Bond Ionicity Research Articles

Metal-Oxygen Bond Ionicity as an Efficient Descriptor for Doped NiOOH Photocatalytic Activity.

The computational design of solid catalysts has become a very "hot" field during the last decades, especially with the recent increase in computational tool performance. However, theoretical techniques are still very time demanding because they require the consideration of many adsorption configurations of the reaction intermediates on the surface. Herein, we propose to use the metal-oxygen (M-O) bond ionicity as a descriptor for the photocatalytic activity of one of the best catalysts for the oxygen evolution reaction (OER). Ionicity is a bulk property and thus carries the advantage of being easily obtainable from a simple Bader charge analysis by using density functional theory (DFT). We will show that this criterion can be used successfully to design efficient dopants for NiOOH material. This catalyst is known to exhibit interesting photoelectrochemical properties for OER if it is doped with specific transition metals. Finally, we demonstrate that other electronic properties that relate to bulk calculation, such as oxidation states and density of states, are not alone sufficient to explain the photocatalytic activity of the material. Thus, M-O bond ionicity attracts significant interest compared with other bulk observables obtained by using DFT computations.

Dispersion Properties of (Y0.06GA0.94)2O3 Thin Films

We have studied the refractive index dispersion in (Y0.06Ga0.94)2O3 thin films obtained by high-frequency ion plasma sputtering. We show that the films are formed with mixed orientation, and the maxima of the diffraction patterns correspond to the monoclinic crystal structure of β-Ga2O3. We have established that as for β-Ga2O3 films, we observe anomalous dispersion in the near UV region for (Y0.06Ga0.94)2O3 films annealed in hydrogen, while we observe normal dispersion for films annealed in oxygen or argon. For the studied films with normal dispersion, we compare two single-oscillator approximation models, and we fi nd the parameters of the approximation, the dispersion energy, the chemical bond ionicity, the coordination number, the static refractive index, the characteristic wavelength for UV absorption, and the average oscillator absorption energy. We established that the static refractive index in the studied films increases as the number of oxygen vacancies increases, and that it is inversely proportional to the average oscillator absorption energy.

Infrared study of α–SiO2 single crystal under high pressure

We have measured high pressure α-quartz reflectance spectra in the mid infrared. We used single crystals, taking full profit of polarization. The quality of the spectra allows fitting the reflectance spectra. We have characterized the pressure evolution of E and A2 modes with increased precision, even in the spectral regions where they overlap. In addition, we have determined the TO-LO splitting of each mode. Some of the A2 modes show dramatic pressure variations of the LO-TO splitting, which cannot be explained only by changes in length and ionicity of individual bonds, requiring a new mechanism. We suggest that rotation of the SiO4 tetrahedra plays a fundamental role. We have also determined the evolution of the electronic dielectric constant under high pressure. We find that its pressure increment is mainly a volume effect, although the small increase in birefringence points to secondary changes associated to the electronic resonances.

Characterization of low loss microwave dielectric materials Li3Mg2NbO6 based on the chemical bond theory

The low loss microwave dielectric materials Li3Mg2NbO6 with orthorhombic structure were prepared by the conventional solid-state method. The microwave dielectric properties of Li3Mg2NbO6 ceramics exhibited significant dependence on the sintering conditions, microstructure and compositions. Single phase of Li3Mg2NbO6 could be obtained throughout the range of sintering temperatures. The saturated samples sinterted at 1225 °C for 4 h showed excellent microwave properties of εr value of 14.94, Q·ƒ value of 100,965 GHz and τf value of −21.96 ppm/°C. The chemical bond theory was used to calculate the lattice energy, ionicity energy and the coefficient of thermal expansion for the characterization of correlations between properties and structures of Li3Mg2NbO6 ceramics. The microwave dielectric properties of Li3Mg2NbO6 were strongly dependent on the chemical bond ionicity, lattice energy and coefficient of thermal expansion.

The chemical bond properties and ferroelectricity studies of SrBi4Ti4O15

Spontaneous polarization as the most immediate parameter in ferroelectricity is always an emphasis in ferroelectric research. Some ferroelectric microscopic theory such as Berry-phase method and first principles calculation are used to study the spontaneous polarization of perovskite type ferroelectrics. SrBi4Ti4O15 is a typical bismuth layered structure ferroelectric, the complexity of its crystal structure makes the ferroelectric research more difficult. This study, from the perspective of chemical bond, analyzes the relationship between the chemical bond properties and the spontaneous polarization, and further explores the atomic bonding state in ferroelectric crystal and its impact on ferroelectricity.By starting from the crystal structure data of SrBi4Ti4O15, the atomic local cluster structure including bond length, atomic coordination situation and the number of atoms in a crystal structure unit are obtained by the atomic environment calculation (AEC). Calculation results show that there are 13 atomic local cluster structures in SrBi4Ti4O15. Then combined with the crystal decomposition method, the SrBi4Ti4O15crystal is decomposed into 38 pseudo-binary crystals, and each pseudo-binary crystal corresponds to a chemical bond. Accordding to the dielectric theory of chemical bond that used in binary crystal, chemical bond properties such as the number of effective valence electron, the effective valence electron density and the bond ionicity are calculated. Through improvement of the bond dipole moment model, the relationship among bond dipole moment, bond properties, and bond parameter is established, and the bond dipole moment of each bond in SrBi4Ti4O15 is calculated.The spontaneous polarization of an ferroelectric can be approximated as the superposition of the spontaneous polarization of various chemical bonds in the crystal, where the spontaneous polarization of chemical bond is due to the bond dipole moment. Based on the traditional polarization theory, the spontaneous polarization can be expressed as the average bond dipole moment per unit volume, and considering the number of molecules in unit cell (Z) and the atomic site occupation factor (SOF), the correlation between bond dipole moment and spontaneous polarization of the bismuth layered ferroelectrics is established. On the basis of this, the calculated spontaneous polarization along a axis in the ferroelectric SrBi4Ti4O15 is 28.03 C/cm2.This study simplifies the complex crystal research by AEC and crystal decomposition method, and studies the ferroelectricity of the bismuth layered ferroelectrics from the viewpoint of chemical bond. The bond dipole moment as the bridge in this study for connecting spontaneous polarization with chemical bond properties, which is a new micro study method in macro-properties of bismuth layered ferroelectrics. Based on this method, the spontaneous polarization of other relevant ferroelectrics such as orthogonal SrBi2Ta2O9, orthogonal Bi4Ti3O12, and tetragonal SrBi4Ti4O15 are calculated, all the results are in good agreement with the experimental values and other theoretically calculated values.

Characterization of crystal structure and microwave dielectric properties of AZrNb2O8 (A=Zn, Co, Mg, Mn) ceramics based on complex bond theory

Single phase AZrNb2O8 (A=Zn,Co,Mg,Mn) ceramics with monoclinic wolframite structure were prepared and characterized by crystalline structure refinement for the investigation on the correlations between crystal structure and microwave dielectric properties. A semiempirical method based on the complex bond theory was used to calculate the bond ionicity, lattice energy, and coefficient of thermal expansion of AZrNb2O8 ceramics. The crystal structure changed with the variation of A site ions and affected the Nb–O octahedral correspondingly. Due to the variation of crystal structure, the observed bond ionicity was helpful to understand the electric polarization. The variation of the Q·ƒ and τƒ values could be attributed to the lattice energy and coefficient of thermal expansion respectively. The microwave properties of AZrNb2O8 ceramics were strongly dependent on the chemical bond ionicity, lattice energy and coefficient of thermal expansion.

Band structures of atomic chains of group IV, III–V, and II–VI elements

The relativistic band structures of AN and ANB8–N chains have been calculated by the linear augmented-cylindrical-wave method, which is an extension of the augmented-plane-wave method for cylindrical polyatomic systems. The band structures of covalent monatomic chains of Group IV elements are characterized by σ(s), π+, and π–, and σ(pz)* bands. The C, Si, Ge, and Sn chains are metallic. There is a considerable difference between the relativistic and nonrelativistic band structures. Because of the cylindrical symmetry of chains in the nonrelativistic model, the π bands crossing the Fermi level are orbitally doubly degenerate (i.e., the π+ and π– band energies are exactly the same). The spin and orbital motion of electrons are coupled in the chains to split π bands, but each π+ and π- band is doubly spin-degenerate. The spin–orbit splitting energy for C and Sn chains varies from 1.7 meV to 0.67 eV. The mass–velocity correction reduces all valence band levels: the level shifts are 2–5 meV for C and up to 2.2 eV for Sn. The Darwin corrections are several-fold lower than the mass–velocity contributions. A sharp change in the band structure is observed in going from covalent to partially ionic chains. The carbon chain has a metallic band structure with a zero gap in the center of the Brillouin zone, and a boron nitride chain is an insulator with an optical gap of 8 eV and optical transitions between the occupied π and vacant π* states at the edge of the Brillouin zone (this is explained by the existence of the antisymmetric component of the electron potential in the BN wire, which mixes even bonding and odd antibonding π states). Going from the BN chain to the AlP, GaAs, and InSb chains is accompanied by a decrease in the chemical bond ionicity, which leads to a gradual decrease in the π–π* gaps.

Complex chemical bond theory, Raman spectra and microwave dielectric properties of low loss ceramics NdNbO4–xAl2O3

In this work, low loss microwave dielectric materials of NdNbO4–x wt% Al2O3 ceramics were prepared via a solid-state reaction method. The complex chemical bond theory, phase composition, standard deviation (σ) of the bond angles, microstructures, microwave dielectric properties and vibrational phonon modes were investigated. Microscopic analysis showed that sintered specimens presented single monoclinic fergusonite phase. The Rietveld refinements and Raman spectra were used to evaluate the correlation between the complex chemical bond theory and the microwave dielectric properties. With an increase of Al2O3 content, the Raman shift of Ag (331 and 808 cm−1) toward to the bigger value direction and the FWHM of Ag (331 and 808 cm−1) decrease, which lead to a decrease in bond ionicity and increase in lattice energy. The microwave dielectric properties of NdNbO4–x wt% Al2O3 exhibit closely relationship with the complex chemical bond theory. The variation trend of dielectric constant was accordance with the bond ionicity. The Q × f value and τ f value were mainly dependence on the lattice energy and bond energy, respectively. Fine microwave dielectric properties for NdNbO4–3 wt% Al2O3 ceramic was obtained with er = 18.1, Q × f = 54,700 GHz (9.1 GHz), τ f = −0.51 ppm/°C sintered at 1150 °C for 4 h.

Pockels Coefficients in Multicomponent Oxide Glasses

In this work, a corrected model for the additivity of the Pockels coefficients in multicomponent oxide glasses is derived. Given some material deformation, changes in density and electronic polarizability are known to be the dominant factors in the photoelastic effect. A trend of Pockels coefficients decreasing with increasing bond ionicity is postulated. This is discussed and shown to be due to the balancing of the relative changes in density and in polarization based on the bond strength and directionality. Alkali silicate data taken from the literature, shown to oppose the observed trends, is analyzed and compared to alkaline earth silicate data. The opposing trend is shown to be due to the bonding of the modifier ions. The alkali ions, associated with a single nonbridging oxygen (NBO), yield an increased relative density change and a decreased relative change in polarization. The alkaline earth ions, being associated with two NBOs, as expected experience significant density and polarizability changes that are more spatially distributed. Greater insights into the structural and chemical origins of the Pockels coefficient in multicomponent oxide glasses will enable the development of new optical fibers for high energy laser applications where photoelasticity‐influenced parasitic effects such as stimulated Brillouin scattering are mitigated.

On EXAFS Debye-Waller factor and recent advances

The effects of structural and vibrational disorder on the EXAFS signals are parameterized in terms of the Debye Waller (DW) factor. Here the vibrational contribution is addressed, which for most systems can be singled out by studying the temperature dependence of the EXAFS DW factor, which corresponds to a good accuracy to the parallel mean square relative displacement (MSRD) around the inter-atomic equilibrium distance. By comparing the first-shell EXAFS thermal expansion with the crystallographic thermal expansion one can evaluate the perpendicular MSRD. The results of recent measurements on copper and on several tetrahedral semiconductors are here critically compared and some properties of the MSRDs are discussed, such as the dependence of correlation, force constants and vibrational anisotropy on crystal structure and bond ionicity as well as the relative merits of the correlated Debye and Einstein models. The anharmonic contribution to the parallel MSRD of CdTe has been evaluated and a quasi-harmonic analysis has been attempted, leading to an estimation of the bond Grüneisen parameter.

Computational discovery of ferromagnetic semiconducting single-layerCrSnTe3

Despite many single-layer materials being reported in the past decade, few of them exhibit magnetism. Here we perform first-principles calculations using accurate hybrid density functional methods (HSE06) to predict that single-layer ${\mathrm{CrSnTe}}_{3}$ (CST) is a ferromagnetic semiconductor, with band gaps of 0.9 and 1.2 eV for the majority and minority spin channels, respectively. We determine the Curie temperature as 170 K, significantly higher than that of single-layer ${\mathrm{CrSiTe}}_{3}$ (90 K) and ${\mathrm{CrGeTe}}_{3}$ (130 K). This is due to the enhanced ionicity of the Sn-Te bond, which in turn increases the superexchange coupling between the magnetic Cr atoms. We further explore the mechanical and dynamical stability and strain response of this single-layer material for possible epitaxial growth. Our study provides an intuitive approach to understand and design single-layer magnetic semiconductors for a wide range of spintronics and energy applications.

Correlations of crystal structure, bond energy and microwave dielectric properties of AZrNb2O8 (A = Zn, Co, Mg, Mn) ceramics

The monoclinical structure AZrNb2O8 (A = Zn, Co, Mg, Mn) ceramics were prepared by the conventional solid-state method. The microwave dielectric properties of AZrNb2O8 (A = Zn, Co, Mg, Mn) ceramics exhibited significant dependence on the sintering conditions, microstructure and compositions. The crystalline structure refinement, chemical bond ionicity, lattice energy were carried out to investigate the correlations between the crystalline structure and microwave dielectric properties for AZrNb2O8 (A = Zn, Co, Mg, Mn) ceramics based on the complex bond theory. The effects of A-site cations (Mn, Zn, Mg and Co) at AZrNb2O8 compounds on structural parameters such as packing fraction and B-site octahedral distortion were studied based on Rietveld refinement. Variations in the dielectric constant (εr) were explained by the ionic polarizability of the compositions. The quality factor (Q·ƒ) and temperature coefficient of resonant frequency (τf) were correlated with the packing fraction and B-site octahedral distortions (δ) in these compositions, respectively. Correlations between the microwave dielectric properties and crystal structure, the chemical bond energy were also discussed based on the variation of A-site ions.

On the theory of elastic properties of two-dimensional hexagonal structures

The properties of graphene and graphene-like materials (GLMs) have been considered using the Harrison bond-orbital method, within which the stability of GLMs with a high bond ionicity has been analyzed. For purely covalent crystals (graphene, silicene, and germanen) and GLMs with a low bond ionicity, the anharmonic elastic properties (Gruneisen constant, pressure and temperature dependences of the bulk modulus) have been considered. Within the Keating force constant model, the second- and third-order elastic constants and the pressure dependence of the second-order elastic constants have been determined. The parameters of the previously proposed empirical potential have been determined. The results obtained are compared with the available experimental data and calculation results of other studies.

The bond ionicity in ANB8−N compounds from maximally localized Wannier functions

The bond ionicity in seventy two ANB8−N compounds is investigated according to the recently introduced first-principles ionicity scale, based on the centers of the maximally localized Wannier functions, which has several interesting features. The obtained bond ionicities (qi) are found to exhibit the expected trends, according to electronegativity arguments. In particular, the bond ionicity in the alkaline-earth oxides increases by going from MgO to BaO. A strong crystal structure dependence of qi is observed. A critical value of qi (of 0.91) that separates between the tetrahedrally and octahedrally coordinated systems is inferred directly from the calculated values of qi. The volume dependence of qi is investigated for all the considered compounds and found to reduce by volume decrease for most of the studied systems. The adopted ionicity scale is established as a very strong competitor to the most widely accepted Phillips and Pauling ionicity measures.

Origin of nonlinear piezoelectricity in III-V semiconductors: Internal strain and bond ionicity from hybrid-functional density functional theory

We derive first- and second-order piezoelectric coefficients for the zinc-blende III-V semiconductors, {Al,Ga,In}-{N,P,As,Sb}. The results are obtained within the Heyd-Scuseria-Ernzerhof hybrid-functional approach in the framework of density functional theory and the Berry-phase theory of electric polarization. To achieve a meaningful interpretation of the results, we build an intuitive phenomenological model based on the description of internal strain and the dynamics of the electronic charge centers. We discuss in detail first- and second-order internal strain effects, together with strain-induced changes in ionicity. This analysis reveals that the relatively large importance in the III-Vs of non-linear piezoelectric effects compared to the linear ones arises because of a delicate balance between the ionic polarization contribution due to internal strain relaxation effects, and the contribution due to the electronic charge redistribution induced by macroscopic and internal strain.

First principles approach to ionicity of fragments

We develop a first principles approach towards the ionicity of fragments. In contrast to the bond ionicity, the fragment ionicity refers to an electronic property of the constituents of a larger system, which may vary from a single atom to a functional group or a unit cell to a crystal. The fragment ionicity is quantitatively defined in terms of the coefficients of contributing charge states in a superposition of valence configurations of the system. Utilizing the constrained density functional theory-based computations, a practical method to compute the fragment ionicity from valence electron charge densities, suitably decomposed according to the Fragment Hamiltonian (FH) model prescription for those electron densities, is presented for the first time. The adopted approach is illustrated using BeO, MgO and CaO diatomic molecules as simple examples. The results are compared and discussed with respect to the bond ionicity scales of Phillips and Pauling.

The relationship between bond ionicity, lattice energy, coefficient of thermal expansion and microwave dielectric properties of Nd(Nb(1-x)Sb(x))O4 ceramics.

The crystalline structure refinement, chemical bond ionicity, lattice energy and coefficient of thermal expansion were carried out for Nd(Nb(1-x)Sb(x))O4 ceramics with a monoclinic fergusonite structure to investigate the correlations between the crystalline structure, phase stability, bond ionicity, lattice energy, coefficient of thermal expansion, and microwave dielectric properties. The bond ionicity, lattice energy, and coefficient of thermal expansion of Nd(Nb(1-x)Sb(x))O4 ceramics were calculated using a semiempirical method based on the complex bond theory. The phase structure stability varied with the lattice energy which was resulted by the substitution constant of Sb(5+). With the increasing of the Sb(5+) contents, the decrease of Nb/Sb-O bond ionicity was observed, which could be contributed to the electric polarization. The ε(r) had a close relationship with the Nb/Sb-O bond ionicity. The increase of the Q×f and |τ(f)| values could be attributed to the lattice energy and the coefficient of thermal expansion. The microwave dielectric properties of Nd(Nb(1-x)Sb(x))O4 ceramics with the monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, lattice energy and coefficient of thermal expansion.

The correlations among bond ionicity, lattice energy and microwave dielectric properties of (Nd(1-x)La(x))NbO4 ceramics.

(Nd1-xLax)NbO4 ceramics were prepared via a conventional solid-state reaction route and the correlations among bond ionicity, lattice energy, phase stability and microwave dielectric properties were investigated. The diffraction patterns showed that the (Nd1-xLax)NbO4 ceramics possessed a monoclinic fergusonite structure. The chemical bond ionicity, bond covalency and lattice energy were calculated using the empirical method. The phase structure stability varied with the lattice energy which resulted due to the substitution content of La(3+) ions. With the increase of La(3+) ion contents, the decrease of Nd/La-O bond ionicity was observed, which could be attributed to the electric polarization. εr has a close relationship with the Nd/La-O bond covalency. The increase of the Q × f values and τf values could be attributed to the change in the lattice energy. The microwave dielectric properties of (Nd1-xLax)NbO4 ceramics with a monoclinic fergusonite structure were strongly dependent on the chemical bond ionicity, bond covalency and lattice energy.

On the mutual solubility in MMoO4-LnVO4 systems, where M = Ca, Cd, Sr, Ba, Pb, Ln = Y, Sc, Ce-Lu

The factors determining the solubility in MMoO4(scheelite)-LnVO4(zircon) systems, where M = Ca, Cd, Sr, Ba, Pb, Ln = Y, Sc, La-Lu, are analyzed. It is shown that primary solid solutions based on REE vanadates are absent due to a high (above 36 kJ/mol) enthalpy of polymorphic scheelite → zircon transition. For solid solutions based on bivalent metal molybdates, the zircon → scheelite transition has a lower enthalpy of 16–22.3 kJ/mol. The differences in bond ionicity in MoO 4 2− and VO 4 3− anions and interatomic distances in MO8 and LnO8 polyhedra restrict mutual solubility of the initial compounds, but are not the cause of its asymmetry.

Effective atomic charges and orbital hybridization in ZnX (X = S, Se, Te) crystals from first principles

We have studied the distribution of Born effective atomic charge amplitudes over orbitals in cubic ZnX semiconductors, analyzed the projected electronic densities of states for the zinc and chalcogen s, p, and d states, and examined zinc and chalcogen orbital hybridization effects. Covalence effects and Zn-X bond ionicity have been assessed in relation to the type of chalcogen.