •OH. Therefore, it provided a theoretical base for the further application of boron catalyst in two-electron oxygen reduction process.

High Carrier Mobility and High Figure of Merit in the CuBiSe2 Alloyed GeTe

AbstractAccording to the Mott's relation, the figure‐of‐merit of a thermoelectric material depends on the charge carrier concentration and carrier mobility. This explains the observation that low thermoelectric properties of GeTe‐based materials suffer from the degraded carrier mobility, on account of the fluctuation of electronegativity and ionicity of various elements. Here, high‐performance CuBiSe2 alloyed GeTe with high carrier mobility due to the small electronegativity difference between Cu and Ge atoms and the weak ionicity of CuTe and BiTe bonds, is developed. Density functional theory calculations indicate that CuBiSe2 alloying increases the formation energy of Ge vacancies and correspondingly reduces the amount of Ge vacancies, leading to an optimized carrier concentration and a high power factor of ≈37.4 µW cm−1 K−2 at 723 K. Moreover, CuBiSe2 alloying induces dense point defects and triggers ubiquitous lattice distortions, leading to a reduced lattice thermal conductivity of 0.39 W m−1 K−1 at 723 K. These synergistic effects result in an optimization of the carrier mobility, the carrier concentration, and the lattice thermal conductivity, which favors an enhanced peak figure‐of‐merit of ≈2.2 at 723 K in (GeTe)0.94(CuBiSe2)0.06. This study provides guidance for the screening of GeTe‐based thermoelectric materials with high carrier mobility.