Electronic band structure and x-ray spectra of boron nitride polytypes

The electronic band structures of boron nitride crystal modifications of the graphite (h-BN), wurtzite (w-BN), and sphalerite (c-BN) types are calculated using the local coherent potential method in the cluster muffin-tin approximation within the framework of the multiple scattering theory. The specific features of the electronic band structure of 2H, 4H, and 3C boron nitride polytypes are compared with those of experimental x-ray photoelectron, x-ray emission, and K x-ray absorption spectra of boron and nitrogen. The features of the experimental x-ray spectra of boron nitride in different crystal modifications are interpreted. It is demonstrated that the short-wavelength peak revealed in the total densities of states (TDOS) in the boron nitride polytypes under consideration can be assigned to the so-called outer collective band formed by 2p electrons of boron and nitrogen atoms. The inference is made that the decrease observed in the band gap when changing over from wurtzite and sphalerite to hexagonal boron nitride is associated with the change in the coordination number of the components, which, in turn, leads to a change in the energy location of the conduction band bottom in the crystal.     

Fine structure of the valence band top in a 3C BN crystal with a nanopore

The electronic band structure of a 3C BN boron nitride crystal with pores (r～0.3 nm) statistically distributed over the crystal is calculated by the local coherent potential method within the multiple scattering approximation. The valence band tops of crystalline (stoichiometric) and porous boron nitride are compared to the x-ray photoelectron spectrum (XPS) of BN and the soft x-ray emission spectra (SXES) of nitrogen. The origin of a short-wavelength shoulder in XPS, NK XES, and NK SXES of binary nitrides is discussed.     

Analytical optimization of the lattice parameter using the binding energy calculated in the quasi-classical approximation

The dependence of the static binding energy of a crystal on the crystal structure as calculated in the initial quasi-classical approximation is shown to allow the equilibrium lattice parameter to be found analytically. The application of this method to boron nitride modifications leads to lattice parameters that coincide with experimental values to within several percent. This method gives lattice parameters of 2.66, 3.49, and 2.44 Å for the BN hexagonal layer and the cubic c-BN and “ideal” wurtzite-like w-BN crystals, respectively. The corresponding binding energies are estimated to be 23.2, 14.1, and 13.6 eV/mol, respectively.     

On the curve of the density of phonon states for cubic boron nitride (from the results of photoluminescence measurements)

The fine structure of the phonon wing associated with the zero-phonon line (ZPL) of the BN1 center in the cubic boron nitride is analyzed in comparison with the structure of the phonon wing of the luminescence center at 3.188 eV in diamond, the second-order Raman scattering spectrum, and the theoretically calculated densities of phonon states of the c-BN compound. Taking into account the similarity of the structures of the phonon wings in the spectra of the above centers and the previously made assumptions that the structure of the phonon wing of the center at 3.188 eV is due to the specific features in the density of phonon states of diamond, it is assumed that, in the observed density of phonon states of cubic boron nitride, the critical points are represented by the specific features of the structure of the phonon wing associated with the zero-phonon line (at 3.294 eV) of the BN1 luminescence center. In turn, these latter specific features coincide accurate to within 5–10cm^?1 with the theoretically calculated lattice vibrations of the c-BN compound and the experimental data obtained from the second-order Raman spectra.     

The nature of a selective peak in the BK-absorption edge spectrum and the electronic energy band structure of the crystal 3C BN0.99

A new interpretation of the nature of the resonance in the quantum-yield K spectra of boron in the crystal 3C BN is proposed. This interpretation is based on calculation of the electronic energy band structure of the nonstoichiometric boron nitride 3C BN_0.99, which is carried out by the local coherent potential method in the multiple-scattering approximation. The tops of the valence band and of the XANES range of nonstoichiometric and perfect crystals of boron nitride are compared with the x-ray photoelectron spectrum of 3C BN and the BK-absorption edge spectrum. The electronic states near the BK-absorption edge are modeled and discussed for the relaxed and metastable states caused by the formation of vacancies in the nitrogen sublattice.     

Physical properties of Ima2-BN under pressure: First principles calculations

A fully orthorhombic boron nitride (BN) polymorph with an orthorhombic symmetry (Ima2-BN, space group: Ima2) was investigated by first-principles calculations. The Ima2-BN under 30 GPa is both mechanically and dynamically stable via elastic constants and phonon spectra. The anisotropic and electronic properties of Ima2-BN under different pressure are investigated in this work. The anisotropic properties calculations show that the Young's modulus of Ima2-BN in (001) plane exhibits the greatest anisotropy under ambient pressure, while in (111) plane it is the greatest when P > 20 GPa, while the (010) plane has always exhibited the minimal anisotropy whether under ambient pressure or high pressure. Ima2-BN is an indirect wider band gap semiconductor material under ambient pressure, and the band gap of Ima2-BN decreases with the increasing pressure. The minimum thermal conductivities κ_min of Ima2-BN is 1.85 W/(cmK), it is slightly higher than of B₄N₄-I and c-BN.     

Analysis of the polarization dependence of the N K edge X-ray absorption fine structure in InN

The polarization dependence of the x-ray absorption near-edge structure (XANES) of InN beyond the N K edge is calculated. The XANES calculations are performed for different values of the angle θ between the XY plane of crystalline indium nitride and the incident x-radiation (θ = 15°, 30°, 45°, 60°, 75°, and 90°). It is shown that, in the case of N K XANES for InN, a strong polarization dependence of the specific features of the spectrum is observed. The calculated spectra are compared with previously measured experimental spectra. The partial densities of the electronic states of InN are calculated near the top of the valence band and near the bottom of the conduction band.     

Effect of the degree of ordering of structural vacancies on the fine structure of the valence-band top in cubic boron nitride

The electronic energy structure of the defect system of c-BN_x with ZnS-type structure is calculated in the multiple-scattering theory by the local coherent potential method. The cluster version of the MT approximation is used to calculate the crystal potential. The effect of the relaxation of the crystal lattice on the electronic structure of nonstoichiometric boron nitride c-BN_0.75 is studied and a comparison is made with the electronic energy structure of c-BN in the same approximation. Fiz. Tverd. Tela (St. Petersburg) 39, 1064–1065 (June 1997)     

X-ray spectra and electronic band structure of nitrogen in Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solid solutions

The electronic energy structure of the valence band and the x-ray absorption near edge structure (XANES) region of nitrogen in Al _x Ga_1?x N solid solutions and binary crystals of gallium nitride GaN and aluminum nitride AlN are calculated using the local coherent potential method and the cluster version of the muffin-tin approximation within the framework of the multiple scattering theory. It is demonstrated that the calculated electron densities of states correlate with the nitrogen K x-ray emission and nitrogen K x-ray absorption spectra. The electronic energy structure of the top of the valence band and the XANES region in Al _x Ga_1?x N solid solutions are compared with those in the binary crystals of the GaN and AlN nitrides, and an interpretation of their specific features is proposed. An analogy is drawn between the evolution of the electronic energy structure of the valence band and the XANES region in the alloys under investigation and the evolution of the electronic band structure in the Al _x B_1?x N and B _x Ga_1?x N alloys. General trends in the transformation of the structure and variations in properties of these alloys are discussed.     

Photo- and cathodoluminescence of cubic boron nitride micropowders activated by Tm,Tb, and Eu rare-earth ions

This work is devoted to the production of cubic boron nitride (c-BN) micropowders that are activated by ions of rare-earth elements, such as europium (Eu), terbium (Tb), and thulium (Tm), as well as to the study of the structural properties and photo- and cathodoluminescence of these micropowders. The micropowders have been synthesized from a hexagonal boron nitride powder in the presence of a catalyst under pressures of 4–6 GPa at temperatures of 1800–1900 K. The activation of the micropowders by the rareearth elements (REEs) has been carried out by introducing the corresponding REE compounds into synthesis precursor. The efficiency of the introduction of an impurity into the c-BN lattice is ~5%. The composition and structure of the samples have been examined using X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive spectrometry. The results obtained during studying c-BN, c-BN:Tm, c-BN:Tb, and c-BN:Eu micropowders using color cathodoluminescence clearly demonstrate their ability to emit light in the wide spectral range, which is of interest for developing new light-emitting devices that are intended for operation in corrosive ambient. An analysis of the photoluminescence spectra of c-BN:REE micropowders has made it possible to find that the observed spectral bands belong to the corresponding transitions between the energy levels of the REEs, as well as to determine the probable positions of Tb³⁺ and Eu³⁺ ions in the cubic boron nitride lattice.     

Infrared reflectance spectrum of BN calculated from first principles

Using the linear response theory, vibrational and dielectric properties are calculated for c-BN, w-BN and h-BN. Calculations of the zone-center optical-mode frequencies (including LO-TO splittings) are reported. All optic modes are identified and excellent agreement is found between the theory and experimental results. The static dielectric tensor is decomposed into contributions arising from individual infrared-active phonon modes. It is found that all the structures have a smaller lattice dielectric constants than those of the electronic contributions. Finally, the infrared reflectance spectrums are presented. Our theoretical results indicate that w-BN shows a similar reflectivity spectrum as c-BN. It is difficult to tell apart the wurtzite structure from the zinc blende phase by IR spectroscopy.     

Two dimensional cubic boron nitride nanosheets converted from hexagonal boron nitride bilayers: electrical conductivity,magnetism and visible absorption properties

The development of novel structure, fabrication methods, formation mechanisms, and versatile applicability of boron nitride (BN) nanomaterials is still one of the research hotspots. In this report, we developed a novel two dimensional cubic boron nitride nanosheets (2D c-BNNSs) based on the first principles calculations. This structure is converted from hexagonal BN (h-BN) bilayers induced by hydroxyl (OH) radical and fluoride (F) atom codoping. The geometrical, electronic, and optical properties of the novel 2D OH radical and F atom codoped c-BNNSs (OH-F-c-BNNSs) have been systematically investigated. The results reveal that the unpaired electrons appear due to the electronegativity difference between OH radical and F atoms, resulting in the excellent electrical and magnetic properties of OH-F-c-BNNSs. In addition, OH-F-c-BNNSs also exhibit a strong response to the visible light with an absorption range covering the whole visible light region. More importantly, when the doping positions of OH radical and F atom are exchanged (F-OH-c-BNNSs), the F-OH-c-BNNS will have only electrical conductivity, which will make us to regulate the intrinsic properties of c-BNNSs for different applications only by adjusting the element doping positions. This work can provide a theoretical and experimental basis/support for designing and fabricating new types of 2D c-BN nanomaterials for different applications.     

Theoretical study of the influence of vacancies on the electronic structure of a hexagonal boron nitride monolayer

The influence of boron and nitrogen vacancies and divacancies on the electronic structure of a hexagonal boron nitride h-BN monolayer is studied. In the presence of vacancies in the structure, the introduced states appear in the forbidden band. The position of an introduced state with respect to the upper occupied level and the lower vacant level depends on deformation. Calculations show that, depending on the defect type and the magnitude of the applied deformation, the introduced state can be both localized and not localized on atoms surrounding the defect. When the state is localized in the system, the inhomogeneous distribution of the spin density is observed, resulting in the appearance of the magnetic moment in the system.     

Electronic structure of the CuBS2 crystal

The band structure and spectra of the total and projected densities of states of a new crystal of the chalcopyrite family, namely, CuBS₂, have been calculated in terms of the density functional theory. It has been found that the crystal is a pseudo-direct-band-gap semiconductor, and the best theoretical estimate of the optical band gap is 3.44 eV. The upper valence band of the CuBS₂ crystal basically consists of the contributions from the p states of S atoms and the d states of Cu atoms. The crystal splitting is 0.2 eV. The bottom of the conduction band is basically formed by the sp states of boron and sulfur atoms with an admixture of the s states of copper atoms.     

Theoretical study on Si-doped hexagonal boron nitride (h-BN) sheet: Electronic,magnetic properties,and reactivity

The properties and reactivity of Si-doped hexagonal boron nitride (h-BN) sheets were studied using density functional theory (DFT) methods. We find that Si impurity is more likely to substitute the boron site (Si_B) due to the low formation energy. Si-doping severely deforms h-BN sheet, resulting in the local curvature changes of h-BN sheet. Moreover, Si-doping introduces two spin localized states within the band gap of h-BN sheet, thus rendering the two doped systems exhibit acceptor properties. The band gap of h  -BN sheet is reduced from ∼4.70 eV$\sim 4.70\text{eV}$ to 1.24 (for Si_B) and 0.84 eV (for Si_N), respectively. In addition, Si-doped one exhibits higher activity than pristine one, endowing them wider application potential.     

Calculations of the electronic structure of crystalline SrZrO<Subscript>3</Subscript> in the framework of the density-functional theory in the LCAO approximation

The electronic structures of four well-known modifications of crystalline SrZrO₃ with different symmetries, namely, the cubic (Pm3m), tetragonal (I4/mcm), and two orthorhombic (Cmcm, Pbnm) modifications, are calculated in the framework of the density-functional theory in the basis set of the linear combination of atomic orbitals (LCAO). A comparative analysis of the electronic properties of the crystals under consideration is performed on the basis of the calculated band structures and densities of states (the total densities of states and the densities of states projected onto the atomic states). The calculated relative stabilities of the different modifications are in good agreement with the experimental data on the phase transitions in the SrZrO₃ crystal: the low-temperature modifications with lower symmetry are more stable. The ionicities of chemical bonding in different modifications of crystalline SrZrO₃ are compared by analyzing the Mulliken populations and constructing the localized Wannier functions for the occupied energy bands.     

Electronic energy structure and x-ray spectra of wide-gap AlN and BN crystals and B<Subscript>x</Subscript>Al<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solutions

The electronic energy structure of 2H and 3C AlN and BN crystals and B_xAl_1?xN solid solutions is calculated on the basis of the local coherent potential method using the cluster version of the MT approximation and the theory of multiple scattering. The features of the electronic structure of 2H-AlN crystals are compared with x-ray K and L absorption and emission spectra of aluminum and nitrogen. An interpretation of these features is given. The concentration dependences of the width of the upper subband of the valence band and the band gap in B_xAl_1?xN solid solutions (x = 0.25, 0.5, 0.75) are investigated. Charge transfer from aluminum to nitrogen atoms is shown to occur and increase with boron doping in both crystallographic modifications.     

X-ray spectra and electronic structure of aluminum in AlN and B x Al1 − x N crystals with a wurtzite structure

The electronic band structure and x-ray spectra of aluminum in wurtzite-type crystals of B_xAl_{1 ? x} N compounds are investigated and compared with those of the binary aluminum nitride AlN. The main features and the features revealed for the first time in the L _{II, III} x-ray emission and absorption spectra are interpreted.     

Tuning the electronic and magnetic properties of boron nitride nanotubes

Density Functional Theory is used to investigate the effect of altering the B/N ratio and carbon doping on the electronic and magnetic structure of zigzag, (7, 0) and armchair (5, 5) boron nitride nanotubes. The calculations indicate that increasing the boron content relative to the nitrogen content significantly reduces the band gap to a value typical of a semiconductor. Calculations of carbon doped semiconducting BN tubes, which have more boron atoms than nitrogen atoms have a net spin and a difference in the density of states at the valence band between the spin up and spin down state.     

Local structure and electronic properties of BaTaO2N with perovskite-type structure

First-principles calculation based on density-functional theory in the pseudo-potential approach have been performed for the total energy and crystal structure of BaTaO₂N. The calculations indicate a random occupation of the anionic positions by O and N in a cubic structure, in agreement with neutron diffraction measurements and infrared spectra. The local symmetry in the crystal is broken, maintaining a space group Pm3?m, as used in structure refinement, which represents only the statistically averaged result. The calculations also indicate displacive disordering in the crystal. The average Ta-N distance is smaller (2.003 Å), while the average Ta-O distance becomes larger (2.089 Å). The local relaxation of the atoms has an influence on the electronic structure, especially on the energy gap. BaTaO₂N is calculated to be a semiconductor with an energy gap of about 0.5 eV. The upper part of the valence band is dominated by N 2p states, while O 2p states are mainly in the lower part. The conduction band is dominated by Ta 5d states.