Zr 4d States Research Articles

First-Principles Study of the Electronic Structure and Spontaneous Polarization of PbZr<sub>0.4</sub>Ti<sub>0.6</sub>O<sub>3</sub>

Density functional theory plane-wave pseudopotential with the general gradient approximation (GGA) was used to investigate electronic structural properties and the bulk spontaneous polarization (Ps) of PbZr0.4Ti0.6O3. It is found that there are strong hybridizations between Ti 3d states or Zr 4d states and O 2p states, which can reduce short-range repulsion in atoms and enhance the stability of the ferroelectric phase of PbZr0.4Ti0.6O3. Compared with cubic ideal structure, the calculated internal electronic structural data indicate that the slightly distorted O6 octahedrons around the central Ti and Zr atoms change to the Ti-O5 and Zr-O5 pyramid in the optimized structure, respectively. The major contribution to the spontaneous polarization along [001] comes from the stronger interaction along the c axis between the Ti and O rather than the Zr and O ions. The Pb atom’s relative displacement of oxygen octahedral implies that the Pb-O bonding interaction is also a key factor impacting the ferroelectricity of PbZr0.4Ti0.6O3. A theoretical spontaneous polarization of 0.78 C/m2 was computed in the tetragonal PbZr0.4Ti0.6O3 along [001] direction, consistent with the single crystal experimental data.

Ab initio calculations of elastic modulus and electronic structures of cubic [formula omitted

Using the plane wave pseudopotential method within the generalized gradient approximation (GGA), we have studied the elastic constants and electronic structures of CaZrO 3 crystallized in cubic perovskite structure. The independent elastic constants of cubic CaZrO 3 , which are derived by calculating the strain-induced stresses, satisfy the requirement of mechanical stability and indicate cubic CaZrO 3 could be stable. It is found that cubic CaZrO 3 is an indirect insulator with the GGA energy band gap of 3.30 eV. The bottom of conduction bands of cubic CaZrO 3 is mainly contributed by Ca-3d states with some mixture of Zr-4d and Ca-4s states, while the top of valence bands is dominated by O-2p states. The Zr–O bond has a significant covalent character while the Ca–O bond is typically ionic.

First-Principles Study of Electronic Structure of the LavesPhase ZrFe2

We perform the ab initio calculation for obtaining the density ofstates and magnetic properties of ZrFe2 Laves phase compoundbased on the method of augmented plane waves plus local orbital. Theresults indicate that the ferromagnetic state is more stable than theparamagnetic one, but with a slightly larger volume. The 3d−4dexchange interactions between Fe and Zr electrons lead to theantiparallel coupling for Fe 3d and Zr 4d states, which isresponsible for the ferrimagnetic ordering of the compound. Theresulting magnetic moment of about 1.98 μB for Fe isspatially localized near the Fe site, while around Zr a small butextended negative spin states causes a moment of about−0.44 μB. Moreover, the resulting magnetic moments with thegeneralized gradient approximation are more consistent withexperimental values than that of the local-spin density approximation.

The atomic and electron structure of ZrO2

The atomic structure of amorphous and crystalline zirconium dioxide (ZrO2) films is studied using X-ray diffraction and extended X-ray absorption fine structure techniques. The electron structure of ZrO2 is experimentally determined using X-ray and UV photoelectron spectroscopy, and the electron energy band structure is theoretically calculated using electron density functional method. According to these data, the valence band of ZrO2 consists of three subbands separated by an ionic gap. The upper subband is formed by the O2p states and Zr4d states; the medium subband is formed by the O2s states; and the narrow lower subband is formed predominantly by the Zr4p states. The bandgap width in amorphous ZrO2, as determined using the electron energy loss spectroscopy data, amounts to 4.7 eV. The electron band structure calculations performed for a cubic ZrO2 phase point to the existence of both light (0.3m 0) and heavy (3.5m 0) holes, where m 0 is the free electron mass. The effective masses of band electrons in ZrO2 fall within (0.6–2.0)m 0.

バルク金属ガラスＺｒ‐ＴＭ‐Ａｌ（ＴＭ＝Ｎｉ，Ｃｕ）の電子構造

The valence-band electronic structures of Zr-TM-Al (TM=Ni, Cu) bulk metallic glasses have been investigated by means of synchrotron-radiation photoelectron spectroscopy. Their valence-band spectra show Zr 4d-, Ni 3d- and Cu 3d-derived bands at the binding energies of 0.5, 2.0 and 3.6 eV, respectively. The Zr 4d-derived band becomes prominent around the excitation photon energy hν of 40 eV. It is found that the wider the supercooled liquid region ΔTx=Tx-Tg (Tx: the crystallization temperature, Tg: the glass transition temperature), the larger the peak binding energy of the Zr 4d-derived band becomes. For the photoexcitation at hν∼18 eV, where the Zr 4d states less contribute to the spectrum, the spectral intensity reduces towards the Fermi level. This may imply the formation of a pseudogap in the sp bands. It is also found that the width of the pseudogap for the occupied states becomes wider as ΔTx is increased. These spectral findings suggest that both the strength of the chemical bonding around Zr and the reduction in the electronic energy because of the pseudogap formation and the chemical bonding contribute to the large glass formation ability of the Zr-Cu-Al metallic glasses.

Electronic structure of bulk metallic glass Zr 55Al 10Cu 30Ni 5

The electronic structure of a bulk metallic glass Zr 55Al 10Cu 30Ni 5 has been studied by means of photoelectron spectroscopy in order to understand the origins of its large glass formation ability and unique mechanical properties from the microscopic point of view. The valence-band photoelectron spectra show three bands ascribed to the Zr 4d, Ni 3d, and Cu 3d states. A remarkable feature of these bands is the highly symmetric spectral shape with the high-binding energy and narrow width in comparison with the d bands of the crystalline transition metals. This is attributed to the lack of the crystalline periodicity in the metallic glass as well as the reduction in the neighbouring atoms to hybridize with those transition metals. A high-resolution valence-band spectrum also reveals the intensity reduction near the Fermi level, which implies that the pseudo-gap in the electronic structure may be one of the important factors for the glass formation.

Spatially-resolved valence-electron energy-loss spectroscopy of Zr-oxide and Zr-silicate films

We examined electronic structures in Zr-oxide (ZrO2) and Zr-silicate (ZrxSi1−xO2) films deposited on Si substrates by using valence-electron energy-loss spectroscopy combined with scanning transmission electron microscopy (the electron probe diameter was about 0.3 nm). Our analysis indicated that both valence-electron excitations in ZrO2 and in SiO2 occurred in the ZrxSi1−xO2 films. Therefore, the band gaps in the ZrxSi1−xO2 films should be dominated by an energy gap between O 2p and Zr 4d states.

Electronic structure of the Zr suboxide layer formed on a ZrC([formula omitted]) surface

Angle-resolved photoemission spectroscopy utilizing synchrotron radiation has been used to study the electronic structure of the ZrC(100) surface covered by a ZrO-like layer. When the ZrC(100) surface exposed to 50 L O2 is heated at 950 °C, a ZrO-like layer is formed on the surface and the surface is activated towards gas adsorption. Angle-resolved photoemission measurements show that the O2p-derived bands show clear dispersion along both the 〈100〉 (ΓM) and 〈011〉 (ΓX) symmetry directions of ZrC(100). Since the surface gives a sharp (1×1) LEED pattern, it is concluded that a well ordered ZrO layer having a (1×1) periodicity is formed on the ZrC(100) surface. The O2p-derived bands show no dispersion along the axis normal to the surface, thus the bands can be viewed as two-dimensional electronic states. In addition to the O2p-derived bands, partially filled states are found around the Fermi level in the ZrO-like layer. The states are removed by water or oxygen adsorption. The origin of the states is discussed and the states are ascribed to partially filled Zr4d states.

Density functional studies of PbZrO3, KTaO3 and KNbO3

Density functional calculations are used to investigate the non-ferroelectric end-points of the KTN and PZT alloy systems and to address the errors in the local approximation for KNbO3. In PbZrO3, substantial hybridization is found between O 2p and Zr 4d states and between O 2p and Pb s and p states. A strong Γ point phonon instability is present for the cubic perovskite structure. Relaxation, allowing only zone center distortions, yields a large energy lowering of 0.253 eV and bond length changes in excess of 0.5 Å. Two related R point instabilities are found; both are weaker. Calculations for 40 atom unit cells with orthorhombic Pba2 and Pbam space groups and previously reported experimental positions, yield energies close to but still above the relaxed Γ structure. However, after relaxation of the oxygen coordinates, the antiferroelectric structure is favored by 0.02 eV per formula unit, in accord with the PZT phase diagram. KTaO3 is found to be stable in the cubic perovskite structure. Phonon frequencies and bulk properties in good agreement with existing measurements are obtained. A highly anharmonic low frequency mode corresponding to the observed incipient ferroelectric behavior is identified. The absence of ferroelectricity in KTaO3 is due to the extreme sensitivity of the soft mode to covalency and the slight chemical differences between Nb and Ta, particularly the higher d binding energy in Nb. Weighted density approximation calculations of bulk properties of perovskite KNbO3 showing an equilibrium volume in substantially better agreement with experiment than with the standard local density approximation are reported.

Structure and energetics of antiferroelectric PbZrO3.

Structural instabilities and electronic properties of ${\mathrm{PrZrO}}_{3}$ are investigated using local-density calculations. Substantial hybridization is found in the electronic band structure, both between O 2p and Zr 4d states and between O 2p and Pb s and p states. A very strong \ensuremath{\Gamma} point phonon instability is present for the cubic perovskite structure. Relaxation, allowing only zone-center distortions, yields a large energy lowering of 0.253 eV, with bond-length changes in excess of 0.5 \AA{}. Two related R point instabilities are found, both of which involve changes in Pb-O distances. The more unstable of these involves a 12\ifmmode^\circ\else\textdegree\fi{} rotation of the O octahedra, with an energy lowering of 0.20 eV per formula unit. Total-energy calculations were performed for 40 atom unit cells with orthorhombic Pba2 and Pbam space groups as reported experimentally. Energies close to but still above the related \ensuremath{\Gamma} structure were obtained with experimentally determined atomic positions. However, after relaxation of the oxygen coordinates, the antiferroelectric structure is favored by 0.02 eV per formula unit. This ordering and the small ferroelectric-antiferroelectric energy difference is in accord with the Pb(Zr,Ti)${\mathrm{O}}_{3}$ phase diagram.

Electronic structure of stoichiometric and Ar+-bombarded ZrO2 determined by resonant photoemission.

The electronic properties of thermally grown ${\mathrm{ZrO}}_{2}$ thin films before and after ${\mathrm{Ar}}^{+}$ bombardment have been studied with resonant photoemission spectroscopy using synchrotron radiation. For stoichiometric ${\mathrm{ZrO}}_{2}$ thin films the experimental valence-band spectra are in good agreement with the calculated density of states for bulk ${\mathrm{ZrO}}_{2}$. For both stoichiometric and ${\mathrm{Ar}}^{+}$-bombarded ${\mathrm{ZrO}}_{2}$ thin films, resonant photoemission from the valence band was observed when the photon energy was swept through the Zr 4p\ensuremath{\rightarrow}4d transition energy. The resonant profile was found to exhibit a maximum at h\ensuremath{\nu}=39 eV, followed by a second well-resolved broad maximum around 50 eV. The feature at 39 eV is consistent with resonant enhancement of the Zr 4d states and has been used to identify those regions of the valence band with an important Zr 4d admixture. The results are in good agreement with the calculated Zr 4d partial density of states. The intensity increase observed at h\ensuremath{\nu}\ensuremath{\sim}45--50 eV is found to be associated with the nonbonding region of the valence band, although a proper interpretation is needed. In addition, it was found that ${\mathrm{Ar}}^{+}$ bombardment induces electronic states in the band-gap region and changes in the O 2p valence band. Three distinct emission bands were identified in the band gap as a function of the ${\mathrm{Ar}}^{+}$ dose. They are associated with the formation of oxygen vacancies and mixed oxidation states due to preferential sputtering of the oxygen atoms. Resonant photoemission of these ${\mathrm{Ar}}^{+}$-bombarded films demonstrates both the cationic character of the band-gap states and the increase of the cationic contribution to the O 2p valence band.

Electronic states of group-IV endohedral atoms in C28.

We have systematically studied the interaction between the C 28 fullerene molecule and the group-IVA endohedral atoms C, Si, Ge, and Sn, using state-of-the-art electronic structure and total-energy calculations based on density-functional theory. We find no covalent bonding between these atoms and the fullerene cage. The nature of the cage-atom interaction involves charge transfer from the endohedral atom to the cage which produces weak, mostly ionic binding. The degree of charge transfer increases with the atomic number of the endohedral atom. By comparing the electronic structure of the group-IVA endohedral cage compounds to Zr@C 8 (a group-IVB endohedral), we find that the latter bonds strongly to the cage due to a covalent interaction betwen the Zr 4d states and the C 28 states

Photoemission studies on metallic glasses using synchrotron radiation

We report on the use of UV synchrotron radiation in the energy range 40–120 eV to probe the electron states of CuZr, FeZr, CoZr and CuHf amorphous alloys. The great advantage of tunable radiation over fixed frequency sources is that it allows identification of the electron states from a knowledge of the energy dependences of their scattering cross-sections. This variation is particularly pronounced in the case of the d states of zirconium which are essentially removed from the observed photoelectron spectra as the photon energy is increased from 40 eV to about 100 eV. The results confirm, for example, that for CuZr alloys it is the d states from zirconium that lie at the Fermi level. By combining these results with those obtained from a technique involving constant initial states the scattering cross-sections for the Zr 4d and Cu 3d states were investigated. This will allow the experimental photoemission data to be corrected to give a more representative picture of the electronic band structure of these alloys, and has laid the foundation for extending the method to other newer alloy systems where the electron states have still not been identified.

Local densities of states of Zr-based transition metal glasses

The local densities of states of TLxZr1-x(x approximately 0.3) metallic glasses, where TL is Co, Ni, Rh or Pd, have been investigated by soft X-ray emission spectroscopy. Unlike the local Rh and Pd 4d states which overlap the part of the Zr 4d band with high binding energy, the Co and Ni 3d states are centred on the Zr 4d states. The contribution to the density of states at the Fermi level from the TL elements is more pronounced for the 3d than for the 4d elements. The results are discussed in relation to density of states calculations on disordered and ordered alloys. Particularly good agreement is obtained with density of states calculations performed for ordered structures, indicating the dominant effect of chemical short-range order.

Electronic structure in amorphous CuxZr1-xalloys

The electronic structure of amorphous CuxZr1-x (x=0.65, 0.50 and 0.35) alloys is calculated for realistic, structurally relaxed models by using the linear muffin-tin orbital method in the atomic sphere approximation and the recursion method. The orbitals of the Cu 3d and 4s and the Zr 4d and 5s electrons are included in the calculation and the electronic charge density is determined self-consistently. The hybridised mixing between the Cu 3d and Zr 4d states can be seen and the calculated density of states agrees well with the observed XPS and UPS data. The observed change of the Zr-Zr distance depending on the Cu contents is attributed to charge transfer into the Zr 4d band.