Modeling oxygen diffusion in barium titanate using molecular dynamics: Comparison between Mg and Sc dopants

The growth kinetics of thin anodic WO 3 films investigated by electrochemical impedance spectroscopy

Diffusion coefficients of oxygen in Nd 2CuO 4−δ

ABSTRACTExperimental results from electrical conductivity relaxation (ECR) measurements on SrFeO3-x are described. Values of Dchem and kchem were obtained by monitoring the variation of the time dependence of the electrical conductivity after an abrupt change in the oxygen partial pressure. Values for the oxygen ion and vacancy diffusion coefficients were calculated from the measured thermodynamic factors and compared with those of other compositions in the series La1-xSrxFeO3-x. The surface exchange coefficients, kchem and kex were also determined.

Spark plasma sintered Bi1−xCaxFe1−yTiyO3−δ (BCFTO) (x = y = 0.05 and 0.1) nanoparticle ceramics are studied for photoconductivity properties. As-prepared (AP) BCFTO hosts a large concentration of grain boundary (GB) oxygen vacancies (OV), whereas air annealed (AA) BCFTO have significantly suppressed GB OV. X-ray absorption near edge spectroscopy study confirms that Fe and Ti remain in 3+ and 4+ oxidation states, respectively. Thus, lattice OV created when only Ca2+ is substituted in BiFeO3 are charge compensated in Ca and Ti codoped BiFeO3. This ascertains that BCFTO is devoid of lattice OV. Photoconductivity studies show four orders of more photocurrent arising from GB OV contributions in BCFTO-AP compared to that in BCFTO-AA samples. A large increase in the activation energy for the AA samples (0.4 eV to 1.6 eV) compared to that for the AP samples (0.06 eV to 0.5 eV) is obtained from ln ω vs 1/T Arrhenius plots. This further substantiates the suppression of GB OV resulting in poor photoconductivity. Diffuse band edges observed in Kubelka-Munk plots of BCFTO-AP samples are a consequence of OV defect states occupying the bulk bandgap. In the absence of OV defect states, band edge becomes sharper. Density functional theory (DFT) calculations further support the experimental observations. DFT study shows that the presence of Ca and Ti does not enhance the photocurrent as these codopants do not produce mid-bandgap states. The mid-bandgap defect states are attributed only to the unsaturated bonds and OV at the GB in BCFTO. These studies manifest a critical role of OV residing at the GB in tuning the photoconductivity and, hence, the photoresponse of BCFTO.

The oxygen permeation flux through the perovskite membrane made of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) has been measured as a function of both the temperature and the oxygen pressure gradient across the membrane. A simple model for the surface exchange fluxes was used to understand the mechanism of the oxygen transport. The limiting step of the oxygen transport was found to be the bulk diffusion for the BSCF perovskite at temperatures above 700 oC. Furthermore, the oxygen vacancy diffusion coefficient (Dv) can be deduced from the dependence of the oxygen permeation flux on the oxygen pressure gradient. From permeation measurement on the BSCF membrane tube, Dv is between 2.82×10-9 m2/s at 900 oC and 0.82×10-9 m2/s at 700 oC.

Oxygen permeation study in a tubular Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3- δ oxygen permeable membrane

Oxygen permeation rates are reported for 2 mm thick La1-xSrxCoO3-δ(0 ≤x≤0.8) in the temperature range 700 ≤T1100 °C and oxygen partial pressure range 7.0 x 103- 1.0 x 105Pa. With the exception ofx= 0.6 permeation rates increase with an increase in Sr doping level. The oxygen permeation can be described by Wagner theory. An anomaly is observed in the oxygen permeation between 750 - 775 °C, which is attributed to an order-disorder transition. It is further shown that the partial pressure at the low partial pressure side of the membrane has an important influence on the permeation rate. The (calculated) ionic conductivities and oxygen vacancy diffusion coefficients are among the highest reported in literature.

La2Mo2O9-based oxide-ion conductors have attracted more and more attention since the high ionic conductivity was reported in 2000 by Lacorre et al. in this kind of material. In this paper the recent investigation on the novel La2Mo2O9-based oxide-ion conductors is reviewed. It is focused on the structure and phase transition, oxide-ion conductivity, mechanism of oxygen vacancy diffusion, chemical stability and thermal expansion coefficient in the pure and doped La2Mo2O9 samples. It is pointed out that the La2Mo2O9-based oxide-ion conductors could be possibly applied in the fields such as solid oxide fuel cells, oxygen sensors and oxygen permeable membranes.

Much recent attention has focused on the voltage-driven reversible topotactic transformation between the ferromagnetic metallic perovskite (P) SrCoO3-δ and oxygen-vacancy-ordered antiferromagnetic insulating brownmillerite (BM) SrCoO2.5. This is emerging as a paradigmatic example of the power of electrochemical gating (using, e.g., ionic liquids/gels), the wide modulation of electronic, magnetic, and optical properties generating clear application potential. SrCoO3 films are challenging with respect to stability, however, and there has been little exploration of alternate compositions. Here, we present the first study of ion-gel-gating-induced P → BM transformations across almost the entire La1-xSrxCoO3 phase diagram (0 ≤ x ≤ 0.70), under both tensile and compressive epitaxial strain. Electronic transport, magnetometry, and operando synchrotron X-ray diffraction establish that voltage-induced P → BM transformations are possible at essentially all x, including x ≤ 0.50, where both P and BM phases are highly stable. Under small compressive strain, the transformation threshold voltage decreases from approximately +2.7 V at x = 0 to negligible at x = 0.70. Both larger compressive strain and tensile strain induce further threshold voltage lowering, particularly at low x. The P → BM threshold voltage is thus tunable, via both composition and strain. At x = 0.50, voltage-controlled ferromagnetism, transport, and optical transmittance are then demonstrated, achieving Curie temperature and resistivity modulations of ∼220 K and at least 5 orders of magnitude, respectively, and enabling estimation of the voltage-dependent Co valence. The results are analyzed in the context of doping- and strain-dependent oxygen vacancy formation energies and diffusion coefficients, establishing that it is thermodynamic factors, not kinetics, that underpin the decrease in the threshold voltage with x, that is, with increasing formal Co valence. These findings substantially advance the practical and mechanistic understanding of this voltage-driven transformation, with fundamental and technological implications.

Barium titanate, which is characterized by a positive temperature coefficient of resistance (PTCR), is widely used in practice. At the same time, it is unknown why only a small percentage of the introduced donor dopant takes part in the formation of PTCR effect, which phases appear at grain boundaries, how the introduced acceptor dopants affect the properties of grains. Elucidation of the above questions is of considerable scientific and practical interest. It has been shown that the phases Bа6Ti17O40 and Y2Ti2O7 precipitate on grains of barium titanate doped with donor dopant (yttrium). We identified paramagnetic impurities (iron, manganese, chromium) in starting reagents. These impurities can occupy titanium sites. Therefore, the part of the donor dopant that is spent on the charge exchange of acceptor dopants does not participate in the charge exchange of titanium Ti4+ → Ti3+, which is responsible for the appearance of PTCR effect in barium titanate. It has been found that an extra acceptor dopant (manganese) is distributed mainly at grain boundaries and in the grain outer layer. It has been shown that manganese ions introduced additionally (as acceptor dopants) increase the potential barrier at grain boundaries and form a high-resistance outer layer in PTCR ceramics. The resistance of grains, outer layers, and grain boundaries as a function of the manganese content has been investigated.

While thermally stimulated depolarization currents, sometimes in conjunction with the thermal slicing technique, are often used to determine the activation energies and relaxation times of the processes occurring in a material, little attention has been paid to the question of whether the results do represent the true properties of the system. In this paper, a theoretical analysis and the results of calculations are presented for systems whose isothermal behaviour can be described in terms of a distribution of relaxation times (DRT). It is found that for a DRT with a common activation energy E0, the activation energy EBO derived from the Bucci or BFG relaxation time is often less than E0, white if the DRT is associated with a distribution of activation energies the prefactor of the BFG relaxation time often differs from its true value. Simple criteria are proposed for establishing whether the parameters derived from any given set of measurements do in fact correspond to those of the system, and for distinguishing between a DRT with a distribution of activation energies and one with a single activation energy.

The diffusions of He atom and small He clusters in Ti at different temperatures are simulated by molecular dynamics. The prefactors and the activation energies of diffusion coefficients are calculated. It can be concluded that the diffusion is anisotropic. Simulations show that prefactors of diffusion coefficients are different from those of He species diffusing in different directions, but the activation energies are the same. The result demonstrates that it is insufficient to predict the diffusion behavior of He in metal when only the energy barrier in static lattice is considered. Dynamics calculations are necessary to obtain the correct prefactors. Another counterintuitive observation is that He-dimer migrates more quickly than single He atom does at room temperature. The results emphasize the importance of dynamics simulations in predicting diffusion behavior of He in metals.

The thermodynamic and kinetic properties of mono- and di-vacancy defects in cubic (para-electric) barium titanate BaTiO3 are studied by means of density-functional theory calculations. It is determined which vacancy types prevail for given thermodynamic boundary conditions. The calculations confirm the established picture that vacancies occur in their nominal charge states almost over the entire band gap. For the dominating range of the band gap the di-vacancy binding energies are constant and negative. The system, therefore, strives to achieve a state in which, under metal-rich (oxygen-rich) conditions, all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration barriers are calculated for mono-vacancies in different charge states. As oxygen vacancies are found to readily migrate at typical growth temperatures, di-vacancies can be formed at ease. The key results of the present study with respect to the thermodynamic behavior of mono- and di-vacancies influence the initial defect distribution in the ferroelectric phases and therefore the conditions for aging.

Bipolar resistive switching and synaptic characteristics modulation at sub-μA current level using novel Ni/SiOx/W cross-point structure

Proton conductors are attractive materials with a wide range of potential applications such as proton-conducting fuel cells (PCFCs). The conventional strategy to enhance the proton conductivity is acceptor doping into oxides without oxygen vacancies. However, the acceptor doping results in proton trapping near dopants, leading to the high apparent activation energy and low proton conductivity at intermediate and low temperatures. The hypothetical cubic perovskite BaScO2.5 may have intrinsic oxygen vacancies without the acceptor doping. Herein, we report that the cubic perovskite-type BaSc0.8Mo0.2O2.8 stabilized by Mo donor-doing into BaScO2.5 exhibits high proton conductivity within the ‘Norby gap’ (e.g., 0.01 S cm−1 at 320 °C) and high chemical stability under oxidizing, reducing and CO2 atmospheres. The high proton conductivity of BaSc0.8Mo0.2O2.8 at intermediate and low temperatures is attributable to high proton concentration, high proton mobility due to reduced proton trapping, and three-dimensional proton diffusion in the cubic perovskite stabilized by the Mo-doping into BaScO2.5. The donor doping into the perovskite with disordered intrinsic oxygen vacancies would be a viable strategy towards high proton conductivity at intermediate and low temperatures.