Compensating Oxygen Vacancies Research Articles

The strange case of enhancing the resistivity by extended sintering time in (Bi0.5Na0.5)TiO3–BaTiO3 solid solution

With the presence of volatile elements in the composition of a ceramic material, longer sintering time presumably creates more vacancies. In the case of (Bi0.5Na0.5)TiO3–BaTiO3 lead-free piezoelectric ceramics and when batched and calcined with a Bi2O3 deficient stoichiometry, there can be a surprising increase in resistivity with increasing sintering time. It is deduced that the extended sintering time reduces the concentration of bismuth vacancies (and the ionically compensating oxygen vacancies). This is also associated with a decrease of depolarization temperature, Td, and possibly a change in dielectric polarization phenomena from normal to relaxor ferroelectric. These observations do not occur in pure (Bi0.5Na0.5)TiO3, suggesting that they are actually facilitated by the Ba2+ content in the solid solution BNT−BT. Microstructural observations indicate the formation of BaTi2O5 secondary phase which considerably grows as sintering goes on. With more Ti4+ being incorporated to the secondary phase, the stoichiometry of the solid solution can be restored, so is the resistivity and the observed trends in decreased Td.

Impact of post-nitridation annealing in CO2 ambient on threshold voltage stability in 4H-SiC metal-oxide-semiconductor field-effect transistors

The combination of NO annealing and subsequent post-nitridation annealing (PNA) in CO2 ambient for SiO2/SiC structures has been demonstrated to be effective in obtaining both high channel mobility and superior threshold voltage stability in SiC-based metal-oxide-semiconductor field-effect transistors (MOSFETs). N atoms on the SiO2 side of the SiO2/SiC interface incorporated by NO annealing, which are plausible causes of charge trapping sites, could be selectively removed by CO2-PNA at 1300 °C without oxidizing the SiC. CO2-PNA was also effective in compensating oxygen vacancies in SiO2, resulting in high immunity against both positive and negative bias-temperature stresses.

Defects and polaronic electron transport in Fe2WO6.

We report the synthesis of phase pure Fe2WO6 and its structural characterization by high quality synchrotron X-ray powder diffraction, followed by studies of electric and thermoelectric properties as a function of temperature (200-950 °C) and pO2 (1-10-3 bar). The results are shown to be in accordance with a defect chemical model comprising formation of oxygen vacancies and charge compensating electrons at high temperatures. The standard enthalpy and entropy of formation of an oxygen vacancy and two electrons in Fe2WO6 are found to be 113(5) kJ mol-1 and 41(5) J mol-1 K-1, respectively. Electrons residing as Fe2+ in the Fe3+ host structure act as charge carriers in a small polaron conducting manner. A freezing-in of oxygen vacancies below approximately 650 °C results in a region of constant charge carrier concentration, corresponding to an iron site fraction of XFe2+≅ 0.03. By decoupling of mobility from conductivity, we find a polaron hopping activation energy of 0.34(1) eV and a charge mobility pre-exponential u0 = 400(50) cm2 kV-1 s-1. We report thermal conductivity for the first time for Fe2WO6. The relatively high conductivity, large negative Seebeck coefficient and low thermal conductivity make Fe2WO6 an interesting candidate as an n-type thermoelectric in air, for which we report a maximum zT of 0.027 at 900 °C.

Control of electrochemical reduction behavior in nonequilibrium Al-doped TiO2 thin films

To develop resistive switches using TiO2, the control of their switching voltage is a crucial issue. This study shows how acceptor and compensating oxygen vacancies modify the resistive switching behavior from both the experimental and theoretical points of view. Nonequilibrium TiO2 thin films doped with acceptor Al with oxygen vacancies were prepared by pulsed laser deposition. Al doping results in a clear reduction in the switching voltage when the TiO2 thin films are operated as a unipolar-type switch but not when they are operated as bipolar-type switches. Density functional theory calculations revealed that the doped Al and compensating oxygen vacancies ([Al′Ti]=2[VO⋅⋅]) promote the formation of oxygen vacancies compensated by electrons (2[VO⋅⋅]=n) due to the defects association of 2Al′Ti−2VO⋅⋅−2e′. This defect cluster functions like the formation center of conductive nanofilaments for the electrochemical reduction. It was concluded that by promoting the formation of oxygen vacancies compensated by electrons, Al doping results in a decrease in energy consumption of the unipolar-type resistive switching.

Re- and Cs-Copromoted Silver Catalysts for Ethylene Epoxidation: A Theoretical Study

The effect of Re- and Cs-copromotion of Ag catalysts for ethylene epoxidation is studied computationally using density functional theory approach. The models reflecting Re involvement in the form of oxyanion and as a component of Ag-Re alloys are considered. The synergistic effect of Cs and Re is caused by the formation of the CsReOx species, with Cs stabilizing the Re oxyanion to promote ethylene epoxidation over the Ag catalysts. The role of Re is proposed to be related to altering the electron density distribution, compensating oxygen vacancies, covering non-selective sites, and providing the optimal retention time for initial compounds, products, and oxametallacycles on the catalyst surface.

Double doping effect on ferroelectric and leakage current behavior of Pb(Zr0.52Ti0.48)O3 thin film

To study the double doping effect on PZT, following thin films were prepared with stoichiometric compositions of Pb(Zr0·52Ti0·48)O3(PZT), Pb0·92La0·08(Zr0·52Ti0·48)0.98O3(PLZT) and Pb0·92(La0.5Yb0.5)0·08(Zr0·52Ti0·48)0.98O3(PLYZT) by chemical solution deposition method. XRD results revealed tetragonal crystal structure of all the three thin film compositions with significant changes in lattice parameters for La3+and La3+-Yb3+ doped PZT film. Raman study confirmed XRD results. The influence of structural changes on ferroelectric behavior was investigated by studying ferroelectric switching current, remanent polarization and coercive field. An increase in ferroelectric switching current and decrease in coercive field was observed in double doped (La3+ and Yb3+) PZT thin films than the pure one. This result indicates easy domain switching in doped PZT thin films. The double doping has reduced leakage current in PZT by compensating oxygen vacancy and thereby improved the polarization fatigue.

Effect of Cu doping on the anatase-to-rutile phase transition in TiO2 photocatalysts: Theory and experiments

This paper shows that incorporation of Cu inhibits the anatase to rutile phase transition at temperatures above 500 °C. The control sample, with 0% Cu contained 34.3% anatase at 600 °C and transitioned to 100% rutile by 650 °C. All copper doped samples maintained 100% anatase up to 600 °C. With 2% Cu doping, anatase fully transformed to rutile at 650 °C, at higher Cu contents of 4% & 8% mixed phased samples, with 27.3% anatase and 74.3% anatase respectively, are present at 650 °C. All samples had fully transformed to rutile by 700 °C. 0%, 4% and 8% Cu were evaluated for photocatalytic degradation of 1, 4 dioxane. Without any catalyst, 15.8% of the 1,4 dioxane degraded upon irradiation with light for 4 h. Cu doped TiO2 shows poor photocatalytic degradation ability compared to the control samples. Density functional theory (DFT) studies of Cu-doped rutile and anatase show formation of charge compensating oxygen vacancies and a Cu2+ oxidation state. Reduction of Cu2+ to Cu+ and Ti4+ to Ti3+ was detected by XPS after being calcined to 650–700 °C. This reduction was also shown in DFT studies. Cu 3d states are present in the valence to conduction band energy gap upon doping. We suggest that the poor photocatalytic activity of Cu-doped TiO2, despite the high anatase content, arises from the charge recombination at defect sites that result from incorporation of copper into TiO2.

Re- and Cs-copromoted silver catalysts for ethylene epoxidation: A theoretical study

The effect of Re and Cs co-promotion of Ag catalysts for ethylene epoxidation is studied computationally using density functional theory approach. The models reflecting Re involvement in the form of oxyanion and as a component of Ag—Re alloys are considered. The synergistic effect of Cs and Re is caused by the formation of the CsReOx species, with Cs stabilizing the Re oxyanion to promote ethylene epoxidation over Ag catalysts. The role of Re is proposed to be related to altering the electron density distribution, compensating oxygen vacancies, co­vering non-selective sites, and providing the optimal retention time for initial compounds, pro­ducts, and oxametallacycles on the catalyst surface.

Valence and spin fluctuations in the Mn-doped ferroelectric BaTiO3

We study Mn substitution for Ti in BaTiO3 with and without compensating oxygen vacancies using density functional theory (DFT) in combination with dynamical mean field theory (DMFT). We find strong charge and spin fluctuations. Without compensating oxygen vacancies, the ground state is found to be a quantum superposition of two distinct atomic valences, 3d4 and 3d5. Introducing a compensating oxygen vacancy at a neighboring site reduces both charge and spin fluctuations due to the reduction of electron hopping from Mn to its ligands. As a consequence, valence fluctuations are reduced, and is closely fixed to the high spin 3d5 state. Here we show that inclusion of charge and spin fluctuations is necessary to obtain an accurate ground state of transition metal doped ferroelectrics.

Colossal Dielectric Permittivity in (Nb+Al) Codoped Rutile TiO2 Ceramics: Compositional Gradient and Local Structure

(Nb+Al) codoped rutile TiO2 ceramics with nominal composition Ti4+0.995Nb5+0.005yAl3+0.005zO2, z = (4–5y)/3 and y = 0.4, 0.5, 0.6, 0.7, and Ti4+0.90Nb5+0.05Al3+0.05O2 have been synthesized. The resultant samples in ceramic pellet form exhibit a colossal dielectric permittivity (>∼104) with an acceptably low dielectric loss (∼10–1) after optimization of the processing conditions. It is found that a conventional surface barrier layer capacitor (SBLC) effect, while it contributes significantly to the observed colossal permittivity, is not the dominant effect. Rather, there exists a subtle chemical compositional gradient inward from the pellet surface, involving the concentration of Ti3+ cations gradually increasing from zero at the surface without the introduction of any charge compensating oxygen vacancies. Instead, well-defined Gr ± 1/3[011]* satellite reflections with the modulation wave-vector q = 1/3[011]r* and sharp diffuse streaking running along the Gr ± e[011]* direction from electron diffraction su...

Effects of manganese addition on the electronic structure ofBaTiO3

Mn is used as a dopant to improve the electromechanical properties of perovskite oxides. We investigate the effects of Mn defects and associated vacancies on the electronic and atomic properties of ${\mathrm{BaTiO}}_{3}$. Using density functional theory (DFT) and $\mathrm{DFT}+U$ we investigate the equilibrium geometry and electronic properties of the Mn ion on $A$ or $B$ sites and with compensating oxygen vacancies. We study the change in the oxidation state of Mn in response to local environment changes, such as the presence of oxygen vacancies.

Chemical Descriptors of Yttria-Stabilized Zirconia at Low Defect Concentration: An ab Initio Study.

Yttria-stabilized zirconia (YSZ) is an important oxide ion conductor with applications in solid oxide fuel cells (SOFCs) and oxygen sensing devices. Doping the cubic phase of zirconia (c-ZrO2) with yttria (Y2O3) is isoelectronic, as two Zr(4+) ions are replaced by two Y(3+) ions, plus a charge compensating oxygen vacancy (Ovac). Typical doping concentrations include 3, 8, 10, and 12 mol %. For these concentrations, and all below 40 mol %, no phase with long-range order has been observed in either X-ray or neutron diffraction experiments. The prediction of local defect structure and the interaction between defects is therefore of great interest. This has not been possible to date as the number of possible defect topologies is very large and to perform reliable total energy calculations for all of them would be prohibitively expensive. Previous theoretical studies have only considered a selection of representative structures. In this study, a comprehensive search for low-energy defect structures using a combined classical modeling and density functional theory approach is used to identify the low-energy isolated defect structures at the dilute limit, 3.2 mol %. Through analysis of energetics computed using the best available Born-Mayer-Huggins empirical potential model, a point charge model, DFT, and a local strain energy estimated in the harmonic approximation, the main chemical and physical descriptors that correlate to the low-energy DFT structures are discussed. It is found that the empirical potential model reproduces a general trend of increasing DFT energetics across a series of locally strain relaxed structures but is unreliable both in predicting some incorrect low-energy structures and in finding some metastable structures to be unstable. A better predictor of low-energy defect structures is found to be the total electrostatic energy of a simple point charge model calculated at the unrelaxed geometries of the defects. In addition, the strain relaxation energy is estimated effectively in the harmonic approximation to the imaginary phonon modes of undoped c-ZrO2 but is found to be unimportant in determining the low-energy defect structures. These results allow us to propose a set of easily computed descriptors that can be used to identify the low-energy YSZ defect structures, negating the combinatorial complexity and number of defect structures that need to be considered.

Cationic surface segregation in doped LaMnO3

The surface cation chemistry in (La, A)MnO3 (A = Ca, Sr and Ba) is investigated using first-principles thermodynamics. We find that, all three dopants tend to segregate to the surface over a wide range of T–\( p_{{{\text{O}}_{2} }} \) conditions and the tendency for segregation increases with the increase in the dopant cationic size. Moving toward the low oxygen pressure, dopants prefer to remain in the surface regions accompanied by the appropriate number of charge compensating oxygen vacancies. The situation when dopants remain in the bulk regions tends to occur close to the thermodynamic conditions that also favor the decomposition of LaMnO3. The present work serves as an important step toward understanding of factors governing the cationic surface segregation in doped LaMnO3 and opens a pathway to study other important chemical environments (such as water- and CO2-containing air) which are crucially given the fact that the ‘real-world’ air enhances cationic segregation.

X-ray absorption spectroscopy based investigation of local structure in yttria stabilized zirconia nanoparticles generated by laser evaporation method: Effect of pulsed vs CW mode of laser operation

Two order higher ionic conductivity was reported by us (J. Khare et al., Ceram. Int. 40 (2014) 14677) in pellet made from yttria stabilized zirconia nanoparticles grown under pulsed mode in comparison with pellets made from yttria stabilized zirconia nanoparticles grown under CW mode of CO2 laser based vaporization method. To understand possible cause of difference in conductivity, the local structure around Zr+4 ion and Y+3 ion has been investigated using X-ray absorption fine structure spectroscopy. The local structure around Zr has been found to depend on the mode of laser vaporization for nanoparticles generation. In case of pulsed mode of vaporization method the charge compensating oxygen vacancies created by the substitution of Y are preferentially forming dipole type defect configuration leading to high ionic conductivity. While formation of immobile tripole type defects are responsible for low ionic conductivity in pellets made from nanoparticles generated under CW mode of laser vaporization.

Suppressing the atmosphere-induced performance instability of solution-grown zinc oxide-nanowire ultra-violet photodetector by hydrothermal treatment in water

The tunable recovery time and photosensitivity are achieved by controlling the concentration of oxygen vacancies on the surface of ZnO-nanowire (ZnO-NW) photodetector (PD) through a hydrothermal treatment in pure water. Performance instability originates from the interaction of oxygen vacancies near ZnO surface with the atmosphere during service. A novel conception of decreasing the concentration of oxygen vacancy for decreasing photoresponse time is proposed and achieved by a hydrothermal treatment. The rationale can be widely used for compensating oxygen vacancies of nanostructures with complex morphology at low temperature.

Dielectric and piezoelectric properties of low-temperature sintered lead zirconate titanate ceramics with 0.78PbO-0.22CuO flux addition

Liquid phase sintering of lead zirconate titanate (abbreviated as PZT) ceramics with a 0.78 PbO-0.22 CuO (10:1 in weight ratio) flux was investigated. A small amount sintering flux consisting of a stoichiometrically mixed oxide of PbO-CuO with a eutectic composition successfully accelerated densification of the PZT ceramics far below the conventional sintering temperature. With the 3 wt% flux additions, full densifications of the PZT ceramics were achieved at temperatures as low as 825 °C. The results obtained in the crystal structure, microstructure, and electrical property analyses suggest that the Cu2+ impurity ion substitutes as an acceptor center for the perovskite B-site without forming isolated secondary phases. Defect associates of the Cu2+ impurities and charge compensating oxygen vacancies appear to affect dielectric and piezoelectric properties of the sintered PZT samples in both positive and negative ways by forming defect dipoles. When the Cu2+ content is small, slightly improved dielectric and piezoelectric performances were achieved, though ferroelectric relaxation was obviously developed at the higher Cu2+ contents. For the PZT samples sintered at 825 °C with 3 wt% flux addition, relative dielectric permittivity was 2200 and dielectric loss was less than 2%. Remnant polarization, coercive fields, and piezoelectric coefficient (d33) were 32 μC/cm2, 8.9 kV/cm and 373 pC/N, respectively.

Ce3+ Ions Determine Redox-Dependent Anti-apoptotic Effect of Cerium Oxide Nanoparticles

Antioxidant therapy is the novel frontier to prevent and treat an impressive series of severe human diseases, and the search for adequate antioxidant drugs is fervent. Cerium oxide nanoparticles (nanoceria) are redox-active owing to the coexistence of Ce(3+) and Ce(4+) oxidation states and to the fact that Ce(3+) defects, and the compensating oxygen vacancies, are more abundant at the surface. Nanoceria particles exert outstanding antioxidant effects in vivo acting as well-tolerated anti-age and anti-inflammatory agents, potentially being innovative therapeutic tools. However, the biological antioxidant mechanisms are still unclear. Here, the analysis on two leukocyte cell lines undergoing apoptosis via redox-dependent or independent mechanisms revealed that the intracellular antioxidant effect is the direct cause of the anti-apoptotic and prosurvival effects of nanoceria. Doping with increasing concentrations of Sm(3+), which progressively decreased Ce(3+) without affecting oxygen vacancies, blunted these effects, demonstrating that Ce(3+)/Ce(4+) redox reactions are responsible for the outstanding biological properties of nanoceria.

Charge Compensation and Ce3+Formation in Trivalent Doping of the CeO2(110) Surface: The Key Role of Dopant Ionic Radius

In this paper, we use density functional theory corrected for on-site Coulomb interactions (DFT + U) and hybrid DFT (HSE06 functional) to study the defects formed when the ceria (110) surface is doped with a series of trivalent dopants, namely, Al3+, Sc3+, Y3+, and In3+. Using the hybrid DFT HSE06 exchange-correlation functional as a benchmark, we show that doping the (110) surface with a single trivalent ion leads to formation of a localized MCe/ + OO• (M = the 3+ dopant), O− hole state, confirming the description found with DFT + U. We use DFT + U to investigate the energetics of dopant compensation through formation of the 2MCe′ +VO·· defect, that is, compensation of two dopants with an oxygen vacancy. In conjunction with earlier work on La-doped CeO2, we find that the stability of the compensating anion vacancy depends on the dopant ionic radius. For Al3+, which has the smallest ionic radius, and Sc3+ and In3+, with intermediate ionic radii, formation of a compensating oxygen vacancy is stable. On the...

Frequency and temperature dependent dielectric and conductivity behavior of 0.95(K 0.5Na 0.5)NbO 3–0.05BaTiO 3 ceramic

Dielectric and conductivity measurements were carried out on the 0.95(K 0.5Na 0.5)NbO 3–0.05BaTiO 3 (KNN–5BT) ceramic both as a function of temperature (∼350–850 K) and frequency (40 Hz to 100 MHz). A high-temperature dielectric relaxation above Curie temperature (∼590 K) was observed and analyzed with the Cole–Cole function. Frequency dependent conductivity was analyzed with an augmented Jonscher relation and exhibited a universal conductivity behavior. The activation energy of dielectric relaxation was estimated to be 1.09 eV. It could be attributed to the thermal motion of double ionized oxygen vacancy or to the formation of defect dipoles between the acceptor ion and charge compensating oxygen vacancies. The mechanism for both the high-temperature dielectric relaxation and the frequency dependent conductivity was proposed based on a possible mode of incorporation of Ba 2+ and Ti 4+ ions into the KNN lattice sites.

Defect structure and materials “hardening” in Fe2O3-doped [Bi0.5Na0.5]TiO3 ferroelectrics

The effect of Fe2O3-doping on the defect structure and piezoelectric properties of [Bi0.5Na0.5]TiO3 ceramics is studied by means of electron paramagnetic resonance spectroscopy. The results show that the Fe3+ functional centers are incorporated at the perovskite B-site, and clearly can be attributed to the formation of (FeTi′–VO••)• defect complexes with charge compensating oxygen vacancies. Correspondingly, they act as acceptors which promotes materials “hardening.”