Bi 6p States Research Articles

Electronic structures and phase instabilities of cubic Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3： a first-principles comparative study

The electronic structures, potential energy surfaces for the displacement of ions along the tetragonal [001] and Γ phonon properties of Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3 are investigated by employing the first-principles method based on density functional theory. The results indicate that the electronic structures of Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3 are very similar. The valence band is dominated by O 2p states with an admixture of Ti 3d and Bi 6p states. The lower energy region of the conduction band is mainly composed of Ti 3d orbitals. The bond strengths of Ti–O and Bi–O increase when Na is substituted by K. Moreover, minor differences are observed from the potential energy surface for the displacement of ions along the tetragonal [001], which indicates that the phase instabilities of the two compounds mainly come from the displacement of O ions, which plays a more important role when Na is substituted by K. There are three soft modes for Na1/2Bi1/2TiO3 and K1/2Bi1/2TiO3, which mainly originate from the vibration of O6 group. The A2u soft mode becomes harder when Na is substituted by K.

DFT+U predictions: The effect of oxygen vacancy on the structural, electronic and photocatalytic properties of Mn-doped BiOCl

In order to provide fundamental understanding and guidance for the effective Mn doping BiOCl and enhanced physicochemical properties derived from the interplay between Mn dopant and oxygen defect, the density functional theory (DFT)+U calculations were adopted to evaluate the effects of Mn doping and oxygen vacancy defects on the geometric structure, electronic property, optical absorption, Mulliken charge and effective mass of BiOCl. The suitable theoretical models were built up and their relaxed structural parameters agree well with the experiment values. It is found that both substitutional and interstitial Mn doping BiOCl not only are energetically favorable but also modify band structures to achieve remarkable optical red-shift of BiOCl, and the substitutional Mn-doped BiOCl show better feasible Mn doping behavior due to the beneficial p-type doping characteristic and p–d hybridization between substitutional Mn and adjacent Bi (or O) atoms in the CB (or VBM). The occurrence of oxygen vacancy in BiOCl crystal significantly influences the neighboring Bi 6p states, forming a capture center of photo-excited electrons in the forbidden band, which enhances the efficient mobility of photo-generated carriers and improves the effective separation rate of electron–hole pairs. The better negative formation energy of substitutional Mn-doped BiOCl with oxygen vacancy (BiOCl:VO) demonstrates thermodynamically its favorable structural stability. Besides, the higher relative ratio value for the effective masses of activated holes and electrons in the Mn-doped BiOCl:VO system implies the lower recombination rate of electron–hole pairs. Furthermore, our calculated optical absorption spectra exhibit that the spectral absorption edge of BiOCl is obviously red-shifted and extends to visible, red and infrared light region by the synergistic effect of oxygen vacancy and Mn dopant. Therefore, we theoretically predicted that Mn-doped BiOCl:VO system is expected to become highly promising semiconductor material applied in photocatalytic field for effectively photochemical decomposition of organics or water splitting under the sunlight irradiation.

Cu3BiS3 as a potential photovoltaic absorber with high optical efficiency

Cu3BiS3 is a potential photovoltaic material. Employing a first-principles approach, we calculate the structural, electronic, and optical properties of Cu3BiS3, and we demonstrate that Cu3BiS3 is an indirect band gap semiconductor in contrast to similar chalcogenide semiconductors. The fundamental band gap energy is estimated to be Eg ≈ 1.5–1.7 eV. The analysis reveals that Cu3BiS3 has a much stronger absorption coefficient (>105 cm−1) compared to other Cu-S based materials like CuInS2 and Cu2ZnSnS4. This is explained by the presence of localized Bi 6p states in the band gap region, generating a flat lowest conduction band.

Electronic band structure and Fermi surface of tetragonal low-temperature superconductor Bi2Pd as predicted from first principles

We have examined theoretically the electronic band structure and Fermi surface (FS) of the tetragonal low-temperature superconductor Bi2Pd. Our main results are that (i) the Pd 4d and Bi 6p states determine the main peculiarities of the multiple-sheet FS topology, thus for this material a complicated superconducting gap structure with different energy gaps on different FS sheets should be assumed; (ii) the effect of spin-orbit coupling is of minor importance for the distributions of the near-Fermi electronic states; and (iii) this phase adopts a 3D-like type owing to directional bonds between the adjacent atomic sheets.

First-principles study on electronic structure and optical properties of Ca4Bi6O13 crystal

The electronic structure of Ca4Bi6O13 is calculated by the GGA approach. The valence band maximum and the conduction band minimum of Ca4Bi6O13 are located at Γ-point. This means that Ca4Bi6O13 is a direct band gap material. The conduction band in the lower energy region is mainly constructed with the admixture of Bi 6p and O 2p. The conduction band in the higher energy region consists of Ca 3d states. The valence band in the lower energy region is mainly formed from Bi 6s states, and the valence band in the upper energy region consists of O 2p states hybridizing with Bi 6s and 6p states. The optical properties of Ca4Bi6O13 are calculated from spectral dependence of the complex dielectric function, ε(ω)=ε1(ω)+iε2(ω). A highly polarized peak is observed below 3.6eV in ε2(ω) function.

First-principles study on the structural, electronic and optical properties of BiOX (X=Cl, Br, I) crystals

In order to investigate systematically the structural, electronic and optical properties of bismuth oxyhalides BiOX (X=Cl, Br, I) semiconductors, the lattice constants, structural characteristics, band structures, densities of states, atomic charge populations and optical properties of BiOX crystals have been calculated using first-principles based on DFT. The calculated indirect band gaps of BiOCl, BiOBr and BiOI crystals are 2.50, 2.10 and 1.59eV, respectively. The analysis of densities of states and atomic charge populations for BiOX crystals indicates that, (a) the valance band maximum is mainly contributed to O 2p and X np states and the Bi 6p states dominate the conduction band minimum; (b) the contribution of X ns states obviously increases with the increase of X atomic numbers, and the dispersive energy level becomes more and more significant and (c) the sequence of covalent bonding strength between atoms is Bi–O >Bi–I>Bi–Br>Bi–Cl. In addition, the calculated absorption edges of the absorption coefficients I(ω) for BiOCl, BiOBr and BiOI crystals are 355, 448 and 645nm, respectively, which agree well with our experimental measurements of 376, 442 and 628nm and the previous reported results of 370, 440 and 670nm.

ChemInform Abstract: Revisiting the Zintl—Klemm Concept: A2AuBi (A: Li or Na)

Abstractfirst principles quantum mechanical calculations

First-principles study on electronic structure of Sr 2Bi 2O 5 crystal

The electronic structure of Sr 2Bi 2O 5 is calculated by the GGA approach. Both of the valence band maximum and the conduction band minimum are located at Γ-point. This means that Sr 2Bi 2O 5 is a direct band-gap material. The wide energy-band dispersions near the valence band maximum and the conduction band minimum predict that holes and electrons generated by band gap excitation have a high mobility. The conduction band is composed of Bi 6p, Sr 4d and O 2p energy states. On the other hand, the valence band can be divided into two energy regions ranging from −9.5 to −7.9 eV (lower valence band) and from −4.13 to 0 eV (upper valence band). The former mainly consists of Bi 6s states hybridizing with O 2s and O 2p states, and the latter is mainly constructed from O 2p states strongly interacting with Bi 6s and Bi 6p states.

Revisiting the Zintl–Klemm Concept: A2AuBi (A = Li or Na)

AbstractAlkali metal gold bismuthides, A2AuBi, are isoelectronic with alkali metal thallides, ATl = A2TlTl, and yet Na2AuBi adopts an orthorhombic structure with a 1‐D zigzag “ribbon” structural motif rather than the cubic double diamond structure type of NaTl as well as Li2AuBi. Using first principles quantum mechanical calculations applied to A2AuBi, hypothetical “A2HgPb,” and A2TlTl, and comprehensively decomposing the total energies into metallicity, ionicity, and covalency components to establish parallels with the qualitative Zintl–Klemm formalism, the factors determining the relative stability between the zigzag “ribbon” and the diamond network are examined. An interplay between volume‐dependent energy terms, i.e., metallicity or ionicity, and covalency among the electronegative components determines which structural motif is favored. In Na2AuBi, there are two factors stabilizing the zigzag “ribbon.” Au 5d states significantly interact with Bi 6p states, especially Au 5d with Bi 6pz to promote stronger Au–Bi covalent interactions than in the diamond network. This factor does not exist in Na2TlTl and “A2HgPb,” where Hg, Tl, and Pb 5d states are well localized. Secondly, the zigzag ribbons provide effective covalent interactions at larger volumes, as in Na2AuBi, while effective covalent interactions occur in the diamond network only at smaller volume, as in Li2AuBi.

First-principles study of the structural, elastic, electronic and optical properties of the monoclinic BiScO 3

The structural, elastic, electronic and optical properties of the monoclinic BiScO 3 are investigated in the framework of the density functional theory. The calculated structural parameters are in agreement with the experimental values. Moreover, the structural stability of BiScO 3 system has been confirmed by the calculated elastic constants. The band structure, density of states, charge transfers and bond populations are also given. The results indicate that BiScO 3 has a direct band gap of 3.36 eV between the occupied O 2p states and unoccupied Bi 6p states, and its bonding behavior is a combination of covalent and ionic nature. Finally, the absorption spectrum, refractive index, extinction coefficient, reflectivity, energy-loss function and dielectric function of the monoclinic BiScO 3 are calculated. In addition, the variation of the static dielectric constants ε 1(0) as a function of pressure for BiScO 3 is also discussed.

BaM(BS3)S (M = Sb, Bi): Two New Thioborate Compounds with One-Dimensional Polymeric Chain Structure

Two new quaternary thioborate compounds with strongly one-dimensional growth, BaSb(BS(3))S and BaBi(BS(3))S, have been synthesized using the conventional solid state reaction method in closed tubes at 1100 K. The single crystal X-ray diffraction analysis has shown that compound BaSb(BS(3))S crystallizes in space group Pnma of the orthorhombic system with unit cell parameters of a = 9.6898(15) A, b = 6.2293(13) A, c = 11.670(2) A, V = 704.4(2) A(3), and Z = 4, while compound BaBi(BS(3))S crystallizes in space group C2/m with unit cell parameters of a = 14.9890(17) A, b = 6.2457(6) A, c = 7.5591(9) A, beta = 101.604(5) degrees , V = 693.19(13) A(3), and Z = 4. The two compounds both crystallize in the structure of infinite one-dimensional chains with [BS(3)](3-) trigonal plane coordination alternately bridged by [MS(3)](3-) (M = Sb, Bi) trigonal pyramids through sharing two sulfur atoms along the crystallographic b axis. First-principles electronic structure calculations performed with the density functional theory (DFT) method show that the calculated band gaps of BaSb(BS(3))S and BaBi(BS(3))S are 2.29 and 2.16 eV, respectively, which are in good agreement with the experimental values estimated from UV-vis absorption spectra using the Kubelka-Munk equation, and the observed absorption peak is assigned as charge transfers from S-3p states to Sb-5p (Bi6p) states.

Ab initio study of structural, electronic and optical properties of the Bi 12TiO 20 sillenite crystal

It is presented an analysis of some important structural, electronic and optical characteristics of the pure Bi 12TiO 20 sillenite crystal, based on the ab initio calculations of its electronic structure and complex dielectric tensor. The band gap is calculated to be 2.3 eV and found to be direct. The contributions to the valence band top and conduction band bottom come predominantly from O 2p- and Bi 6p-states, respectively. The optical absorption spectrum is calculated and interpreted in terms of electronic band structure for incident radiation energy up to 40 eV. The principal absorption occurs within the energy range from 2.5 to 10 eV, originating manly from the electronic transitions from the O 2p- to the Bi 6p-states.

First-principles energy band calculation for CaBi 2O 4 with monoclinic structure

The electronic structure of CaBi 2O 4 is calculated by a GGA approach. The valence band maximum is approximately located at the Γ-point or the Y-point and the conduction band minimum at the V-point. This means that CaBi 2O 4 is an indirect energy gap material. The conduction band is composed of Bi 6p–O 2p interaction. On the other hand, the valence band can be divided into two energy regions ranging from −9.92 to −7.40 eV (lower valence band) and −4.69 to 0 eV (upper valence band). The former is mainly constructed from Bi 6s states interacting slightly with O 2s and 2p states, and the latter consists of O 2p states hybridizing with Bi 6s and 6p states. The states near the valence band maximum are strongly localized and the mobility of holes generated by band gap excitation is predicted to be fairly low.

Multiple scattering approach to XMCD for Bi substituted Gd iron garnet

We have studied Bi L-edge X-ray magnetic circular dichroism (XMCD) of Bi substituted Gd iron garnet (BiGd-IG) by using the relativistic multiple scattering calculations. BiGd-IG has giant magneto-optical response with substituted Bi. The XMCD analysis is suitable to understand the interesting behavior; it is important to understand the local magnetic and electronic structure around absorption atom Bi. Comparing the experimental spectra, we find that the proportion of magnetic atoms to nonmagnetic ones around Bi strongly influence the Bi XMCD spectra. We also investigate the effects of the spin polarization of the Bi 6p states.

Theoretical prediction on the structural, electronic, and polarization properties of tetragonal Bi2ZnTiO6

We present first-principles investigations on the structural, electronic, and polarization properties of Bi2ZnTiO6 using density-functional theory within the generalized gradient approximation. The theoretical structure we obtained confirms the extra large tetragonality observed by experiment. The materials exhibit a semiconductor behavior with an indirect band gap determined by the occupied O 2p and unoccupied Bi 6p states. There are strong hybridization effects between Bi–O ions, as well as Ti–O and Zn–O ones. The resulting covalent bondings, having a PbTiO3-type two-dimensional character, strengthen each other and favor the coupling between the tetragonal distortion of unit cell and the off-center displacement of A and B-site cations and O anions due to the existence of Zn, and result in the large tetragonality of this compound. Berry-phase calculation gives the polarization as high as 122 μC/cm2.

Structural, elastic, electronic and optical properties of the cubic perovskite BiAlO 3

We report first-principles study of structural, elastic, electronic and optical properties of the cubic perovskite-type BiAlO 3 using the pseudopotential plane waves method within the local density approximation. The calculated structural parameters are in good agreement with previous calculations. The elastic constants and their pressure dependence are calculated using the static finite strain technique. A linear pressure dependence of the elastic stiffness is found. Band structures show that BiAlO 3 has an indirect band gap between the occupied O 2p and unoccupied Bi 6p states. The density of states and Mulliken charge populations analysis shows that Al–O and Bi–O bonds are covalent with a strong hybridization. The variation of the gap versus pressure is well fitted to a quadratic function and an indirect to direct band gap transition occurs at 15.5 GPa. Furthermore, in order to understand the optical properties of BiAlO 3, the dielectric function, absorption coefficient, refractive index, extinction coefficient, optical reflectivity and electron energy loss are calculated for radiation up to 30 eV.

DFT calculations on the electronic structures of BiOX (X = F, Cl, Br, I) photocatalysts with and without semicore Bi 5d states

The electronic structures of BiOX (X = F, Cl, Br, I) photocatalysts have been calculated with and without Bi 5d states using the experimental lattice parameters, via the plane-wave pseudopotential method based on density functional theory (DFT). BiOF exhibits a direct band gap of 3.22 or 3.12 eV corresponding to the adoption of Bi 5d states or not. The indirect band gaps of BiOCl, BiOBr, and BiOI are 2.80, 2.36, and 1.75 eV, respectively, if calculated with Bi 5d states, whereas the absence of Bi 5d states reduces them to 2.59, 2.13, and 1.53 eV successively. The calculated gap characteristics and the falling trend of gap width with the increasing X atomic number agree with the experimental results, despite the common DFT underestimation of gap values. The shapes of valence-band tops and conduction-band bottoms are almost independent of the involvement of Bi 5d states. The indirect characteristic becomes more remarkable, and the conduction-band bottom flattens in the sequence of BiOCl, BiOBr, and BiOI. Both O 2p and X np (n = 2, 3, 4, and 5 for X = F, Cl, Br, and I, respectively) states dominate the valence bands, whereas Bi 6p states contribute the most to the conduction bands. With the growing X atomic number, the localized X np states shift closer toward the valence-band tops, and the valence and conduction bandwidths evolve in opposite trends. Atomic and bond populations have also been explored to elucidate the atomic interactions, along with the spatial distribution of orbital density.

Electronic structures of relaxed BiOX (X = F, Cl, Br, I) photocatalysts

For photocatalysts BiOX (X=F, Cl, Br, I), the atomic sites have been relaxed and the electronic structures have been calculated via the density functional theory (DFT) with or without the adoption of Bi 5d states. BiOF exhibits a direct band gap while the other three species present the indirect feature. The consideration of Bi 5d states results in apparent expansion of the gaps, which are closer to the experimental results. The transition positions are almost independent of the incorporation of Bi 5d states, and the conduction-band bottom flattens with the increase in X atomic number. Both O 2p and X np (n=2, 3, 4 and 5 for X=F, Cl, Br and I, respectively) states dominate the valence-bands while Bi 6p states contribute most to the conduction-bands. The density peak of the localized X np states in the valence-band shifts towards the valence-band top with the increasing X atomic number, along with certain changes in the valence and conduction bandwidths. Atomic and bond populations, as well as the spatial distribution of orbital density have also been investigated.

Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst

A novel bismuth oxychloride (BiOCl) was synthesized by a hydrolysis method. The powder sample was characterized by X-ray diffraction, field emission scanning electron microscope and UV–vis spectrophotometer. The layered compound BiOCl was the first oxyhalide to be used as a photocatalyst. The prepared material has an optical indirect band-gap of 3.46eV. For its photocatalytic reactivity, the methyl orange (MO) dye degradation was chosen to be investigated. In a comparison of a three-cycle measurement of BiOCl with that of TiO2 (P25, Degussa), it was found that BiOCl had better performance than P25 at every recycle. The calculated electronic structure of BiOCl confirms that it has an indirect band-gap, and the Cl 3p and Bi 6p states dominate the highest occupied molecular orbitals (HOMO) and the lowest unoccupied orbitals (LUMO), respectively. The open crystal structure and indirect optical transitions of BiOCl play important roles in its excellent photocatalytic activity.

A theoretical and experimental study of the distorted pyrochlore Bi2Sn2O7

The pyrochlore based bismuth stannate, Bi2Sn2O7, is a material with important applications in catalysis and gas sensing. The thermodynamically stable α phase has 352 atoms in the unit cell and is one of the most complex oxide crystal structures to have been solved by powder diffraction. We have performed a full atomic relaxation and calculated the electronic structure, using gradient corrected density functional theory, with the resulting structure in very good agreement with the previous experimentally determined unit cell. The computed density of states is in excellent agreement with our valence band X-ray photoelectron spectra. The combined results shed new light on the bonding and lone pair activity in this material. A mixture of Bi 6s and O 2p states are found to dominate the top of the valence band while Sn 5s, O 2p and Bi 6p states dominate the bottom of the conduction band. The differing contributions of Sn 5s and Bi 6s states to the valence and conduction bands reflect both differences in atomic binding energies and differences in the strength of the metal s interactions with O 2p. The preference of this material for a distorted structure and its unique catalytic and gas sensing activity are discussed.