Bond Ionicities Research Articles

Atomic scale structure and bond stretching force constants in stoichiometric and off-stoichiometric kesterites.

The deviation from stoichiometry and the understanding of its consequences are key factors for the application of kesterites as solar cell absorbers. Therefore, this study investigates the local atomic structure of off-stoichiometric Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe) and Cu2ZnGeSe4 (CZGSe) by means of Extended X-ray Absorption Fine Structure Spectroscopy. Temperature dependent measurements yield the bond stretching force constants of all cation-anion bonds in stoichiometric CZTS and CZTSe and nearly stoichiometric CZGSe. Low temperature measurements allow high precision analysis of the influence of off-stoichiometry on the element specific average bond lengths and their variances. The overall comparison between the materials is in excellent agreement with measures like ionic/atomic radii and bond ionicities. Furthermore, the small uncertainties allow the identification of systematic trends in the Cu-Se and Zn-Se bond lengths of CZTSe and CZGSe. These trends are discussed in context of the types and concentrations of certain point defects, which gives insight into the possible local configurations and their influence on the average structural parameters. The findings complement the understanding of the effect of off-stoichiometry on the local structure of kesterites, which affects their electronic properties and thus their application for solar cells.

Comparative study of the pressure dependence of optical-phonon transverse-effective charges and linewidths in wurtzite InN

We investigate the hydrostatic pressure dependence of the zone center optical phonons of $c$-plane and $a$-plane wurtzite InN epilayers grown on GaN substrates. The longitudinal to transverse mode splitting for the ${A}_{1}$ and ${E}_{1}$ modes was found to increase with increasing pressure, whereas the associated transverse effective charge decreases for both modes as ${e}_{T}^{*}({A}_{1})=2.93--9.9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}P$ and ${e}_{T}^{*}({E}_{1})=2.80--10.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}P$ (in units of elementary charge and $P$ in GPa). These observations are well in line with results for other II--VI, III--V, and group-IV semiconductor compounds as far as the relation between the magnitude and sign of the pressure derivative of ${e}_{T}^{*}$ and the bond ionicity is concerned. As the latter increases so does $|\ensuremath{\partial}{e}_{T}^{*}/\ensuremath{\partial}P|$ with a sign change from positive to negative for bond ionicities around ${f}_{i}=0.46$ for compounds with anions belonging to the first row of the Periodic Table. A comparison of the results for InN and other nine tetrahedrally bonded compounds indicate that the pressure behavior of the transverse effective charge is mainly determined by the strength of the Pauli repulsion between cation valence electrons and those of the anion core. We also perform ab initio calculations in order to address the origin of the observed increase in linewidth of the ${E}_{2}^{\text{high}}$ mode which is found to arise from a pressure-induced increase in the rate of two-phonon decay processes. This broadening is associated with tuning into resonance of a steep edge in the two-phonon density of states around 460 ${\mathrm{cm}}^{\ensuremath{-}1}$ with the frequency of the ${E}_{2}^{\text{high}}$ mode.

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.

Elastic constants and critical thicknesses of ScGaN and ScAlN

Elastic constants of hexagonal ScxGa1−xN and ScxAl1−xN up to x = 0.375 were calculated using a stress-strain approach. C11, C33, C44, and C66 decreased while C12 and C13 increased slightly with increasing x. The biaxial [0001] Poisson ratios increased from 0.21 for GaN to 0.38 for Sc0.375Ga0.625 N and from 0.22 for AlN to 0.40 for Sc0.375Al0.625N, due to greater u values, in-plane bond lengths and bond ionicities. Subsequently, critical thicknesses for stress relaxation were calculated for ScxAl1−xN/AlN, ScxGa1−xN/GaN, and ScxAl1−xN/GaN heterostructures using an energy balance model. These range from 2 nm for Sc0.375Al0.625N/AlN and Sc0.375Ga0.625N/GaN to infinity for lattice-matched Sc0.18Al0.82N/GaN.

Investigation of medium range order in silicate glasses by infrared spectroscopy

A new decomposition method of infrared spectra which takes into account the configurational and dynamical origins of the disorder allows retrieving structural information on short and medium range orders in potassium silicate glasses. The distribution of tetrahedral units, the occupation of cation sites, the ratio of SiO bond ionicities involving bridging and non bridging oxygen, and a measure of the impact of low frequency floppy modes on the high frequency dynamics are byproducts of the modeling process. The composition dependence of two vibrational modes clearly identified as spectral components signing medium range order, shows that the disruption of the silicate network follows selective schemes. The 3D silica like network completely disappears in glasses with K2O amounts greater than 11mol% and above this threshold a progressive appearance of 2D silicate sheets is evidenced.

Phonon deformation potentials in wurtzite GaN and ZnO determined by uniaxial pressure dependent Raman measurements

We report the phonon deformation potentials of wurtzite GaN and ZnO for all zone center optical phonon modes determined by Raman measurements as a function of uniaxial pressure. Despite all the structural and optical similarities between these two material systems, the pressure dependency of their vibrational spectra exhibits fundamental distinctions, which is attributed to their different bond ionicities. In addition, the LO-TO splitting of the A1 and E1 phonon modes is analyzed which yields insight into the uniaxial pressure dependency of Born’s transverse effective charge eT∗.

Covalent bond indices and ionicities from similarity measures

This work proposes the use of the mathematical concepts of similarity and distance in metric spaces to relate the covalency and ionicity indices of chemical bondings. Numerical results arising from the Bray–Curtis and divergence measures are reported and discussed in selected molecules. The procedure opens new possibilities to define and to relate complementary quantities.

A Systematic Study of the Structure and Bonding of Halogens on Low-Index Transition Metal Surfaces

The structure and bonding of halogens on various transition metal low-index surfaces has been studied by means of density functional theory (DFT) calculations using periodic slabs to model the surface. This approach is shown to be capable of reproducing available experimental data of naked and halogen-covered surfaces. Periodic trends are discerned and discussed for several properties, including metal-halogen bond distances and vibrational frequencies, adsorption energies, and bond ionicities, which have been evaluated by a Bader population analysis of the corresponding density. A simple correlation is discerned, relating the bond ionicity to the metal work function, so that higher work function surfaces are associated with more covalent bonding. Periodic trends in bond ionicities and metal-halogen vibrational frequencies are in harmony with corresponding data derived in an electrochemical environment, indicating that the metal-halogen bonding in vacuum share some features with the electrode metal surface-halogen bonding.

Hydride transfer reactivity of tetrakis(trimethylphosphine)(hydrido)(nitrosyl)molybdenum(0)

The tetrakis(trimethylphosphine) molybdenum nitrosyl hydrido complex trans-Mo(PMe(3))(4)(H)(NO) (2) and the related deuteride complex trans-Mo(PMe(3))(4)(D)(NO) (2a) were prepared from trans-Mo(PMe(3))(4)(Cl)(NO) (1). From (2)H T(1 min) measurements and solid-state (2)H NMR the bond ionicities of 2a could be determined and were found to be 80.0% and 75.3%, respectively, indicating a very polar Mo--D bond. The enhanced hydridicity of 2 is reflected in its very high propensity to undergo hydride transfer reactions. 2 was thus reacted with acetone, acetophenone, and benzophenone to afford the corresponding alkoxide complexes trans-Mo(NO)(PMe(3))(4)(OCHR'R'') (R' = R'' = Me (3); R' = Me, R'' = Ph (4); R' = R'' = Ph (5)). The reaction of 2 with CO(2) led to the formation of the formato-O-complex Mo(NO)(OCHO)(PMe(3))(4) (6). The reaction of with HOSO(2)CF(3) produced the anion coordinated complex Mo(NO)(PMe(3))(4)(OSO(2)CF(3)) (7), and the reaction with [H(Et(2)O)(2)][BAr(F)(4)] with an excess of PMe(3) produced the pentakis(trimethylphosphine) coordinated compound [Mo(NO)(PMe(3))(5)][BAr(F)(4)] (8). Imine insertions into the Mo-H bond of 2 were also accomplished. PhCH[double bond, length as m-dash]NPh (N-benzylideneaniline) and C(10)H(7)CH=NPh (N-1-naphthylideneaniline) afforded the amido compounds Mo(NO)(PMe(3))(4)[NR'(CH(2)R'')] (R' = R'' = Ph (9), R' = Ph, R'' = naphthyl (11)). 9 could not be obtained in pure form, however, its structure was assigned by spectroscopic means. At room temperature 11 reacted further to lose one PMe(3) forming 12 (Mo(NO)PMe(3))(3)[N(Ph)CH(2)C(10)H(6))]) with agostic stabilization. In a subsequent step oxidative addition of the agostic naphthyl C-H bond to the molybdenum centre occurred. Then hydrogen migration took place giving the chelate amine complex Mo(NO)(PMe(3))(3)[NH(Ph)(CH(2)C(10)H(6))] (15). The insertion reaction of 2 with C(10)H(7)N=CHPh led to formation of the agostic compound Mo(NO)(PMe(3))(3)[N(CH(2)Ph)(C(10)H(7))] (10). Based on the knowledge of facile formation of agostic compounds the catalytic hydrogenation of C(10)H(7)N=CHPh and PhN=CHC(10)H(7) with 2 (5 mol%) was tested. The best conversion rates were obtained in the presence of an excess of PMe(3), which were 18.4% and 100% for C(10)H(7)N=CHPh and PhN=CHC(10)H(7), respectively.

Energetics of hydride and electron pair attachment to EX 30/+ (E = B, C, Al, Si and X = F, Cl, Br, I) and the study of bonding trends among EX 30/+, EX 32−/−, and EX 3H −/0 by use of ELF and NBO analyses

A theoretical gas-phase “ligand-free” or “electron pair affinity” (EPA) approach, based on CCSD(T)/(SDB-)cc-pVTZ//MP2/(SDB-)cc-pVTZ electronic structure calculations, is introduced as a possible means for determining Lewis acidity trends among planar EX 3 0/+ (E = B, C, Al, Si; X = F, Cl, Br, I) species. In this treatment, the free electron pair is considered to be an extreme Lewis base. The calculated EPA values are compared with experimental Lewis acidities, previously calculated fluoride ion affinity (FIA) and hydride ion affinity (HA) trends, and are found to exhibit reasonable correlations in all cases. The bonding in the planar and trigonal pyramidal conformations of EX 3 0/+ and of the trigonal pyramidal Lewis base EX 3 2−/− anions are assessed by use of natural bond orbital (NBO) and natural resonance theory (NRT) analyses. The NBO charges of the CX 3 + (X = Cl, Br, I, OTeF 5) cations are shown to correlate with the cation–anion and cation–solvent contacts in the recently determined crystal structures of [CCl 3][Sb(OTeF 5) 6], [CBr 3][Sb(OTeF 5) 6]·SO 2ClF, [CI 3][Al(OC(CF 3) 3) 4], and [C(OTeF 5) 3][Sb(OTeF 5) 6]·3SO 2ClF and known fluoro-carbocation structures. Topological electron localization function (ELF) basin lobe isosurfaces and volumes are used to rationalize the Lewis acidity trends and bond ionicities of the EX 3 0/+ species, and Lewis basicities of the EX 3 2−/− species.

Contributions to the infrared effective charges of oxides and chalcogenides from equilibrium charge and dynamic charge redistribution during normal mode motions

Empirical force constant models for infrared (IR) and Raman scattering have emphasized differences between non-crystalline oxides and chalcogenides, attributing them to different bond angles at the two-fold coordinated O- and S(Se)-atoms in spite of the fact that both classes of materials form continuous random networks (CRNs) with equivalent 8N rule bonding coordination. This article applies ab initio electronic structure calculations to determine (i) equilibrium bonding geometries, and (ii) infrared effective charges for normal mode motions of O- and S-atoms in SiO2, and GeS2 and As2S3, respectively. Differences in equilibrium bond angles and normal mode effective charges are shown to result from quantitative differences in SiO, and GeS and GeSe bond ionicities that optimize the SiO2, and GeS2 and As2S3 total energies at markedly different bond angles.

Pauling's electrostatic rule for partially covalent/partially ionic crystals

Second Pauling's rule, known as the electrostatic rule, has been reformulated with regard to partial net charges in covalent–ionic crystals. For more uniform treatment, an idea of atomic ionicity coefficients (AIC) related to the respective bond ionicities (BI) is introduced. The proposed definitions are consistent with crystal-chemistry principles. Examples are presented, showing how the reformulated Pauling's rule works for chosen compounds. Some other possibilities of applications of the model are also discussed. In particular, for the chosen crystals, the net charges of the ions have been calculated from a knowledge of the ionicities of the respective bonds and the BIs have been estimated from the ionic charges. A case concerning structures with non-equivalent anions is also discussed.

Bond ionicities inCuBC2chalcogenides(B=Al,Ga, In;C=S,Se, Te)

The bond ionicities, ${f}_{i,\mathrm{Cu}\ensuremath{-}C}$ and ${f}_{i,BC}$ of several $\mathrm{Cu}{\mathrm{BC}}_{2}$ $(B=\mathrm{Al},$ Ga, In, and $C=\mathrm{S},$ Se, Te) chalcopyrite compounds are estimated by means of the Phillips--Van Vechten dielectric theory for binary tetrahedral compounds and Levine's extension to multibond crystals in the same way as Neumann [Crystal Res. Technol. 18, 1299 (1983)]. The influence of $\mathrm{Cu} 3d$ electrons has been taken into account considering Jaffe-Zunger [Phys. Rev. B 29, 1882 (1984)] band-structure calculations in chalcopyrite compounds, and performing a simple extrapolation for Te compounds. The $\mathrm{Cu}\ensuremath{-}C$ bond susceptibilities have been estimated from the static dielectric constant values obtained by M\'arquez and Rinc\'on [Phys. Status Solidi B 191, 115 (1995)], and a set of $\mathrm{Cu}\ensuremath{-}C$ bond susceptibilities is proposed. The evaluated ${f}_{i,\mathrm{Cu}\ensuremath{-}C}$ bond ionicities were found to increase with the atomic fractional coordinate of the C atom, x[anion], indicating that the anion position is a good estimation of the $\mathrm{Cu}\ensuremath{-}C$ bond ionicity.

Determination of electric-field gradients in semiconductors with high precision and high sensitivity

Wurtzite-type semiconductors such as GaN and ZnO exhibit intrinsic electric-field gradients at the nuclear sites due to the hexagonal lattice symmetry. We have determined these field gradients ${\mathrm{V}}_{\mathrm{ZZ}}$ for all the nuclear sites by a magnetic double-resonance scheme. Shallow donors, which are present in these semiconductors either unintentionally or by doping, are used to enhance the nuclear magnetic resonance (NMR) sensitivity through the double-resonance technique. The value ${\mathrm{V}}_{\mathrm{ZZ}}$ for $^{67}$ Zn in ZnO is in excellent agreement with the value obtained by M\"ossbauer spectroscopy. For the Ga position in GaN and for the O position in ZnO we improved the precision by an order of magnitude compared to previous NMR results. The ${\mathrm{V}}_{\mathrm{ZZ}}$ value for the N position in GaN was determined. The field gradients are directly related to the bond ionicities in these III-V and II-VI semiconductors.

A study of Si compounds by Zr Lα photoelectron spectroscopy

Zirconium Lα radiation has been utilized to investigate the chemical bonding in silicon compounds by x-ray photoelectron spectroscopy. The compounds studied in the present investigation are SiO2, SiO, SiC, Si3N4, and silicon-oxynitrides. The spectra in the Si 1s and KLL Auger regions were recorded for these compounds and compared with the elemental spectra. In order to estimate the change in chemical environment of Si in these compounds, the Auger parameter was determined in each case as the difference between the binding energies for the 1s and the KLL peaks. The trend in the value of the Auger parameter is correlated with the degree of atomic relaxation due to the increasing neutralization of charge around the Si cations in these compounds. A good agreement was found between the change in the Auger parameter and the bond ionicity calculated for the Si cation in these compounds. Using this plot, the bond ionicities were determined for the oxynitrides. A model is also developed to estimate these ionicities from the concentrations of oxygen and nitrogen present in these compounds. The values of the ionicities obtained by these two different methods give a satisfactory agreement. We have shown that the plot between the change in the Auger parameter and the bond ionicity could be used to estimate the bond ionicities.

Effective bond charges from infrared intensities: ab initio calculations

Effective bond charges are calculated for a number of molecules from theoretically evaluated atomic polar tensors. The initial dipole moment derivatives are corrected for rotational and translational contributions and dependencies following a recently developed theoretical approach. Three levels of ab initio calculations are employed: HF/6-31G(d,p), HF/6-311+G(d,p) and MP2/6-31G(d,p). The effective bond charges obtained possess well-expressed relationships with molecular parameters that are determined from the electric charge distribution in molecules, such as the electronegativities of the constituent atoms, the bond ionicities and the hybridization state of the atoms. Such dependencies are most clearly expressed in structural analogs with identical symmetry.

X‐Ray spectral determination of the effective charges on phosphorus, sulfur, and chlorine atoms in chemical compounds with a nonempirical charge scale

AbstractLinear dependence between the experimental Ka line shifts and Mulliken atomic charges obtained by SCF ab initio calculations with 4‐31G, 4‐31G*, and 4‐31G** basis sets for the P, S, and Cl atoms have been found. The electron density redistribution of PCl3 and SPCl3 free ligands on their complexation with AlBr3 has been investigated. It has been shown that a significant transfer of the total ligand electron density to the acceptor is absent, but there is an essential growth of the ionicity of all bonds on complexation. The P = S and PS bond ionicities are found to increase linearly with the growth of the positive charge on the central phosphorus atom. © John Wiley & Sons, Inc.

29Si NMR structural studies of ionically conductive silicon chalcogenide glasses and model compounds

The structure of ionically conductive glasses in the systems (Li 2 S) x (SiS 2 ) 1− x (0.2 ≤ x ≤ 0.6), (Na 2 S) x (SiS 2 ) 1− x (0.1 ≤ x ≤ 0.6), (Ag 2 S) x (SiS 2 ) 1− x (0.5 ≤ x ≤ 0.6) and (Li 2 S) y (Na 2 S) 1− y ) 0.5 (SiS 2 ) 0.5 (0 ≤ y ≤ 1), (Li 2 Se) x (SiSe 2 ) 1− x (0.23 ≤ x ≤ 0.70), and (Na 2 Se) x (SiSe 2 ) 1− x (0.4 ≤ x ≤ 0.6), prepared by twin-roller quenching, is discussed on the basis of solid-state 29 Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) results. As in the well known stoichiometrically analogous oxide systems, the 29 Si chemical shifts are sensitively affected by differences in the structural environments present. For both the sulfide and selenide systems, the NMR spectra permit easy distinction between corner- and edge-shared silicon tetrahedra. In addition, secondary chemical shift effects are observed, reflecting the number of bridging versus non-bridging chalcogen atoms. The sign and magnitude of these chemical shift trends can be rationalized on the basis of bond ionicities using a semi-empirical theory approach. The main conclusion concerning the structures of these glasses are the following. (1) The introduction of alkali chalcogenides into the network of silicon chalcogenide glasses generates non-bridging sulfur and selenium sites, with preferential destruction of edge-sharing SiX 4 2 tetrahedra. (2) The distribution of the non-bridging selenium sites is closer to random than to ordered. (3) The tendency of forming edge-sharing units decreases in the order S → Se → O and Na → Li → Ag. (4) Mixed LiNa thiosilicate glasses are structurally more closely related to binary lithium thiosilicate glasses than to binary sodium thiosilicate glasses.

Photon-stimulated desorption of halogens

Photon-stimulated desorption of positive halogen ions is studied for systems than span a range of bond ionicities. It is found that only excitations of cation core-level electrons lead to ion desorption for very ionic systems, while only excitations of anion core electrons produce ions from more covalently bonded materials. For systems of intermediate bonding character, both cation and anion excitations can lead to ion desorption. These observations are explained within the context of Auger-stimulated desorption combined with nuclear motion.

Bond ionicities of high- Tc oxides

A semiempirical method for the calculation of the bond ionicity, which is applicable even to extremely anisotropic systems such as copper-oxide superconductors, is proposed as a generalization of the Phillips-Van Vechten-Levine scheme. The value of the ionicity calculated for oxides generally tends to increase as the crystal strain becomes more tensile. It is characteristic of cuprate superconductors that the values of the ionicity are high compared with other 3d transition-metal oxides. Also significant are the extremely high ionicities in the direction normal to the CuO 2 planes and the relatively high covalencies of the intraplanar bonds. These crystal-chemical characteristics may be intimately related to the remarkable insulator-to-metal transition and the associated high- T c superconductivity in the layered copper-oxide systems.