Valence Electron Charge Density Research Articles

Compton spectroscopy of hydrogen storage material LiAlH4: Experiment and DFT strategies

Electronic structure and momentum densities of three possible phases (α, β and γ) of hydrogen storage material, LiAlH4, are reported using the linear combination of atomic orbitals (LCAO) scheme within the density functional theory (DFT). Energy bands and density of states computed using generalised gradient approximation and hybrid (Hartree-Fock + DFT) potentials show that the studied phases of LiAlH4 possess wide band gaps ranging from 3.59 to 6.64 eV. The experimental Compton profile (CP) of the most stable α-phase was measured using 100 mCi 241Am γ-ray Compton spectrometer. The experimental CP is used to validate the applicability of exchange and correlation energies by comparing it with theoretical CPs deduced using various exchange and correlation potentials. The B3LYP hybrid functional-based CP is found to be in good agreement with the present experimental Compton data. Based on Mulliken's population and valence electron charge density, the trend of ionicity is found in the descending order as γ-phase > α-phase > β-phase. Moreover, the modified Becke-Johnson (mBJ) scheme of the full potential linearized augmented plane wave (FP-LAPW) method is also used to compute the electronic and optical response of LiAlH4, which show its use as absorptive material for ultraviolet radiations. The bonding nature of the Al-H bond and Li+-[AlH4]¯ is also presented using the electron localization function.

Compton profile and charge density reconstruction by the maximum entropy method

The aim of this work is to propose a theoretical procedure to determine the electron momentum density and Compton profile of valence electrons in solids. The procedure consists in a hybrid methodology that combines the maximum entropy method and Dirac-Hartree–Fock formalism, which allows including exchange and correlation effects on valence electrons for distances near the atomic nucleus and that are approximated using the Breit-Wigner distribution function. This technique can be applied for a wide range of crystalline solids. The results and comparisons reported here are for: lithium, beryllium, aluminum, silicon and copper. The application of the model only requires prior knowledge of the Fermi momentum values. In addition, a reconstruction of the valence electron charge density distribution via simple expression of the Compton profile is derived.

Unusual oxidation-induced core-level shifts at the HfO2/InP interface

X-ray photoelectron spectroscopy (XPS) is one of the most used methods in a diverse field of materials science and engineering. The elemental core-level binding energies (BE) and core-level shifts (CLS) are determined and interpreted in the XPS. Oxidation is commonly considered to increase the BE of the core electrons of metal and semiconductor elements (i.e., positive BE shift due to O bonds), because valence electron charge density moves toward electronegative O atoms in the intuitive charge-transfer model. Here we demonstrate that this BE hypothesis is not generally valid by presenting XPS spectra and a consistent model of atomic processes occurring at HfO2/InP interface including negative In CLSs. It is shown theoretically for abrupt HfO2/InP model structures that there is no correlation between the In CLSs and the number of oxygen neighbors. However, the P CLSs can be estimated using the number of close O neighbors. First native oxide model interfaces for III-V semiconductors are introduced. The results obtained from ab initio calculations and synchrotron XPS measurements emphasize the importance of complementary analyses in various academic and industrial investigations where CLSs are at the heart of advancing knowledge.

Energy band structure, optical properties and chemical bonding of Cu<sub>7</sub>Sis<sub>5</sub>I crystal

Purpose: Interest in the study of crystalline Cu 7 SiS 5 I caused by the presence of high ionic conductivity, which nature has not yet been proper explained. The calculation of Cu 7 SiS 5 I electronic structure can be very useful in this regard, its knowledge will help to establish the nature of chemical bonding between Cu and S(I), which is necessary for the better understanding of ionic conductivity mechanism. Methods: This paper presents the results of calculations of the band structure, the state density distribution of the electron density and such optical functions as real e 1 and imaginary e 2 part of the dielectric permittivity, reflectivity R , absorption coefficient a and refractive index n of Cu 7 SiS 5 I crystal. Results: The total valence band with width of 13.85 eV for Cu 7 SiS 5 I consists from four subbands separated by forbidding intervals. The analysis of partial contributions into the density of electronic states allowed to identify the genetic origin of different subbands of the valence band, and also to obtain the formation of a chemical bond in the crystal under study. The most important feature of the electronic spectrum of the Cu 7 SiS 5 I crystal is the presence of an intense peak in the density of states N ( E ) formed by Cu3 d- like zone. The calculated spectral dependences of the absorption and reflection coefficients show the main features of the e 2 ( Е ) dependence, and the maxima of the energy distribution of the refractive index practically coincide in position with the centers of gravity of the bands of the real part of the complex permittivity. The electron density r( r ) is calculated as well as the maps of the spatial distribution of charge of the valence electrons in the tetrahedron [SiS 4 ], triangle [CuS 3 ] and bipyramid [Cu 2 S 3 I 2 ] are plotted. Conclusion: According to charge density of valence electrons, calculated using pseudo waves functions, the features of the chemical bond in the crystal Cu 7 SiS 5 I are studied. The observed features of the optical functions of Cu 7 SiS 5 I are identified taking into account the nature of the electronic states. Analysis of the distribution of electron density indicates that a mixed ion-covalent type of chemical bond is realized in this compound. The ion component is due to the transfer of the charge density between cations and anions due to the difference in their electronegativities.

First−principles study: Structural, mechanical, electronic and thermodynamic properties of simple−cubic−perovskite (Ba0.62K0.38)(Bi0.92Mg0.08)O3

A new simple cubic perovskite superconductor (Ba0.62K0.38)(Bi0.92Mg0.08)O3 with a Tcmag ∼30 K has been recently synthesized by utilizing facile hydrothermal conditions at 220 °C. Here, we have employed the pseudo-potential plane-wave (PP–PW) approach based on the density functional (DFT) theory, within the generalized gradient approximation (GGA) to calculate and investigate the physical and electronic properties of this new superconductor. The calculated elastic constants satisfy the Born mechanical stability criterion. Based on the calculated data of Cauchy pressure, Pugh ratio and Poisson's ratio, the compound under investigation is found to be brittle nature. The calculated Peierls stress indicates easier movement of dislocations between glide planes. The energy bands crossing the Fermi level (EF) in electronic band structure reveal metallic behavior of the newly synthesized compound. The contribution of O-2p orbital electrons at the EF is higher than those of other orbital electrons as seen from partial density of states. The formation of saddle point owing to the flatness of the energy band may enhance the electron pairing which subsequently leads to high-TC superconductivity. The valence electron charge density map shows the isotropic nature of charge distribution. Coexistence of electron and hole like pockets form the multi sheets Fermi surfaces and exhibit the multi-band nature of the compound. Importantly, Fermi surfaces with flat faces are considered as favorable condition for Fermi surface nesting. The calculated value of electron-phonon coupling constant (λ = 1.61) implies that the studied compound can be treated as a strongly coupled superconductor. Further, the thermodynamic properties at elevated temperature and pressure are calculated and analyzed for the first time by using quasi-harmonic model.

Revealing the electronic structure and optical properties of K6[Mo4O8F10] novel molybdenum oxyfluoride materials

The electronic structure and optical properties of the molybdenum oxyfluoride K6[Mo4O8F10] single crystal were investigated. Based on X-ray diffraction data, the crystal structure was optimised so as to minimise the forces acting on each atom. The calculation reveal that K6[Mo4O8F10] is an indirect band gap semiconductor of about 1.51 eV, using the local density approximation, 1.72 eV, using gradient approximation (PBE-GGA), 1.81 eV using Engel–Vosko generalized gradient approximation and 2.01 eV, from the recently modified Becke–Johnson potential (TB-GGA-mBJ). Partial density of states reveals the orbitals’ contribution and the degree of the hybridisations between the orbitals. The contours of the valence electronic charge density of each atom in K6[Mo4O8F10] reveal that there are some electrons from Mo, K, F and O orbitals that are transferred to the valence bands and participate in the interactions between the Mo, K, F and O atoms to form covalent bonding. The strength of the covalent bond depends on the degree of the hybridisation and the electro-negativity differences between Mo, K, F and O atoms. The calculated bond lengths exhibit good agreement with the experimental data. The optical properties help to get deep insight into the electronic structure and reveal the types of the orbitals that participate in the optical transitions. It has been found that the investigated crystal possesses negative uniaxial anisotropy (δε = −0.102) and positive birefringence ( = 0.2765).

Exploring the electronic structure of Pb2+ ions containing material Pb16(OH)16(NO3)16

A theoretical band structure calculation for lead nitrate hydroxide Pb16(OH)16(NO3)16 single crystal was performed based on the experimental crystallographic data obtained by Chang et al. Calculations exhibit that the conduction band minimum (CBM) is situated at Γ the center of the Brillouin zone (BZ) while the valence band maximum (VBM) is located between Γ and Y points of the BZ, resulting in an indirect energy band gap of about 3.70eV in close agreement to the measured one (3.78eV). The angular momentum resolved projected density of states reveals the existence of the strong hybridization between the orbitals and the VBM is originated from Pb-6s/6p and O-2p orbitals while the CBM from N-2p and Pb-6p orbitals. The calculated valence electronic charge density distribution explore the bond characters and the dominancy of the covalent bonding between Pb–O of PbOn ployhedra and N–O of [NO3]− triangle. The calculated bond lengths and angles show good agreement with the experimental data.

Electronic structure and transport properties of Ba2Cd2Pn3 (Pn = As and Sb): An efficient materials for energy conversion

The full potential method within the recently modified Becke-Johnson potential explore that the Ba2Cd2Pn3 (Pn = As and Sb) compounds are narrow band gap semiconductors of about 0.49 and 0.32 eV, which confute the finding of the previous TB-LMTO-ASA calculation that Ba2Cd2Sb3 is a poor metal. It has been found that there are subtle difference in band desperations of the two compounds, resulting in significant influence on the electronic and transport properties, taking into account the size and the electro-negativity differences between As and Sb atoms. Calculation show that there exists a strong hybridization between the orbitals which may lead to form covalent bonding which is more favorable for the transport of the carriers than ionic one. The electronic structure, the anisotropy and the inter-atomic interactions are further analyzed by calculating the valence electronic charge density distribution in two crystallographic planes. The semi-classical Boltzmann theory as incorporated in BoltzTraP code was used to calculate the transport properties of Ba2Cd2As3 and Ba2Cd2Sb3 at different temperatures and chemical potentials to ascertain the influence of temperatures and substituting As by Sb on the transport properties. The carries mobility decreases with increasing the temperature also with increasing the carriers concentration. We have observed that substituting As by Sb lead to increase the carries mobility of Ba2Cd2Sb3 along the whole temperature interval and the carries concentration range. It has been found that Ba2Cd2As3 exhibit higher carriers concentration, electronic electrical conductivity and Seebeck coefficient than that of Ba2Cd2Sb3 along the investigated temperature range. The highest value of Seebeck coefficient occurs at 300 K, which show good agreement with the experimental data. The power factor increases linearly with increasing the temperature and Ba2Cd2As3 exhibit a bit higher power factor than that of Ba2Cd2Sb3 up to 500 K. Above this temperature both compounds are alternating. Based on the results our finding that the Ba2Cd2Pn3 (Pn = As and Sb) compounds are efficient materials for energy conversion.

NaMgH3 a perovskite-type hydride as advanced hydrogen storage systems: Electronic structure features

The physical properties of perovskite NaMgH3 has been comprehensively investigated by means of density functional theory. Calculations reveals that NaMgH3 is a wide band gap insulator. The angular momentum projected density of states and the valence electronic charge density explore that the NaMgH3 has high gravimetric and volumetric H densities. The calculated angular momentum projected density of states reveals that the hydrogen posses the highest density among Mg and Na. It has been found that a charge transfer occurs towards H atom. Complex first-order linear optical properties helps to further understand the physical properties of NaMgH3. Analyzing the optical properties reveals the NaMgH3 posses negative uniaxial anisotropy and the optical properties give deep insight into the electronic structure.

Electronic structure and optical properties of CdO from bulk to nanosheet: DFT approach

First principle calculations of CdO nanosheet were performed with the help of full potential linear augmented plane wave (FPLAPW) method within the generalized gradient approximation (GGA) as implemented in WIEN2k program. Electronic structure and optical properties of CdO nanosheet have been calculated and discussed in this work. The calculated band structure shows that the CdO nanosheet is direct band gap EgΓ-Γ semiconductor. The band gap value so obtained is 0.8eV. The contribution of different bands was analyzed on the basis of total (TDOS) and partial density of state (PDOS). A number of optical parameters have been calculated and discussed in details. Static dielectric constants are found to be 2.1 and 1.5 for ε1xx(ω) and ε1zz(ω). The static refractive index n(0) value for nxx(ω) and nzz(ω) are found to be 1.439 and 0.113 respectively. In the high energy region the reflectivity is about 26%. The average value of dielectric constant ε2average(ω) for CdO-bulk and nanosheet shows the critical point at 2.3eV and 3.4eV respectively. Calculated static birefringence values for CdO (bulk) and CdO nanosheet are −0.04 and −0.31, respectively. Valence electron charge density plot show mixed ionic covalent character having 38.71% ionicity. Scissor correction has been done in order to get the optical gap equivalent to experimental band gap (3.4eV). CdO nanosheet show high efficiency for linear and non linear optical devices.

First principle investigation of electronic structure, chemical bonding and optical properties of tetrabarium gallium trinitride oxide single crystal

The electronic band structure, valence electron charge density and optical susceptibilities of tetrabarium gallium trinitride (TGT) were calculated via first principle study. The electronic band structure calculation describes TGT as semiconductor having direct band gap of 1.38eV. The valence electronic charge density contour verified the non-polar covalent nature of the bond. The absorption edge and first peak of dielectric tensor components showed electrons transition from N-p state to Ba-d state. The calculated uniaxial anisotropy (0.4842) and birefringence (−0.0061) of present paper is prearranged as follow the spectral components of the dielectric tensor. The first peak in energy loss function (ELOS) shows the energy loss of fast traveling electrons in the material. The first sharp peak produced in ELOS around 10.5eV show plasmon loss having plasma frequencies 0.1536, 0.004 and 0.066 of dielectric tensor components. This plasmon loss also cause decrease in reflectivity spectra.

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.

First principle investigation of electronic structure and optical behaviors of 2-amino-4-fluorododec-4-encarbolic acid

The electronic structure and dispersion of optical susceptibilities of 2-amino-4-fluorododec-4-encarbolic acid were calculated using all-electron full potential linear augmented plane wave method based on density functional theory. Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and Engel Vosko generalized gradient approximation (EV-GGA) were used to solve the exchange correlation functional. The EV-GGA scheme was selected to obtain better band gap value (4.91eV) as compared to GGA-PBE (4.75eV). The valence electron charge density plots show foremost covalent nature of the bonds. The C–C bond is pure covalent bond while C–H, C–F, C–O, N–H and C–N configuration show dominant covalent bond with small percentage of ionicity. The calculated spectral peaks in ε2(ω) show electron transition between the occupied and unoccupied bands. The calculated absorption coefficient I(ω) and reflectivity spectra R(ω) of 2-amino-4-fluorododec-4-encarbolic acid show response to electromagnetic radiation in extreme ultraviolet region (UV-C). The dispersion of calculated second order dielectric tensor components ε2(ω), ε1(ω), I(ω) and L(ω)show considerable anisotropy.

First principles treatment of structural, optical, and thermoelectric properties of Li7MnN4 as electrode for a Li secondary battery

The electronic structure, electronic charge density and linear optical properties of the metallic Li7MnN4 compound, having cubic symmetry, are calculated using the full potential linearized augmented plane wave (FP-LAPW) method. The calculated band structure and density of states using the local density, generalized gradient and Engel–Vosko approximations, depict the metallic nature of the cubic Li7MnN4 compound. The bands crossing the Fermi level in the calculated band structure are mainly from the Mn-d states with small support of N-p states. In addition, the Mn-d states at the Fermi level enhance the density of states, which is very useful for the electronic transport properties. The valence electronic charge density depicts strong covalent bond between Mn and two N atoms and polar covalent bond between Mn and Li atoms. The frequency dependent linear optical properties like real and imaginary part of the dielectric function, optical conductivity, reflectivity and energy loss function are calculated on the basis of the computed band structure. Both intra-band and inter-band transitions contribute to the calculated optical parameters. Using the BoltzTraP code, the thermoelectric properties like electrical and thermal conductivity, Seebeck coefficient, power coefficient and heat capacity of the Li7MnN4 are also calculated as a function of temperature and studied.

Electronic structure of low-pressure and high-pressure phases of silicon disulfide

Self-consistent density functional theory calculations of band spectra, densities of states as well as the spatial distribution of valence electron charge density were carried out for the low-pressure α-phase and the high-pressure β-phase of SiS2. Group-theoretical analysis performed for both phases enabled the symmetry of wave functions in a number of high-symmetry points of the Brillouin zone as well as the structure of valence band representations to be found. Based on the calculations of the band structure, the orthorhombic α-phase of SiS2 was determined to be an indirect-gap semiconductor with the band gap Egi = 2.44 eV (T1 → X8 transition), while the β-phase was shown to be direct gap with Egd = 2.95 eV (Г3 → Г2 transition). The calculated energy distribution of the total density of states in the valence band of α-SiS2 qualitatively and quantitatively correlates with the main experimental features of the X-ray photoelectron spectrum.

Electronic band structure and specific features of AA- and AB-stacking of carbon nitride (C3N4): DFT calculation

The state-of-the-art all-electron full potential linear augmented plane wave based on density functional theory was applied for calculating the structural, electronic band structure, effective mass, density of state, and valence electron charge density of AA- and AB-stacking of carbon nitride (C3N4). Calculations were performed using four schemes, namely local density approximation (LDA-CA) by Ceperley–Alder, generalized gradient approximation developed by Perdew Burke and Ernzerhof (GGA-PBE), Engel–Vosko generalized gradient approximation (EV-GGA) and the recently developed modified Becke–Johnson (mBJ) potentials. The calculated band structure and total density of states verify the semiconducting nature of the two configurations with band gap of about 2.589 eV and 2.990 eV for AA- and AB-stacking of C3N4. The effective mass ratio of electron, heavy hole and light hole were calculated for both AA- and AB-stacking. The upper valence band in AB-stacking is much flatter as compared to AA-stacking, and elucidates a dramatic increase in the effective mass ratio of heavy holes which cause a reduction in group velocity of the wave packet and consequently decrease the mobility of the charge carrier. The partial density of state shows the variation in the orbital state and its strength as one moves from AA- to AB-stacking. The strong hybridization among the orbitals of both stacking (AA- and AB-) shows covalent nature of the bonds. The calculated valence electron charge density contour shows the prevailing covalent nature of C and N bonds with negligible percentage (8.08%) of ionicity.

Electronic structure, Compton profiles and cohesive properties of Cs-halides

We have reported energy bands, density of states, valence electron charge densities and Compton profiles of CsCl, CsBr and CsI using linear combination of atomic orbitals with Hartree–Fock and density functional theories. We have also computed these properties, except the momentum densities, using full potential linearized augmented plane wave method. The general features of the energy bands and the density of states in these halides are found to be almost similar. To interpret the theoretical data on Compton line shapes, we have also measured the Compton profiles using our 20 Ci 137Cs spectrometer. It is seen that the Hartree–Fock calculations give relatively a better agreement with the experimental momentum densities. On the basis of equal-valence-electron-density profiles, a comparison of relative nature of bonding is made which is in agreement with the valence charge densities and atomic charges by means of Mulliken analysis. Using our experimental and theoretical Compton profiles, we have also computed the cohesive energy of the halides.

First-principles study of structural, elastic, electronic and optical properties of SrMO 3 (M=Ti and Sn)

We present structural, elastic, electronic and optical properties of the perovskites SrMO 3 (M=Ti, and Sn) for different pressure. The computational method is based on the pseudo-potential plane wave method (PP-PW). The exchange-correlation energy is described in the generalized gradient approximation (GGA). The calculated equilibrium lattice parameters are in reasonable agreement with the available experimental data. This work shows that the perovskites SrTiO 3, and SrSnO 3 are mechanically stable and present an indirect band gaps at the Fermi level. Applied pressure does not change the shape of the total valence electronic charge density and most of the electronic charge density is shifted toward O atom. Furthermore, in order to understand the optical properties of SrMO 3, the dielectric function, absorption coefficient, optical reflectivity, refractive index, extinction coefficient and electron energy-loss are calculated for radiation up to 80 eV. The enhancement of pressure decreases the dielectric function and refractive indices of SrTiO 3 and SrSnO 3.

Theoretical studies of positron annihilation in YN and ScN

Valence electron and positron charge densities in YN and ScN are obtained from wave functions derived in a model pseudopotential band structure calculation coupled with the independent particle model (IPM). It is observed that the positron density is maximum in the open interstices and is excluded not only, from the ion cores but also to a considerable degree from the valence bonds. Electron–positron momentum densities are calculated for the (0 0 1) and (1 1 0) planes. The results are used to analyze the positron effect in binary alloy semiconductors.

Structural stability ofFe5Si3andNi2Sistudied by high-pressure x-ray diffraction andab initiototal-energy calculations

We performed high-pressure angle dispersive x-ray diffraction measurements on Fe5Si3 and Ni2Si up to 75 GPa. Both materials were synthesized in bulk quantities via a solid-state reaction. In the pressure range covered by the experiments, no evidence of the occurrence of phase transitions was observed. On top of that, Fe5Si3 was found to compress isotropically, whereas an anisotropic compression was observed in Ni2Si. The linear incompressibility of Ni2Si along the c-axis is similar in magnitude to the linear incompressibility of diamond. This fact is related to the higher valence-electron charge density of Ni2Si along the c-axis. The observed anisotropic compression of Ni2Si is also related to the layered structure of Ni2Si where hexagonal layers of Ni2+ cations alternate with graphite-like layers formed by (NiSi)2- entities. The experimental results are supported by ab initio total-energy calculations carried out using density functional theory and the pseudopotential method. For Fe5Si3, the calculations also predicted a phase transition at 283 GPa from the hexagonal P63/mcm phase to the cubic structure adopted by Fe and Si in the garnet Fe5Si3O12. The room-temperature equations of state for Fe5Si3 and Ni2Si are also reported and a possible correlation between the bulk modulus of iron silicides and the coordination number of their minority element is discussed. Finally, we report novel descriptions of these structures, in particular of the predicted high-pressure phase of Fe5Si3 (the cation subarray in the garnet Fe5Si3O12), which can be derived from spinel Fe2SiO4 (Fe6Si3O12).