Oxygen Ion Site Research Articles

Temperature-dependent phonon Raman scattering and spectroscopic ellipsometry of pure and Ca-doped SrxBa1−xNb2O6 ferroelectric ceramics across the phase transition region

Optical phonons and the phase transition of relaxor ferroelectric ceramics SrxBa1−xNb2O6 (SBN) and Cay(Sr0.5Ba0.5)1−yNb2O6 (CSBN) with different composition (, ) have been investigated by variable-temperature Raman scattering and spectroscopic ellipsometry. The anomalous temperature dependence of Tauc gap energy (Et) is used to fit the phonon energy dependence of the permittivity, and the Raman intensity of some interesting optical phonons can be ascribed to the phase transition from a ferroelectric to a paraelectric structure. The Curie temperature of SBN decreases from 556 to 359 K with increasing Sr composition, which can be attributed to the substitution of smaller Sr2+ for Ba2+. On increasing the Ca composition, however, the phase transition temperature of CSBN remains nearly unchanged at about 350 K. This could be due to the fact that most doped Ca2+ ions move into the oxygen ion site and exhibit no obvious effect on the vibrational properties. Therefore, the general disorder which results from Sr2+ substituting Ba2+ , dominates the phase transition process for SBN-based ferroelectric oxides. Meanwhile, the dielectric functions from 200 to 600 K have been evaluated with the aid of the Tauc–Lorentz model. The electronic transition is located at about 5 eV and decreases with increasing temperature for all the samples. Moreover, the phase transition temperature range derived from the spectroscopic ellipsometry agrees well with that from the Raman scattering. It reveals that the variation of the fundamental energy gap may be associated with the phase transition of SBN ceramics. Both Raman scattering and spectroscopic ellipsometry are proven to be a effective method of exploring the phase transition of ferroelectric oxides.

Resistance switching behaviors of amorphous (ZrTiNi)Ox films for nonvolatile memory devices

In this study, the resistive switching characteristics of a multicomponent oxide (ZrTiNi)Ox film with a thickness of 50 nm for resistive random access memory applications are investigated and discussed, and are intended to be used to complement the studies of simple and binary oxides reported in the literature. Analysis reveals that the deposited films are amorphous and highly oxygen-deficient (ZrTiNi)Ox with a chemical formula of (Zr0.74Ti0.15Ni0.11)O1.12□0.78 in which the concentration of the existing oxygen vacancies are found to be 41% of the total oxygen ion sites. The prepared Pt/(ZrTiNi)Ox/Pt device exhibits excellent unipolar switching behavior marked with a substantial resistance ratio (larger than 102), good endurance, and long retention time. The low resistance state exhibits a linear ohmic behavior as a result of the conductive filament of the aligned oxygen vacancies, while the high resistance state conduction contributes to the space-charge-limited current conduction mechanism.

Calculation of chemical bonds and diffusion path of Li ion in (LaLi)Ti2O6 by DV-Xα cluster method

The chemical bonds and lithium diffusion of La4/3−y Li3y Ti2O6 (y = 0.21) were investigated by using the DV-Xα cluster method. The cluster model used is the formula La8Li2Ti2O11. A Li ion was moved on the ab plane at z = 1/2. The Na ion was moved along the x axis in the cluster model La8Na2Ti2O11 for comparison. The total bond overlap population (BOP) between the moving Li ion and the other ions was calculated on the ab plane at z = 1/2. The total BOP of the Li ion along the x axis increased near the oxygen ion site, whereas the BOP of the Na ion decreased. The decrease in total BOP indicates the decrease in covalent interaction between the Na and the other ions. The change of the net charge of the Li ion was almost the same as that of the Na ion. This suggests that the smaller change of covalent interaction in the mobile Li ion determines the diffusion path of Li ion.

High-Temperature Superconductivity: The Attractive Up Regime

We present the results of an infinite order unitary transformation applied to a multiband Hubbard Hamiltonian by which it can be shown rigorously that an attractive interaction term appears at the oxygen ion sites as a result of oxygen–copper virtual charge excitations. This exact result yields convincing evidence that the pairing mechanism in two and/or three band Hubbard models results precisely from this attraction.

Fast oxygen ion conductors—from doped to ordered systems

This contribution briefly introduces the traditional acceptor doping of oxides, charge compensated by oxygen vacancies to bring about the oxygen ion conduction utilised in solid oxide electrolytes. It is next shown how a simple defect–chemical approach to defect–defect interactions is a useful first-approximation tool in the interpretation of data and design of improved materials, e.g. by optimising dopants. Moreover, it is shown how the trapping of vacancies at acceptor defects can be avoided altogether if the cation sublattice can be made to order completely. This is realised in a few known systems and we illustrate the principle using the oxygen deficient complex perovskite Sr4(Sr2Nb2)O11 as an example, in which the Sr and Nb on the B-site sublattice can be perfectly ordered into a NaCl-structure-type arrangement, leaving all oxygen ion sites equivalent, without trapping sites. The design of new materials should thus aim at cation order, and the possibility of achieving this is discussed also for pyrochlore type oxides. A Kröger–Vink type notation for an inherently deficient sublattice without defects in the other sublattices is proposed.

Relationship between the local structure and spontaneous emission probability of Er3+ in silicate, borate, and phosphate glasses

Extended X-ray absorption fine structure (EXAFS) measurements have been performed on Er31 in silicate, borate, and phosphate glasses in order to investigate the local structure surrounding the Er31.Er31 ions coordinate to non-bridging oxygen ion sites, where alkali or alkaline earth ions terminate the network structure of silicate glasses. In borate glasses, the local structure surrounding Er31 ions is altered by the structural change ofthe borate anion. Er31 ions coordinate to non-bridging oxygen ion sites and BO4 structural units in the cases with and without the formation of non-bridging oxygen, respectively. The former is similar to the case in silicate glasses. Er31 ions selectively coordinate to the PyO site regardless of the glass composition variation. A correlation was observed between the spontaneous emission probability for 4I13/2 → 4I15/2 transition of Er3+ and the average Er–O distance calculated by EXAFS analysis. It shows the maximum value near 2.32 Å, and we conclude that the overlapping radial integral of the 4f and 5d orbitals of Er3+ would be the largest at the optimum Er31– O22 distance 2.3 Å.

Interaction of small molecules with Mg and MgO surfaces studied with MIES and UPS(HeI)

Metastable Impact Electron Spectroscopy (MIES) in combination with UPS(HeI) was used to study the interaction of molecular (CO 2, CO, N 2O) adsorbates with magnesium and magnesia (MgO) surfaces at room temperature (RT). Spectra obtained for Mg on Si(100) surfaces and for MgO surfaces are discussed. CO does not chemisorb on magnesium at RT. Dosing the Mg surface with N 2O leads to the dissociation of the N 2O molecule in N 2 and O and to the subsequent oxidation of the surface. Mg surfaces exposed to CO 2 show the formation of CO 3 2−. From the comparison of MIES and UPS spectra the following reaction mechanism is suggested: (i) dissociation of CO 2 in front of Mg; (ii) formation of a Mg O bond: and (iii) CO 2 chemisorption at low-coordinated oxygen ion sites of the surface. Spectra for MgO exposed to CO 2 show only weak CO 3 2− features. This can be explained by CO 3 2− formation at defect sites only. Dosing MgO with CO or N 2O leads to no new features in the MgO spectrum.

Anharmonic oxygen vibration and mode softening in Tl2Ba2Ca2Cu3O10 superconductor

The softening of the IR active 300 cm−1 phonon mode in Tl-2223 superconductor, aroundT c has been explained by considering a sixth order polarization potential at the off-center oxygen ion site in the Tl-O plane, and by using a nonlinear lattice dynamical theory. The present theory explains, more or less satisfactorily, the unusual temperature dependence of oxygen ion vibration and frequency shifts at higher temperature. The existence of strong nonlinear electron-phonon interaction atT c has been emphasized.

Raman spectroscopic study of the pressure-induced phase transition in TiO 2

Pressure dependence of Raman spectra in TiO 2 was measured up to 40 GPa using diamondanvil cell at room temperature. Two phase transitions were observed with increasing pressure. Rutile phase begins to transform into α-PbO 2 phase at about 10 GPa and the α-PbO 2 phase coexists with rutile phase up to 15 GPa. The α-PbO 2 phase disappears above 15 GPa and single phase of different structure is observed at 20 GPa. This new high-pressure phase is stable up to 40 GPa. Comparing the Raman spectra of the new high-pressure phase of TiO 2 measured at 20 GPa with those of ZrO 2 and HfO 2, the crystal structure of the high-pressure phase has been determined as the baddeleyite structure. The line widening of Raman band observed for the high-pressure phase is explained by the disorder in oxygen ion site predicted from the proposed mechanism for the pressure-induced phase transition in TiO 2.

Semiempirical structural model of the electric field gradients at oxygen nuclei in YBa 2Cu 3O 7

The electric field gradients at the oxygen ion sites in YBa 2Cu 3O 7 are calculated as a sum of ionic, overlap, and valence terms using a semiemoirical model in which the O ion holes that determine the valence terms are the principal adjustable parameters. Good fits to the experimental results are obtained for reasonable values of fractional hole character in O 2pσ orbitals only.

Non-superconductivity of La 2−xBa xCaCu 2O 6

Oxygen defect intergrowths copper perovskite, La 2−xBa xCaCu 2O 6, was prepared by rapid heating and quenching, and subsequently by high O 2- pressure annealing. The oxygen ion site between two-dimensional Cu-O planes is thought to be completely vacant. Although copper valency is +2.15, high enough to produce superconductivity, superconductivity cannot be observed. In the electrical property, carrier localization was still observed at low temperatures, as seen in the same type of compounds with low copper valency (+2.05). The temperature dependence of magnetic susceptibility was characterized by Curie-like behavior, and neither a 2D- nor 3D-antiferromagnetic correlation was observed.

Atomistic simulation of ionic and electronic defects in YBa2Cu3O7.

An empirical two-body potential model has been developed in order to describe the electronic and ionic defect properties of orthorhombic Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The potential model was derived starting from known potentials and calculated potentials and fits the crystal structure to better than 0.03 \AA{} for the major bonds. Oxygen vacancy formation and hole trapping as a localized polaron are most favorable at the chain oxygen ion site 1. Hole trapping on ${\mathrm{Cu}}^{2+}$ ions is most favorable for ions in the copper-oxygen plane. Divalent impurity ions such as ${\mathrm{Ni}}^{2+}$, ${\mathrm{Zn}}^{2+}$, and ${\mathrm{Cd}}^{2+}$ can dissolve in the crystal preferentially at the ${\mathrm{Cu}}^{2+}$ plane site 2. On the other hand, trivalent impurities such as ${\mathrm{Al}}^{3+}$, ${\mathrm{Fe}}^{3+}$, and ${\mathrm{Ga}}^{3+}$ dissolve in the crystal at the chain ${\mathrm{Cu}}^{2+}$ site 1 along with the incorporation of oxygen ions. Anions such as ${\mathrm{S}}^{2\ensuremath{-}}$, ${\mathrm{Cl}}^{\ensuremath{-}}$, or ${\mathrm{F}}^{\ensuremath{-}}$ would preferentially substitute for ${\mathrm{O}}^{2\ensuremath{-}}$ at sites near the ${\mathrm{Cu}}^{2+}$ chain position.

Atomistic Simulation of Ionic and Electronic Defects in YBa 2 Cu3O7

AbstractAn empirical two-body potential model has been developed in order to describe the electronic and ionic defect properties of orthorhombic YBa 2 Cu307. The potential model was derived starting from known potentials and calculated potentials and fits the crystal structure to better than 0.03 A for the major bonds. Oxygen vacancy formation and hole trapping as a localized polaron are most favorable at the chain oxygen ion site. Hole trapping on Cu + ions is most favorable for ions in the copper-oxygen plane. Divalent impurity ions such as Ni+2, Zn+2 and Cd+2 can dissolve in the crystal preferentially at the Cu+2 plane site.

Solubility of Hydrogen Defects in Doped and Undoped BaTiO3

Thermal desorption studies and electrical conductivity measurements have been used to investigate the solubility of hydrogen defects in barium titanate ceramics in oxidizing atmospheres. Hydrogen is found to dissolve in the perovskite lattice of undoped and acceptor‐doped BaTiO3 by the formation of hydroxide ions on regular oxygen ion sites. The hydrogen defects act as mobile donors and may substitute the oxygen vacancies in compensating fixed acceptors. From the dependence of the solubility on the water vapor pressure and the dopant concentration, the incorporation/desorption equilibrium reaction is deduced. A defect model is derived which consistently extends previously proposed models. The influence of Ba2TiO4 and Ba6Ti17O40 second phases on the solubility of hydrogen defects is discussed.

Mechanisms for hydrogen diffusion in TiO2

The predominant hydrogen-containing species in Ti${\mathrm{O}}_{2}$ (rutile) are O${\mathrm{H}}^{\ensuremath{-}}$ (hydroxyl) ions, in which the oxygen occupies a regular oxygen ion site, and the O-H bond is perpendicular to the $c$ axis. It is proposed that diffusion of hydrogen parallel to the $c$ axis proceeds by a proton jump from one ${\mathrm{O}}^{2\ensuremath{-}}$ ion to another along the channel as represented by O${\mathrm{H}}^{\ensuremath{-}}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{O}}^{2\ensuremath{-}}$ \ensuremath{\rightarrow} ${\mathrm{O}}^{2\ensuremath{-}}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{H}}^{+}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{O}}^{2\ensuremath{-}}$ \ensuremath{\rightarrow} ${\mathrm{O}}^{2\ensuremath{-}}$\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H${\mathrm{O}}^{\ensuremath{-}}$. It is also proposed that diffusion perpendicular to the $c$ axis proceeds by a rotation of the O${\mathrm{H}}^{\ensuremath{-}}$ bond to move the proton from one channel to an adjacent channel, followed by a proton jump to another ${\mathrm{O}}^{2\ensuremath{-}}$ ion in the same channel. From a potential-energy model, which includes a Morse function to represent the O${\mathrm{H}}^{\ensuremath{-}}$ bond, as well as electrostatic and repulsive terms, the activation energies for hydrogen and tritium diffusion parallel to the $c$ axis were calculated to be (including a zero-point energy correction) 0.60 and 0.69 eV, respectively, in good agreement with the respective experimental values of 0.59 and 0.75 eV. The calculated activation energy for diffusion perpendicular to the $c$ axis was 1.23 eV (no zero-point energy correction), as compared to the experimental values of 1.28 and 1.11 eV, respectively, for hydrogen and tritium. The calculated equilibrium orientation of the O${\mathrm{H}}^{\ensuremath{-}}$ ion in Ti${\mathrm{O}}_{2}$ and the calculated stretching frequency of this species were also in good agreement with the respective experimental results.

EPR and optical absorption studies of radiation-produced defects in sodium beta -alumina

The EPR and optical absorption spectra of X-ray irradiated single crystal of Na beta -alumina have been examined. The most prominent absorptions are tentatively ascribed to an F+ centre at the O(5) oxygen ion site in the conduction plane. To this defect the authors assign an axially symmetric EPR spectrum which shows a resolved hyperfine structure due to interaction of the single unpaired spin with two equivalent 27Al(I=5/2) nuclei. The measured spin-Hamiltonian parameters are g/sub ///=2.0076(2), gperpendicular to =2.0032(2), A/sub ///=1.665(2) mT, Aperpendicular to =1.617(2) mT, the defect symmetry axis coinciding with the crystal c axis. From the EPR linewidth it is inferred that the hyperfine interaction with three equivalent 23Na nuclei is of order 0.2 mT. This defect is also optically active having a strongly polarised s to pz transition with mean energy EF=4.15(5) eV. The corresponding s to px,y optical transitions are not observed in the range of photon energies from 4.2-6.8 eV.