2p Hybridization Research Articles

Crystalline and electronic structures of lithium silicates: A density functional theory study

Lithium silicates, such as Li 2SiO 3 and Li 4SiO 4, are considered as favorable candidates for the tritium breeding materials of a deuterium–tritium type nuclear fusion reactor. Their bulk structural and electronic properties are investigated using pseudopotential plane wave (PPW) method within density functional theory (DFT). The optimized crystal structure parameters are well consistent with the experimental results. The results indicate that Li 2SiO 3 and Li 4SiO 4 are insulators with band gaps of about 5.36 and 5.53 eV, respectively. The valence electrons density of state reveal the covalency properties mainly resulting from the overlapping of O 2p and Si 3p orbital electrons in both lithium silicates. The nonbridging O (NBO) atoms and bridging O (BO) atoms of Li 2SiO 3 exhibit significantly different electron distributions. The Si 3s and 3p hybridization is observed in Li 2SiO 3, but not in Li 4SiO 4. The mechanical elastic constants reflected Li 4SiO 4 might be unstable in some degrees.

Room Temperature Ferromagnetism and its "Switch" Behaviour in some Dilute Magnetic Oxides: an Electronic Structure and Magnetization Study

ZnO doped with a few per cent of magnetic ions such as Ni, Fe, Co exhibits room temperature ferromagnetism (RTFM), transforming it into a very promising candidate for future spintronic applications. Two samples i.e. ZnO doped with Ni and Cr (5% each) have been investigated in the present work. The samples were characterized by Rietveld refinement of X-ray diffraction (XRD) patterns and the superconducting quantum interference device (SQUID) magnetometry. Rietveld analysis confirms that both the polycrystalline samples possess wurtzite structure with no evidence of any secondary phase. The SQUID measurements exhibit a diamagnetic state for the pristine ZnO and a paramagnetic state for the as-synthesized (Cr and Ni)-doped ZnO samples. However, the post annealing in H2 and vacuum drive them to a remarkable ferromagnetic state at room temperature. No element specific signature for ferromagnetism was seen. Then the X-ray photoelectron spectroscopic (XPS) measurements were performed to investigate their electronic structure and exploring the origin of ferromagnetism in these diluted magnetic semiconductor materials. The XPS results confirm the creation of oxygen vacancies upon Hydrogen/ vacuum annealing, owned to the (Ni/Cr) 3d¬−O 2p hybridization. The findings suggest oxygen vacancies as the intrinsic origin for ferromagnetism in doped ZnO. The important feature of this work is that the ferromagnetism and the consequent electronic property changes are found to be reversible with regard to re-heating the samples in air, showing a switch “on” and “off” ferromagnetic ordering in the ZnO matrix.

A COMPARATIVE STUDY OF OXYGEN LOSS ON IN SITU HEATING IN PrMnO3 AND BaMnO3

We have thoroughly investigated the oxygen loss in PrMnO 3 and BaMnO 3, the end members of the AMnO 3 system, on in situ heating in a reducing atmosphere. This was done to drive some oxygen out from them and thus possibly alter the valence of the Mn cation. Sample characterization was done through X-ray diffraction and SEM measurements. The core-level photoemission point to oxygen loss from only BaMnO 3 changing some of Mn 4+ to Mn 3+ in it, transfer of some spectral weight to the highly localized Fehrenbacher–Rice states and an increased Mn 3d – O 2p hybridization. Magnetization measurements show that at low temperatures, the samples depict a canted antiferromagnetic ordering.

Local structure of Ba(Ti,Zr)O3perovskite-like solid solutions and its relation to the band-gap behavior

Local structures in BaTi{sub 1-x}Zr{sub x}O{sub 3} solid solutions were analyzed using x-ray absorption fine structure (XAFS) measurements and density-functional theory (DFT) calculations. We demonstrate that for low concentrations of Ti, isolated Ti atoms in the relatively large octahedral sites of the BaZrO{sub 3} lattice acquire centrosymmetric coordination with average Ti-O distances shorter than those in BaTiO{sub 3}. In contrast for higher concentrations of Ti, Ti atoms having one or more Ti as their B-site nearest neighbors undergo polar off-center displacements. Our DFT calculations confirm both effects. These results combined with the previously published data suggest that isolated polarizable ions on the B sites of a relatively expanded host perovskite lattice remain nonpolar by symmetric relaxation of the nearest-neighbor oxygen atoms to yield nearly ideal bond lengths around the dopant species. For neighboring Ti atoms, such symmetric relaxation is impossible, and these atoms are displaced off center. Our XAFS measurements did not detect any significant deviations from a random distribution of Ti and Zr in the present samples except for compositions close to BaTiO{sub 3}. The DFT calculations suggest that the dominant effect of the local displacements on band-gap values for this system is determined by the shortest Ti-O bondsmore » due to strong Ti 3d-O 2p hybridization; however, local displacements have only a secondary effect on the band-gap behavior.« less

Study of electronic structure and magnetization correlations in hydrogenated and vacuum annealed Ni doped ZnO

The influence of hydrogen and vacuum annealing on the structural, electronic, and magnetic properties of Ni-doped ZnO pellets viz. Zn0.95Ni0.05O has been investigated using x-ray diffraction (XRD), magnetization, resistance measurements, and x-ray photoelectron spectroscopy (XPS). Rietveld refinement of the XRD patterns confirms that all the polycrystalline samples possess wurtzite type hexagonal structure with no evidence of secondary phases. The superconducting quantum interference device magnetometry measurements exhibit a paramagnetic state for the as-synthesized Zn0.95Ni0.05O. However, the post annealing in H2 as well as in vacuum, drive it to a ferromagnetic state at 300 K. The obtained values of coercivity are 177 and 270 Oe, and the saturation magnetizations are 1.67 and 1.91 emu/g, respectively, for the H2 and vacuum annealed samples. Temperature dependent magnetization results show that the Curie temperatures are nearly the same (∼550 K) for both the annealed samples. Resistance of the annealed samples is three orders of magnitude lower than their as-synthesized counterparts. The XPS results confirm bivalency of Ni ions and creation of O vacancies upon annealing, due to the Ni (3d)–O (2p) hybridization. The ferromagnetism and the consequent electronic property changes are reversible with regard to reheating the samples in air, in agreement with the reported switch “on” and “off” ferromagnetic ordering in the doped ZnO.

Evidence of defect-induced ferromagnetism and its “switch” action in pristine bulk TiO2

In this work, we demonstrate that room temperature ferromagnetism can be induced in pristine anatase TiO2 paramagnetic bulk powder through extended hydrogenation. Defect complexes, Ti3+–VO (Ti3+ ions accompanied by oxygen vacancies) are clearly identified in hydrogenated TiO2 by combining x-ray diffraction and photoemission spectroscopy. The observed ferromagnetic ordering is reversible that can be switched between “on” and “off” by inducing or removing, respectively, these defect complex. We convincingly elucidate that the factors (i) Ti 3d–O 2p hybridization (iii) F+ centers (the electrons in singly occupied oxygen vacancies), and (iii) oxygen vacancy assisted fragmentation of grains, compositely contribute to the ferromagnetic ordering.

First-Principles Calculations on Structural, Electronic, and Optical Properties of 2H-CuAlO<sub>2</sub>

Structural, electronic and optical properties of 2H-CuAlO2 were computed, using the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory (DFT). The equilibrium lattice parameters, band structure, densities of states (DOS) and charge densities of 2H-CuAlO2 have been obtained. The equilibrium lattice parameters, band structure and DOS are found to be in good agreement with the available experimental and calculational values. The charge densities and the chemical bonding of 2H-CuAlO2 are analyzed, which show that bonding between Cu and O is mainly covalent due to Cu 3d and O 2p hybridization and that bonding between Al and O is mainly ionic. The complex dielectric function, refractive index and absorption coefficient of 2H-CuAlO2 have been predicted. The calculated static dielectric constant and static refractive index of 2H-CuAlO2 is 7.1 and 2.66, respectively.

Electronic structure and magnetization correlations in Ni doped ZnO

Effect of hydrogen and vacuum annealing on electronic and magnetic properties of 5% Ni-doped ZnO pellets has been investigated using X-ray diffraction (XRD), magnetization, resistance, and X-ray photoelectron spectroscopy (XPS). Rietveld refinement results confirm that the samples have wurtzite structure with no secondary phase. The SQUID measurements exhibit a paramagnetic state for the as-synthesized Zn 0.95Ni 0.05O, however, the post annealing in H 2 and in vacuum, drives it to a ferromagnetic state at 300 K. Resistance of annealed samples is three orders of magnitude lower than their as-synthesized counterparts. The XPS results confirm bivalency of Ni ions and creation of O vacancies upon annealing, owned to Ni ( 3d)–O ( 2p) hybridization. The ferromagnetism and the consequent electronic changes are retraceable on re-heating the annealed samples in air.

First-principles calculations on electronic, chemical bonding and optical properties of tetragonal SrHfO 3

Electronic, chemical bonding and optical properties of tetragonal SrHfO 3 were investigated using the plane-wave ultrasoft pseudopotential technique based on the first-principles density functional theory (DFT). The optimized equilibrium lattice parameters of tetragonal SrHfO 3 are in good agreement with experimental values. Band structure, densities of states (DOS), charge densities and molecular-orbital bonding structure of tetragonal SrHfO 3 have been obtained. The band structure shows that tetragonal SrHfO 3 has direct band gap. The DOS and charge densities indicate that bonding between Hf and O is mainly covalent due to Hf 5d and O 2p hybridization and that bonding between Sr and O is mainly ionic. The complex dielectric function, absorption coefficient, energy-loss spectrum, complex conductivity function and reflectivity of tetragonal SrHfO 3 have been predicted. The imaginary and real parts of the calculated complex dielectric functions are consistent with the experimental observations.

Nature of N 2p, Ti 3d, O 2p Hybridization of N-Doped TiO2 Nanotubes and Superior Photovoltaic Performance through Selective Atomic N Doping

Mixing it up! N 2p states of N-doped TiO2 nanotubes become predominant near the Fermi level, which is a discrete energy level at the top edge of a valence band, whereas it deteriorates the good hybridization between Ti 3d and O 2p. This leads to a reduced band gap for selective atomic N-doped nanotubes and gives enhanced photovoltaic performance (see graphic).

4d−5pOrbital Mixing and Asymmetric In4d–O2pHybridization inInMnO3: A New Bonding Mechanism for Hexagonal Ferroelectricity

Recent studies on the ferroelectricity origin of YMnO(3), a prototype of hexagonal manganites (h-RMnO(3), where R is a rare-earth-metal element), reveal that the d(0)-ness of a Y(3+) ion with an anisotropic Y 4d-O 2p hybridization is the main driving force of ferroelectricity. InMnO(3) (IMO) also belongs to the h-RMnO(3) family. However, the d(0)-ness-driven ferroelectricity cannot be expected because the trivalent In ion is characterized by a fully filled 4d orbital. Here we propose a new bonding mechanism of the hexagonal ferroelectricity in IMO: intra-atomic 4d(z(2))-5p(z) orbital mixing of In followed by asymmetric 4d(z(2))(In)-2p(z)(O) covalent bonding along the c axis.

An X-ray absorption spectroscopic study on mixed conductive La0.6Sr0.4Co0.8Fe0.2O3−δ cathodes. I. Electrical conductivity and electronic structure

The electrical conduction mechanism of mixed conductive perovskite oxides, La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ), for cathode materials of solid oxide fuel cells has been investigated from electronic structural changes during oxygen vacancy formation. La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) was annealed under various oxygen partial pressures p(O(2))s at 1073 K and quenched. Iodometric titration indicated that the oxygen nonstoichiometry of La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) depended on the annealing p(O(2)), with more oxygen vacancies introduced at lower than at higher p(O(2))s. X-Ray absorption spectroscopic measurements were performed at the O K-, Co L-, Fe L-, Co K-, and Fe K-edges. The valence states of the Co and Fe ions were investigated by the X-ray absorption near edge structure (XANES) at the Co and Fe L(III)-edges. While the Fe average valence was almost constant, the valence of the Co ions decreased with oxygen vacancy introduction. The O K-edge XANES spectra indicated that electrons were injected into the Co 3d/O 2p hybridization state with oxygen vacancy introduction. Both absorption edges at the Co and Fe K-edge XANES shifted towards lower energies with oxygen vacancy introduction. The shift at the Co K-edge resulted from the decrease in the Co average valence and that at the Fe K-edge appeared to be caused by changes in the coordination environment around the Fe ions. The total conductivity of La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) decreased with decreasing p(O(2)), due to a decreasing hole concentration.

First-principles studies of the electronic and elastic properties of Ti2GeC

We have performed first-principles studies on electronic structure and elastic properties of Ti2GeC. The calculated band structure shows that this compound is electrical conductor. From the pressure dependence of elastic constants, we find that Ti2GeC is most stable in the pressure range from 0 to 100 GPa. The strong Ti 3d, Ge 4p and C 2p hybridization may stabilize the structure of Ti2GeC. By analyzing the ratio between the bulk and shear moduli, we conclude that Ti2GeC is brittle in nature, and the brittleness of Ti2GeC originated from the large value of Ti atom occupying the internal parameter z.

Interface effects in the electronic structure of TiO 2 deposited on MgO, Al 2O 3 and SiO 2 substrates

We report the Ti 2p X-ray absorption (XAS) and resonant photoemission (RPES) spectra of one equivalent TiO 2 monolayer grown on MgO, Al 2O 3 and SiO 2 substrates. The Ti 2p XAS spectra of these systems were compared to atomic multiplet calculations projected in different octahedral crystal fields. The comparison indicates that the crystal field splitting and the Ti–O hybridization decrease along the MgO, Al 2O 3 and SiO 2 series. The analysis of the RPES spectra provides the Ti 3d contributions to the valence band in these systems. These were compared to configuration interaction calculations of a TiO 6 cluster for different Ti 3d–O 2p hybridizations. The Ti 3d states in the valence band shift to lower binding energies along the MgO, Al 2O 3 and SiO 2 series. These effects are attributed to changes in the electronic structure at the interface, which, in turn, are related to the formation of cross-linking Ti–O–M (M = Mg, Al, and Si) bonds.

A comparative study of Pr substitution at Y and Ba sites in YBa 2Cu 3O 7 − δ

Aimed at identifying the factors which suppress the superconductivity in cuprate perovskites I have investigated the effect of substituting Pr ions at the Y and Ba sites in YBa 2Cu 3O 7 − δ system using structural, transport, iodometric, and photoemission studies. The rate of Tc depression in the case when Pr substitutes the Ba site is much higher than the case when it substitutes the Y. This is explained as being due to a combined effect of the factors such as depletion of itinerant holes due to depletion of the oxygen content, the Pr 4f–O 2p hybridization, shortening of c-axis causing Cooper pair-breaking, and the loss of orthorhombicity.

The multiferroic phase of DyFeO3: an ab initio study

By performing accurate ab initio density functional theory (DFT) calculations, we study the role of 4f electrons in stabilizing the magnetic-field-induced ferroelectric state of DyFeO3. We confirm that the ferroelectric polarization is driven by an exchange-strictive mechanism, working between adjacent spin-polarized Fe and Dy layers, as suggested by Y Tokunaga (2008 Phys. Rev. Lett.101 097205). A careful electronic structure analysis suggests that coupling between Dy and Fe spin sublattices is mediated by Dy–d and O–2p hybridization. Our results are robust with respect to the different computational schemes used for d and f localized states, such as the DFT+U method, the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional and the GW approach. Our findings indicate that the interaction between the f and d sublattices might be used to tailor the ferroelectric and magnetic properties of multiferroic compounds.

Structural stabilities and electronic and elastic properties of CdO: A first-principles study

The structural phase transition and electronic and elastic properties of CdO have been studied by first-principles pseudopotential calculations. Seven possible potential structures have been considered and our calculations show that there exists a structural phase transition from the NaCl-type (B1) structure to the CsCl-type (B2) structure above 86.6 GPa. The moderately high bulk modulus might partially originate from the strong Cd 5s–O 2p hybridization. The variations of elastic constants (Cij) with pressure have been presented; C44 decreases gradually with pressure, which implies that the B1 phase of CdO will be unstable with pressure. Moreover, the pressure dependences of the Debye temperature and the longitudinal and transverse elastic wave velocities have been investigated.

Electronic structure ofBiMO3multiferroics and related oxides

We have performed a systematic study of the electronic structures of BiMeO3 (Me = Sc, Cr, Mn, Fe, Co, Ni) series by soft X-ray emission (XES) and absorption (XAS) spectroscopy. The band gap values were estimated for all compounds in the series. For BiFeO3 a band gap of ~0.9 eV was obtained from the alignment of the O Ka XES and O 1s XAS. The O 1s XAS spectrum of BiNiO3 indicates that the formation of holes is due to a Ni2+ valency rather than a Ni3+ valency. We have found that the O Ka XES and O 1s XAS of BiMeO3 probing partially occupied and vacant O 2p states, respectively, are in agreement with the O 2p densities of states obtained from spin-polarized band structure calculations. The O Ka XES spectra show the same degree of Bi 6s--O 2p hybridization for all compounds in the series. We argue herein that the stereochemical activity of Bi 6s lone pairs must be supplemented with inversion symmetry breaking to allow electric polarization. For BiMnO3 and BiFeO3, two cases of multiferroic materials in this series, the former breaks the inversion symmetry due to the antiferromagnetic order induced by particular orbital ordering in the highly distorted perovskite structure and the latter has rhombohedral crystal structure without inversion symmetry.

Electronic structure of the [formula omitted] surface: Angle-resolved photoemission study

The electronic structure of Ni 2 P ( 10 1 ̄ 0 ) has been investigated by photoemission spectroscopy (PES) utilizing synchrotron radiation. The surface prepared by Ar + ion sputtering (3 kV, 15 min) and annealing (400 °C) gave a c(4 ×2) low-energy electron diffraction pattern. In the PES spectra measured for this surface, a Ni 3d–P 3p hybrid band (main band) and a satellite were observed at 0–4 eV and at 8 eV, respectively. Resonant PES study showed that the satellite is associated with the photoemission process leading to a two-hole bound state. The angle-resolved PES measurements showed that there were at least two surface states; one was observed at about 0.4 eV around the Γ ¯ point and the other was observed at about 0.6 eV along the boundary of the surface Brillouin zone along the [0001] direction.

Angle-Resolved and Resonant Photoelectron Spectroscopy Study of Ni2P (10-10) Single-Crystal Surface

The electronic structure of Ni2P (10−10) has been investigated by photoemission spectroscopy (PES) utilizing synchrotron radiation. The surface which was cleaned by Ar+ ion sputtering (3 kV, 15 min) and annealing (400oC) gave a c (2×4) low-energy electron diffraction pattern. In the PES spectra measured for this surface, a Ni 3d-P 3p hybrid band (main band) and a satellite were observed at 0—4 eV and at 8 eV, respectively. Resonant PES (RPES) study showed that the satellite is associated with the photoemission process leading to a two-hole bound final state. The angle-resolved PES measurements showed that there were at least two surface states ; one was observed at about 0.4 eV around the Γ− point and the other was observed at about 0.6 eV along the boundary of the surface Brillouin zone along the [0001] direction. The results of RPES measurements suggested that the latter state should have little contribution of the Ni 3d component.