Madelung Site Potentials Research Articles

PbBi3O4X3 (X = Cl, Br) with Single/Double Halogen Layers as a Photocatalyst for Visible-Light-Driven Water Splitting: Impact of a Halogen Layer on the Band Structure and Stability

Layered oxyhalides with a Sillén(−Aurivillius) structure have attracted considerable attention as promising photocatalysts for efficient visible-light-induced water splitting due to the elevation of the O-2p valence band maximum (VBM) derived from the strong s–p interaction between Pb/Bi-6s and O-2p electrons and the electrostatic destabilization of oxygen anions in the fluorite layers. However, the role of the halogen layer, consisting of single or double layers, was not known. In this study, we examined a Sillén-type layered oxyhalide PbBi3O4X3 (X = Cl, Br) with alternating stacks of single/double halogen layers and skeletal Bi-based oxide layers. Density functional theory (DFT) calculations of PbBi3O4X3 revealed that the two inequivalent halogen layers contribute differently to the valence band density of states; the halogen p orbitals in double layers are higher in energy and responsible for the VBM, which is understood in terms of Madelung site potentials, where the electrostatic repulsion between halide ions is at play. Another finding from the DFT calculation is the spatial separation of the conduction band minimum and the VBM based on the asymmetry of halogen layers, which may promote the charge separation. While both PbBi3O4Cl3 and PbBi3O4Br3 possess band levels appropriate for visible-light-induced water splitting, only the former showed photocatalytic O2 evolution activity. The stability of PbBi3O4Cl3 is due to the significant contribution of the O-2p orbitals to its VBM, in stark contrast to PbBi3O4Br3, where Br-4p predominately contributes to the VBM. PbBi3O4Cl3 functions as a stable O2-evolving photocatalyst in Z-scheme water splitting under visible light irradiation. These results show that the electrostatic repulsion of halogen layers is a key factor for tuning the valence band structure of oxyhalides while maintaining sufficient stability for water-splitting photocatalysis.

Extended layer-by-layer Madelung potential analysis of layered oxyhalide photocatalysts and other layered systems

Extended layer-by-layer Madelung analysis was developed and applied to various layered materials to reveal the contribution of distant layers to Madelung site potential.

Valence Band Engineering of Layered Bismuth Oxyhalides toward Stable Visible-Light Water Splitting: Madelung Site Potential Analysis.

A layered oxychloride Bi4NbO8Cl is a visible-light responsive catalyst for water splitting, with its remarkable stability ascribed to the highly dispersive O-2p orbitals in the valence band, the origin of which, however, remains unclear. Here, we systematically investigate four series of layered bismuth oxyhalides, BiOX (X = Cl, Br, I), Bi4NbO8X (X = Cl, Br), Bi2GdO4X (X = Cl, Br), and SrBiO2X (X = Cl, Br, I), and found that Madelung site potentials of anions capture essential features of the valence band structures of these materials. The oxide anion in fluorite-like blocks (e.g., [Bi2O2] slab in Bi4NbO8Cl) is responsible for the upward shift of the valence band, and the degree of electrostatic destabilization changes depending on building layers and their stacking sequence. This study suggests that the Madelung analysis enables a prediction and design of the valence band structures of bismuth and other layered oxyhalides and is applicable even to a compound where DFT calculation is difficult to perform.

Structural aspects of the M1 and M2 phases in MoVNbTeO propane ammoxidation catalysts

Abstract The structures of M1 and M2 in MoVNbTeO propane ammoxidation catalysts have been solved using a combination of TEM, neutron powder diffraction, and synchrotron X-ray powder diffraction. The unit cell of M1 is Pba2 (No. 32) with a = 21.134(2) Å, b = 26.658(2) Å, c = 4.0146(3) Å and Z = 4. The formula unit is Mo7.8V1.2NbTe0.937O28.9. The unit cell of M2 is Pmm2 (No. 25) with a = 12.6294(6) Å, b = 7.29156(30) Å, c = 4.02010(7) Å and Z = 4. The formula unit is Mo4.31V1.36Te1.81Nb0.33O19.81. Tellurium sites in hexagonal channels of both phases are displaced toward vanadium-occupied framework sites, whereas Te in the heptagonal channel of M1 is near the channel center. The chemical topology resulting from oxidation states and Madelung site potentials presents active moieties for the ammoxidation of propane in M1 and propene in M2. EPR confirmed the presence of V4+ and possibly Mo5+ in M1 and V4+ in M2.

THE STRUCTURE OF Bi 26Mo 10O 69

The structure of Bi 26Mo 10O 69 was solved using a combination of X-ray and neutron diffraction. The final refinement was completed using high resolution neutron powder diffraction in a monoclinic cell [ a = 1.17456(3) nm, b = 0.57988(1) nm, c = 2.47919(5) nm, β = 102.903(1)°, P2 c ], neglecting a very subtle triclinic distortion resolved only with synchrotron X-ray data. This phase exhibits a solid solubility range of approximately 2.6 ≤ Bi/Mo ≤ 2.8, with the ideal ratio at 2.6 and one of the Bi sites disordered. The structure is rationalized by comparison with other bismuth molybdates using valence bond sums and Madelung site potentials. As with several other bismuth molybdates, there is significant charge transfer between Bi-rich channels and surrounding molybdenum tetrahedra in this fluorite-related structure. © 1997 Elsevier Science Ltd

Assessment of structure and critical temperature correlation in HgBa2CuO4 + δand Ca–La–Ba–Cu–O superconductors

HgBa2CuO4+δand CaxLa1−xBa1.75−xLa0.25+xCu3O7±δsuperconducting systems were structurally investigated employing Rietveld analysis. The doping of holes in Hg compounds was done by oxygenation and reduction methods. In Ca–La–Ba–Cu–O system the doping of holes was introduced via cation cosubstitution of Ca+2for La+3and La+3for Ba+2, so that the nominal charge of non-copper cations remained constant (=7.25). The electronic structure was assessed by calculating Madelung site potentials. It was found that the difference ΔVAof Madelung potentials on the apical and planar oxygen sites correlates with Tcsimilar to the trends reported in the literature. Based on ΔVA}$ analysis it was possible to separate the intrinsic `chemical pressure' effect on Tcfrom the one caused by electron density redistribution in Ca–La–Ba–Cu–O system upon cosubstitution.

Origin of the semiconducting-to-superconducting transition of La2-xCa1+xCu2O6+/- delta under high-oxygen-pressure annealing.

The Madelung energy of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$${\mathrm{Cu}}_{2}$${\mathrm{O}}_{6\ifmmode\pm\else\textpm\fi{}\mathrm{\ensuremath{\delta}}}$ system, whose properties change from semiconductor to superconductor under high-oxygen-pressure annealing, is calculated. We find the increase in \ensuremath{\Delta}${\mathit{V}}_{\mathit{A}}$, the difference in Madelung site potential between the apex and the ${\mathrm{CuO}}_{2}$ planes' oxygen, promotes the superconducting transition in this system. Slight intercalation of oxygen between two ${\mathrm{CuO}}_{2}$ planes is found to be of primary importance in determining \ensuremath{\Delta}${\mathit{V}}_{\mathit{A}}$.

Correlation between the superconducting transition temperature and Madelung site potential difference in YBa 2Cu 3O X

The Madelung site potentials have been calculated in YBa 2Cu 3O X system in order to find the role of the difference in the Madelung site potentials ΔV T and ΔV M within the ionic model. The bond valence sum of each of the ions in the YBa 2Cu 3O X compound is assigned as charge of the corresponding ion in the calculation of the Madelung site potential. Correlations between ΔV = 0.75 ΔV M +0.25 ΔV T obtained and superconducting transition temperature T c are examined and found qualitatively very good agreement with the well-know experimental result for the various oxygen content x. This also supports that the usual believing ΔV M is of primary important parameter to control T c . The reason will be argued in this paper.

Relationship between Madelung Potentials and Bond Valence Sums in Copper-Oxide Superconductors

High-T c copper-oxide superconductors are characterized from the viewpoint of crystal chemistry through comparative analyses of Cu-O bond lengths, Madelung site potentials and bond valence sums. The interrelationships among these structural and electronic parameters are investigated in detail for the elucidation of the crystal-chemical factors governing T c and the species of doped carriers. Two characteristics, the strain parameter for CuO2 planes calculated on the basis of bond valence sums and the Madelung-potential difference between in-plane O and Cu sites, are shown to represent an essentially equivalent physical content. The ability of the present analysis to predict the maximum value of T c of (possible candidates for) cuprate superconductors is also discussed.

Raman scattering and X-ray diffraction study in layered cuprates

Raman scattering and powder X-ray diffraction in the parent compounds of T,T'-phase and (Ca 0.86Sr 0.14)CuO 2 were measured to examine the two-magnon (ω 2m) and highest phonon energy (ω 2p) dependences on Cu-O bond length ( d), and to determine the structural parameters of the T'-phase. We found that both ω 2m and ω 2p exhibit strong power-law dependence on d, ω 2m∝ d -4.6, and ω 2p∝ d -6.3, in the T'-phase except for Gd 2CuO 4, and both energies for Gd 2CuO 4 deviate from the power-law relation. The strong d dependence of ω 2p suggests that this phonon is strongly coupled with charge-transfer electronic state. The observed phonon is assigned to two-phonon scattering of the bond-stretching mode of Cu and O atoms in the basal plane. The depression of ω 2p for Gd 2CuO 4 implies a structural change, such as oxygen distortion in the CuO 2 plane, which influences the charge-transfer energy gap Δ and ω 2m. The Δ values were estimated from the Madelung site potentials V i , which were calculated based on the crystal structural parameters determined by Rietveld analysis. Cu 3d-O 2p transfer energy t pd, and on-site Coulomb repulsive potentials, U d and U p, were determined using a one-dimensional Cu 2O cluster model by fitting ω 2m as a function of d. Finally, we discuss the relationship between crystal structure and carrier doping. The result of ΔV: ( V R - V Cu ) dependence on d suggests that the carrier concentration is not constant in T'-phase and change in the carrier concentration influences T c in these materials.

Arrangement of the oxygen atoms in the BiO-layers of superconducting Bi 2Sr 2CaCu 2O 8.2

We computed the Madelung site potentials of each ion and the total energies per cell of superconducting Bi 2Sr 2CaCu 2O 8.2 for two structural models: (1) insertion of the excess oxygen atoms into the BiO-layers and (2) location of the excess oxygen atoms between the BiO-layers, and various charge distributions. The calculation results indicate that the structural model (2) is stable and has Bi 2.8+ and holes on the oxygen atoms of CuO-layers.

Superconductivity and Madelung potential of YBa2Cu3O6+x ordered superstructures.

The Madelung potential of each possible ordered superlattice of YBa 2 Cu 3 O 6+x is calculated as the oxygen content x varies from zero to 1. ΔV A , the difference in Madelung site potential between apex and in-plane (CuO 2 ) oxygen atoms, is presented as a function of the composition x. The correlation of T c and x is discussed in terms of Ohta's empirical curve of T c vs ΔV A . The theoretical results suggest that the superconductivity depends on the superlattice structure

Pressure dependence of charge-transfer-type gaps in CuO2 layered compounds.

Pressure dependence of the electronic gap has been investigated for insulating analogs of high-${\mathit{T}}_{\mathit{c}}$ superconductors, the so-called T (${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$), ${\mathit{T}}^{\mathrm{*}}$ (${\mathrm{LaTbCuO}}_{4}$), and T' (${\mathrm{Eu}}_{2}$${\mathrm{CuO}}_{4}$) -phase compounds with single-${\mathrm{CuO}}_{2}$-layered structures. With application of hydrostatic pressure, the optical gap energy was observed to increase for the T'- and ${\mathit{T}}^{\mathrm{*}}$-phase compounds and decrease for the T-phase compound. The results can be explained in terms of two competing contributions to the charge-transfer-type gap, i.e., Madelung site potentials and electron hopping energies which respectively tend to increase and decrease the gap energy under high pressures.

DETERMINATION OF PREFERRED STRUCTURE OF Bi-2212 CUPRATE BY THEORETICAL METHOD

Firstly, we calculated the Madelung site potentials and Madelung constants of Bi 2 Sr 2 CaCu 2 O 8 cuprate for two different structural models, i.e., (1) bismuth and oxygen on the same x-y plane, and, (2) on the different x-y plane, and indicated that the latter structural model is more stable. Furthermore, we proposed a modulation structural model for the Bi 2 Sr 2 CaCu 2 O 8.2 superconductor. These theoretical results explained some experimental observations.

Tc versus Δ VA correlation in Cu-oxide superconductors

It is proposed that ΔVA, the difference in Madelung site potential between apex and in-plane-oxygens defined independently to the doping rate, is a primary parameter for the electronic states of the CuO2 plane and governs the optimum Tc's throughout all the families of Cu-oxide superconductors. Microscopic origin of the Tc versus ΔVA correlation is discussed in relation to the stability of Zhang-Rice singlet.

Role of the Madelung energy in hole conductivity in copper oxides: Difference between semiconductors and high-Tc superconductors.

The difference in Madelung site potential, \ensuremath{\Delta}${V}_{M}$, between a hole on a copper site and one on an oxygen site can control the hole's site preference and ability to delocalize. Among a variety of copper oxide structures, e\ensuremath{\Delta}${V}_{M}$ is shown to vary by \ensuremath{\sim}3 eV and to clearly separate two classes of compounds: Those with e\ensuremath{\Delta}${V}_{M}$>47 eV are metallic with high ${T}_{c}$ and delocalized holes, whereas those with lower \ensuremath{\Delta}${V}_{M}$ are semiconducting because their holes are self-trapped on oxygen.

Comparison of the Hole Conductivities in Ceramic Cuprate Semiconductors and High-Tc Superconductors, as Reflected in the Difference between Oxygen and Copper Madelung Site Potentials

ABSTRACTThe Madelung energies of 18 cuprates and 1 nickelate with sheets or chains of Cu and O have been computed, along with the relevant Madelung site potentials for O and Cu. The difference between these site potentials has been shown to correlate extremely well with the conductivity of these cuprates: only if the site potential difference is intermediate can the holes (the presumed charge carriers) move more easily between Cu and O sites in the CuO2 sheets.

Crystal Coulomb energies. VII. The electrostatic binding energy defect in Tetrathiofulvalinium 7,7,8,8,-tetracyanoquinodimethanide

The Madelung energy EC of TTF TCNQ was computed by Ewald’s method using the 300 and 100 °K crystal structures and several charge models based on CNDO/2, PPP, and INDO atom charge densities, and as a function of charge transfer. In all cases EC compares unfavorably, by several electron volts, with the cost of ionizing the lattice. This had been noted previously for NMP TCNQ. This binding energy defect is too large to be explained by metallic binding or exchange forces, or by electron correlation, even in the Wigner crystal limit. We speculate that polarization forces, or covalent effects may play a large role in stabilizing the lattice. The Madelung site potentials in the TTF+1TCNQ−1 lattice are sufficient to explain the observed core level splittings in the photoelectron spectrum.