Mobile Oxygen Vacancies Research Articles

Mechanical loss of Zr 0.8− xCe xY 0.2O 1.9 ( x=0–0.4)

The solid solution oxides of CeO 2–Y 2O 3 and ZrO 2–Y 2O 3 are useful crystals as solid state ionics, which are applied to sensors, solid electrolytes and other advanced devices such as SOFC. However, in the ternary system of CeO 2–ZrO 2–Y 2O 3, the decrease in oxide ion conductivity is observed by mixing ZrO 2 and CeO 2. In this work, the oxygen movement and the doping effect of CeO 2 to ZrO 2–Y 2O 2 are examined by the mechanical loss measurement of polycrystalline bodies. We discuss the difference of the mobility of oxygen, which is the origin of mechanical loss in the crystals, by comparing the mechanical loss peaks in a series of solid solutions, Zr 0.8− x Ce x Y 0.2O 1.9 ( x=0–0.4). The low temperature mechanical loss at 300–820 K spectra were measured for Zr 0.8− x Ce x Y 0.2O 1.9 ( x=0–0.4). We observed two loss maxima of Debye-type relaxation which are related to local jumps of oxygen, and their strong Ce-doping level dependence. The activation enthalpy of 1.2 eV suggested the simple relaxation of an oxygen in the pairs of oxygen vacancies and Y ions in nearest neighbor (peak 1). Additional maximum (peak 2) with H=2.2 eV, which appeared at high temperature and are related to defect agglomerates with the same more than one dopant cation, disappeared with the Ce-doping of x=0.2 and 0.4. The loss (peak 1) amplitude also decreased with increasing of Ce-doping content, but with the same activation enthalpy (1.1–0.9 eV). The addition of CeO 2 in Y 2O 3–ZrO 2 was found to reduce the mobility of oxygen vacancies under elastic field.

Proton transport and structural relations in hydroxyl-bearing BaTiO 3 and its doped compositions synthesised by wet-chemical methods

Hydrothermally synthesised powders of BaTiO 3 and its Fe- or Nd-doped analogues contain hydroxyl groups in the lattice substitutional to oxide ion, as confirmed from TGA/DTA, IR spectral analysis of D 2O-treated powders, EGA-MS, the contraction in lattice constant with heat treatment by XRD and surface examination by XPS. Electrical resistivity measurements were carried out on the pellets from 298 to 1000 K by ac impedance spectroscopy and dc methods in dry or moist air and 8% H 2+Ar environments. The electrical conductivity observed for unsintered pellets between 298 and 500 K, is in the range of 10 −3 to 10 −7 S/cm and can be attributed to the extrinsic hydroxyls in BaTiO 3. The acceptor-doped composition, BaTi 0.9Fe 0.1O 3− δ : 2 δ(OH) exhibits higher electrical conductivity than BaTiO 3 or the donor-doped Ba 0.9Nd 0.1TiO 3− δ : 2 δ(OH) in moist air. The hydrothermally prepared powders heat treated below 1000 K having cubic symmetry at room temperature, possess higher proton conductivity and reabsorption capability for hydroxyls on exposure to moisture than the powders sintered at 1673 K (tetragonal symmetry). The conductivity at 298–500 K is due to the mobility of proton along OHO octahedra in the perovskite lattice. The conduction at 550–1000 K is a combined effect of proton as well as oxygen vacancy mobility in BaTiO 3 and Ba 0.9Nd 0.1TiO 3; electron hole (Ti 4+, Ti 3+, Fe 3+, Fe 2+) participation is the additional contribution in acceptor-doped composition in this temperature range.

Improved ferroelectric and pyroelectric properties in Mn-doped lead zirconate titanate thin films

We have investigated the effects of Mn doping on the ferroelectric and pyroelectric properties of Pb(Zr0.3Ti0.7)O3 (PZT) thin films on substrates Pt/Ti/SiO2/Si. The Mn-doped (1 mol %) PZT (PMZT) showed almost no hysteretic fatigue up to 1010 switching bipolar pulse cycles, coupled with excellent retention properties. We present evidence that while a low permittivity interfacial layer forms between the Pt electrode and PZT films, this does not occur in PMZT. We propose that Mn dopants are able to reduce oxygen vacancy mobility in PZT films and Mn2+ ions consume the oxygen vacancies generated during repeated switching, forming Mn4+ ions. These mechanisms are probably responsible for their low observed fatigue characteristics. Mn doping brings additional benefits to the electrical properties of PZT films. The relevant pyroelectric coefficients (p) of a 700 nm thick film are 3.52×10−4 C m−2 K−1 and detectivity figures of merit FD=3.85×10−5 Pa−0.5 at 33 Hz for Mn-doped PZT, compared with p=2.11×10−4 C m−2 K−1 and FD=1.07×10−5 Pa−0.5 for the undoped PZT films. This means that the Mn-doped PZT thin films are excellent candidates as device materials for both memory and pyroelectric applications.

Formation and mobility of oxygen vacancies inRuSr2GdCu2O8

Oxygen vacancies are introduced in the RuO2 and possibly CuO2 planes of RuSr2GdCu2O8 by annealing in vacuum above 600 K. The diffusive jumps of the O vacancies are accompanied by a reorientation of the local distortion, and are probed by measuring the elastic energy loss and modulus versus temperature at 1-10 kHz. An intense acoustic absorption peak develops near 670 K at 1 kHz and finally stabilizes after heating up to 920 K in vacuum. The analysis of the peak shows a barrier for the O diffusion of ~1.4 eV, and a slowing down of Curie-Weiss type, with Tc = 400-470 K, due to the interaction among the O vacancies. A secondary peak is attributed to O vacancies trapped at defects in the RuO2 planes, or to vacancies in the CuO2 planes. No sign of structural phase transformation is found up to 920 K.

Improved Ferroelectric Properties in Mn-Doped PZT Thin Films

We have investigated the effects of Mn doping on the ferroelectric properties of Pb(Zr0.3Ti0.7)O3 (PZT) thin films on substrates Pt/Ti/SiO2/Si. The Mn-doped (1 mol%) PZT (PMZT) showed almost no hysteretic fatigue up to 1010 switching bipolar pulse cycles, coupled with excellent retention properties. We present evidence that while a low permittivity interfacial layer forms between the Pt electrode and PZT films, this does not occur in PMZT. We propose that Mn dopants are able to reduce oxygen vacancy mobility in PZT films and Mn2+ ions consume the oxygen vacancies generated during repeated switching, forming Mn4+ ions. These mechanisms are probably responsible for their low observed fatigue characteristics.

Damage Evolution and Annealing of Au-Irradiated Samarium Titanate Pyrochlore

ABSTRACTDamage evolution and thermal recovery of 1 MeV Au2+ irradiated samarium titanate pyrochlore (Sm2Ti2O7) single crystals were studied by Rutherford backscattering spectroscopy and nuclear reaction analysis. The damage accumulation follows a nonlinear dependence on dose that is well described by a disorder accumulation model, which indicates a predominant role of defect-stimulated amorphization processes. The critical dose for amorphization at 170 and 300 K is ∼0.14 dpa, and a higher dose of ∼ 0.22 dpa is observed for irradiation at 700 K, which agrees with previous in-situ transmission electron microscopy (TEM) data for polycrystalline Sm2Ti2O7. Annealing in an 18O environment reveals a damage recovery stage at ∼ 850 K that coincides with a significant increase in 18O exchange due to oxygen vacancy mobility. This thermal recovery stage is also consistent with the critical temperature for amorphization measured by in-situ TEM in polycrystalline samples.

Fatigue in artificially layered Pb(Zr,Ti)O3 ferroelectric films

We have performed fatigue tests on lead zirconate titanate (PZT) multilayers having stacks of Pb(Zr0.8Ti0.2)O3/Pb(Zr0.2Ti0.8)O3 with repeated distances of 12 formula groups. The results are compared with single-layer n-type (0.5 at. % Ta-doped) PZT films. We conclude that fatigue is dominated by space-charge layers in each case, but that in the multilayer such space charge accumulates at the layer interfaces, rather than at the electrode–dielectric interface. The model, which includes both drift and diffusion, is quantitative and yields a rate-limiting mobility of 6.9±0.9×10−12 cm2/V s, in excellent agreement with the oxygen vacancy mobility for perovskite oxides obtained from Zafar et al.

Microstructure and ionic conductivity of strontium-substituted lanthanum cobaltites

La 0.4Sr 0.6CoO 3− δ powders were synthesized by the glycine nitrate process. X-ray powder diffraction and selected area electron diffraction (SAED) in the transmission electron microscope (TEM) were used to determine the basic crystal structure of the perovskite samples. Additionally, energy-filtering transmission electron microscopy (EFTEM) was used to evaluate the homogeneity of the samples at the nanometer level. The ionic conductivity σ i of La 0.4Sr 0.6CoO 3− δ was obtained from galvanostatic polarization experiments as a function of oxygen non-stoichiometry for 3− δ values between 2.70 and 2.81 (corresponding to p O 2 values between 10 −4 and 10 −1 bar) at 775 and 825 °C. It could be observed that σ i shows a maximum which shifts towards larger values of the oxygen non-stoichiometry with increasing temperature. The maximum value was observed at 3− δ=2.79 and 2.77 for 775 and 825 °C, respectively. This behavior has been reported to be indicative of the formation of vacancy-ordered structures which are expected to lower the mobility of oxygen vacancies. The TEM investigations revealed a superstructure within microdomains which were crystallographically oriented perpendicular to each other and were found to be around 100 nm in size.

Oxygen-related dielectric relaxation and leakage characteristics of Pt/(Ba,Sr)TiO3/Pt thin-film capacitors

The leakage characteristics and dielectric properties of Pt/(Ba,Sr)TiO3/Pt thin-film capacitors were found to be remarkably sensitive to the postannealing temperature in oxygen and nitrogen atmosphere. High leakage currents and low-frequency dielectric relaxation were found in as-deposited capacitors after they had been postannealed in nitrogen at 550 °C and subsequently annealed in oxygen at 350 °C. Such results are related to the mobile oxygen ions and oxygen vacancies accumulated in the (Ba,Sr)TiO3 films. The chemical process of the formation of charged oxygen ions during postannealing is proposed.

Chemical Diffusion and Oxygen Surface Transfer of La1 − xSrxCoO3 − δ Studied with Electrical Conductivity Relaxation

The chemical diffusion coefficient and oxygen-transfer coefficients of selected compositions in the series were studied using the conductivity relaxation technique. Measurements were performed in the temperature range 600-850°C and oxygen partial pressure to 1 bar. Chemical diffusivity and oxygen surface transfer in the perovskites appear to be highly correlated. The general trend displayed is that both parameters decrease with decreasing below about bar at all temperatures. This is attributed to ordering of induced vacancies at low oxygen partial pressures. The observation that the correlation between both parameters extends even to the lowest values in this study suggests a key role of the concentration of mobile oxygen vacancies, rather than of the extent of oxygen nonstoichiometry, in determining the rates of both processes. The characteristic thickness which equals the ratio of the chemical diffusion coefficient to the surface transfer coefficient, shows only a weak dependence on oxygen partial pressure and temperature. For different compositions is found to vary between 50 and 150 μm. © 2002 The Electrochemical Society. All rights reserved.

Effect of temperature and dopant concentration on the conductivity of samaria-doped ceria electrolyte

The conductivity, σ, of a samaria-doped ceria electrolyte is studied as a function of temperature and dopant concentration, x, which was from 5 to 30 mol%. It is shown that a maximum in σ versus x corresponds to a minimum in activation energy. It is found that the conductivity is completely due to oxygen vacancy conduction. The conductivity increases with increasing samaria doping and reaches a maximum for (CeO2)0.8(SmO1.5)0.2, which has a conductivity of 5.6×10–1 S/cm at 800 °C. A curvature at T=Tc, the critical temperature, has been observed in the Arrhenius plot. This phenomenon may be explained by a model which proposed that, below Tc, nucleation of mobile oxygen vacancies into ordered clusters occurs, and, above Tc, all oxygen vacancies appear to be mobile without interaction with dopant cation. In addition, the composition dependences of both the critical temperature and the trapping energy are consistent with that of the activation energy.

Dielectric and mechanoelastic relaxations due to point defects inlayered bismuth titanate ceramics

Complex permittivity and Young's modulus provide relevant information on therole of point defects in the dielectric and mechano-elastic properties offerroelectric materials. Low-frequency measurements as a function of thetemperature performed on Bi4Ti3O12 (BIT) have shown that point anddipole defects are frozen close to domain walls. Low-temperature dipole defectrelaxation processes take place with characteristic times (τ0) of theorder of 10-11 s and 10-12 s and activation energies (Ea) of0.70 eV and 0.65 eV for dielectric and mechano-elastic relaxations,respectively. At higher temperatures new dielectric relaxation peaks appearthat can be attributed to jumps of de-iced oxygen vacancies(τ0≅10-11 s, Ea = 1.08 eV, T≅300 °C) and to vacancymigration (τ0≅10-15 s, Ea = 1.90 eV, T≅450 °C).Elastic relaxation peaks are also present close to 300 °C whoseactivation energy (1.50 eV) and characteristic time (10-15 s) suggest avacancy migration process. Close to 500 °C with Ea = 2.30 eV andτ0≅10-17 s another relaxation peak, which should correspond todomain wall viscous motion near the phase transition temperature, isobserved. The Young's modulus has a smooth step at T≅300 °C thatwe attribute to a change in the mobility of oxygen vacancies with respect tothe domain walls. Below 300 °C the vacancies are frozen in the domainwalls and they are de-iced and distributed throughout the material attemperatures above 300 °C. The experimental results show that thematerial is softer when the vacancies are linked to domain walls than whenthey are distributed throughout the material. The diffusion of vacancies backto the domain wall traps at room temperature takes a long time (days).

Influence of ion energy on titanium oxide formation by vacuum arc deposition and implantation

Titanium oxide thin films were produced by vacuum arc deposition while pulse biasing the substrate. The stoichiometry can be influenced by varying the oxygen partial pressure and the arc current from semiconducting and transparent TiO 1.8 to metallic TiO x ( x≈1.4). Without additional bias voltage, a hyperthermal energy of 25–50 eV of the titanium and oxygen ions leads to the growth of columnar textured rutile films on silicon, with their orientation solely determined by the substrate orientation. While applying negative high voltage pulses between 1 and 5 kV with a duty cycle of 9% no loss of this texture was observed. This can be explained by the rather high displacement energy of 50 eV for rutile. Using atomic force microscopy (AFM) an increasing surface roughness, caused by a larger sputter etching of the grain boundaries, as well as an increase in the column diameter was observed. At 10 kV pulses a broadening and splitting of the rutile (1 1 0) peak in X-ray diffraction (XRD) was observed, indicating enough mobility of oxygen vacancies and titanium interstitials to start the formation of the Magnéli phases Ti n O 2 n−1 .

Dielectric materials for sintering in reducing atmospheres

Abstract Ceramic multilayer capacitors (MLCC) with Ni base metal electrodes (BME) have nowadays achieved the standard of MLCCs with noble metal (Pd/Ag) electrodes. One of the greatest problems to be solved at the industrialization of BME capacitors was the suppression of the electrical degradation, caused by the mobility of charged oxygen vacancies. This paper gives a review of the research and development of BME materials

Ionic conductivity of Gd 2GaSbO 7–Gd 2Zr 2O 7 solid solutions with structural disorder

A new pyrochlore solid solution family, Gd 2GaSbO 7–Gd 2Zr 2O 7 (Gd 2(GaSb) 1− x Zr 2 x O 7; GGSZ) exhibiting oxygen ion conduction due to structural disorder, has been investigated. In this study, the ionic conductivity and structural disorder of GGSZ were analyzed by fitting the electrical conductivity as functions of P O 2 and temperature to a defect chemical model based on oxygen Frenkel disorder. GGSZ with x=0.8 exhibited an ionic conductivity of 2.14×10 −3 S/cm at 1000°C, and an activation energy of 0.51±0.003 eV. The Frenkel disorder enthalpy was found to decrease rapidly with increasing x, dropping to ≈0 eV at x=0.8. The oxygen vacancy mobility was found to increase with x which, together with the increase in carrier density, led to a very sharp increase in ionic conductivity at higher values of x. The GGSZ system is compared with the Gd 2Ti 2O 7–Gd 2Zr 2O 7 and Gd 2Sn 2O 7–Gd 2Zr 2O 7 systems also known to exhibit a structural disorder correlated ionic conductivity.

Electrical and Ionic Conductivity of Gd‐Doped Ceria

Electrical conductivity of , as a function of temperature and oxygen partial pressure, is measured with a complex impedance method. The increase of electrical conductivity with reducing oxygen partial pressure can be described well by a model that assumes constant mobility of both oxygen vacancies and electrons, based on an ideal‐solution model of non‐stoichiometry of Gd‐doped ceria. Ionic conductivity is calculated, and its activation energy is discussed. Electronic conductivity is discussed also. © 2000 The Electrochemical Society. All rights reserved.

Influence of mobile charged defects on the dielectric non-linearity of thin ferroelectric PZT films

The capacity-voltage (C-V) characteristics of thin ferroelectric PZT films are investigated, varying the rate of change of the control voltage in a wide range. It is established that the distance between the maxima of the C-V characteristics decreases as the rate of the voltage change is decreased. This effect is caused by the decrease of the coercive field due to the spatial separation of mobile charge carriers under the action of the control field and the accumulation of charged defects in the near-electrode regions of the films. Parameters characterizing the formation of the bulk charge in the films (concentration and mobility of oxygen vacancies) are estimated. The estimations made are consistent with the literature data.

Niobium Doping and Dielectric Anomalies in Bismuth Titanate

A study of the complex permittivity in bismuth titanate was conducted to reveal the nature of an anomaly in the real part of the permittivity, which occurs below the Curie temperature. This anomaly is frequency dependent and is caused by a combination of two relaxation phenomena that appear in the imaginary part of the permittivity. One of the relaxations showed classic characteristics of an ion‐jump process. Niobium doping suppressed this relaxation and eliminated the nonferroelectric anomaly in permittivity. Niobium is proposed to affect the ion‐jump relaxation through a decrease in the concentration and possibly the mobility of oxygen vacancies.

1 : 2 Long-range ordering and defect mechanism of WO3-doped perovskite Ba(Mg1/3Ta2/3)O3

The nature of the B-site cation ordering and the associated defect process necessary to stabilize the ordered domains were investigated using the WO3-doped Ba〈Mg1/3Ta2/3〉O3 〈BMT〉 system as a typical example of Ba〈B′1/3B″2/3〉O3-type complex perovskites. It was shown that only the 1 : 2 long-range ordering of the B-site cation existed in both undoped and WO3-doped BMT perovskites. The atomic defect mechanism associated with the stoichiometric 1 : 2 long-range ordering was systematically investigated. It is concluded that the substitution of W6+ for Ta5+ in the WO3-doped BMT enhances the degree of the 1 : 2 long-range ordering and produces the positively charged W Ta ⋅ sites with a concomitant generation of tantalum vacancies 〈VTa″″′〉 and mobile oxygen vacancies 〈V⋅⋅ O〉 for the ionic charge compensation.

Atomic Scale Characterization of Oxygen Vacancy Segregation at SrTiO3 Grain Boundaries

AbstractWe have examined the structure, composition and bonding at an un-doped 58° [001] tilt grain-boundary in SrTiO3 in order to investigate the control that the grain boundary exerts over the bulk properties. Room temperature and in-situ heating experiments show that there is a segregation of oxygen vacancies to the grain boundary that is increased at elevated temperatures and is independent of the cation arrangement. These measurements indicate that the widely observed electronic properties of grain boundaries may be due to an excess of mobile oxygen vacancies that cause a highly doped n-type region in the close proximity ( ≍ 1 unit cell) to the boundary. These results are shown to be consistent with both theoretical models and lower resolution chemical analysis.