Mobile Vacancies Research Articles

Ion-Pair Engineering-Induced High Piezoelectricity in Bi4Ti3O12-Based High-Temperature Piezoceramics.

High-temperature piezoceramics are highly desirable for numerous technological applications ranging from the aerospace industry to the nuclear power sector. However, it is a grand challenge to achieve excellent piezoelectricity and high Curie temperature (Tc) simultaneously because there is a contradiction between the large piezoelectric coefficient and high Curie temperature in piezoceramics. Here, we provide a perspective via B-site ion-pair engineering to design piezoceramics with high performance for high-temperature applications. In bismuth-layered Bi4Ti2.93(Zn1/3Nb2/3)0.07O12 ceramics, high piezoelectricity of d33 = 30.5 pC/N (more than four times higher than that of pure Bi4Ti3O12 (d33 = 7.3 pC/N) ceramics) in conjunction with excellent thermal stability, high Curie temperature Tc = 657 °C, and large dc resistivity of ρ = 1.24 × 107 Ω·cm at 500 °C (three orders of magnitude larger than that of the pure Bi4Ti3O12 ceramics) are achieved by B-site Nb5+-Zn2+-Nb5+ ion-pair engineering. Excellent piezoelectricity is ascribed to sufficient orientation of the fine lamellar ferroelectric domain with the introduction of Nb5+-Zn2+-Nb5+ ion-pairs. The good temperature stability of d33 originates from the stability of the crystal structure and the robustness of the oriented ferroelectric domain. The significantly improved resistivity is due to the restricted mobility of oxygen vacancies. This work presents a brand-new technique for achieving high-temperature piezoceramics with high performance by B-site ion-pair engineering.

Characteristic mechanism for fast [formula omitted] conduction in [formula omitted

Solid hydrides are a new class of ionic conductors promising for future applications for hydrogen-carrier, catalysis, and energy conversion. The record-high H− conductivity reported for LaH2.5O0.25 is caused by extraordinarily large pre-exponential factor, which cannot be justified within any simple diffusion picture. To reveal H− dynamics in LaH2.5O0.25 on atomistic level, we developed DFT-based neural-network-potential for large-scale molecular dynamics (MD) simulation and discovered the H− diffusion mechanism: the stoichiometric vacancies are ordered and almost immobile, but they can be rarely kicked-out and become fast mobile vacancies that migrate in a coupled manner with 3H− ring-like rotations. The MD and DFT phonon analysis reveal that the mobile vacancy formation gives rise to an extremely large entropy leading to the anomalously large pre-exponential factor, and that the anharmonicity of H− vibrations causes the large pre-exponential factor. A design concept for faster H− conductor is proposed based on the present finding.

Controlling the A-site deficiency and oxygen vacancies by donor-doping in pre-reductive-sintered thermoelectric SrTiO3 ceramics

Oxygen vacancies are intrinsic defects in ABO3 perovskite structures, and the reductive sintering of SrTiO3 ceramics combined with donor-doping can create mobile carriers and/or vacancies at A-site to improve their thermoelectric performance. By scanning and transmission electron microscopy analyses, A-site deficiency was detected in undoped and donor-doped SrTiO3 ceramics, reaching 19% of A-site vacancies by co-doping La, Ce and Nb respectively into A- and B-sites. Titania with mixed Ti4+/Ti3+ cations were commonly detected as intergranular phases of Ti4+1−mTi3+mO2−m/2, which amounts to 17 vol.% in the undoped SrTiO3, thus revealing their inheritance from the pre-reductive nano-powders via liquid-phase sintering. Oxygen and A-site vacancies are both detectable by cathodoluminescence analysis from either the nano-powders or sintered phases, which reveal that mixed-donor-doping creates more vacancies at A-site, together with more oxygen vacancies to form Schottky pairs. The combination of pre-reductive sintering and mixed-donor-doping creates abundant vacancies as electronic defects in SrTiO3, while carrier transport were not hindered through the grain boundaries which enables zT value of an oxide to exceed 0.38 at 1000 K.

Relationship between ion vacancy mobility and hysteresis of perovskite solar cells

The relation between ion vacancy mobility and hysteresis of perovskite solar cells (PSCs) is investigated by computer simulation. Due to the ion vacancy migration, the hysteresis in PSCs strongly depends on the scan rate of J-V sweeps. The hysteresis only occurs in certain regime of scan rate. We calculate the evolution of ionic charge density stored within the Debye layers to discuss the process of ion vacancy migration and the variation of ion vacancy accumulation during the J-V sweeps. Based on this discussion, we give the physical picture of hysteresis. We discover that the regime of scan rate where hysteresis shows up moves towards higher value with increasing mobility of ion vacancy.

Defect dipole induced improved electrocaloric effect in modified NBT-6BT lead-free ceramics

The Rietveld refinement of the polycrystalline powders of 1% Fe and Mn-doped (Na0.5Bi0.5)0.94Ba0.06Ti0.98V0.02O3 at the Ti-site confirmed a single rhombohedral (R3c) phase. The bandgap, (Eg) was affected by the anti-phase octahedral tilt angle and the spin-orbit splitting energy of Ti4+2p3/2 and Ti4+2p1/2 states. The decrease in Bi loss and increase in the binding energy of Ba due to Fe/Mn doping has been correlated to the strengthening of Bi-O and Ba-O bonds which was revealed from the XPS studies thereby further related to the average A-O bond length from structural studies. Hence, a reduction of oxygen vacancy (VO) for the doped samples has been justified. A significant improvement of the dielectric constant, relaxation time (τ0), and the decrease in conductivity due to doping was revealed from the frequency-dependent (10 Hz-1 MHz) dielectric measurement study. The conduction and relaxation process are dominated by the short-range movement of defects. The activation energy (Ea~1 eV) revealed that there is a presence of double-ionized VOs. The ECE study showed a significant enhancement of the changes in entropy, ΔS, and the adiabatic temperature difference, ΔT, due to doping, with ΔT being highest in the Fe-doped sample. Such improvement of dielectric and ECE properties was confirmed due to the reduction of the mobility of oxygen vacancy because of the formation(MnTi′′−Vo••)/(MnTi′−Vo••)• and FeTi′′−Vo••/(FeTi′−Vo••)• defect dipoles.

Resolving atomic diffusion in Ru(0001)−O(2×2) with spiral high-speed scanning tunneling microscopy

An intermediate state in atomic diffusion processes in the $\mathrm{O}(2\ifmmode\times\else\texttimes\fi{}2)$ layer on Ru(0001) is resolved with spiral high-speed scanning tunneling microscopy (STM). The diffusion of atomic oxygen in the adlayer has been studied by density functional theory and STM. Transition state theory proposes a migration pathway for the diffusion in the oxygen adlayer. With spiral scan geometries---a new approach to high-speed STM---the oxygen vacancy mobility on the highly covered Ru(0001) surface is determined to be in the range of 0.1 to 1 Hz. Experimental evidence for the intermediate state along the oxygen diffusion pathway is provided in real space and real time.

Effect of the sintering temperature on electrical properties of the Na0.54Bi0.46TiO2.96 compound

The Na0.54Bi0.46TiO2.96 compound was prepared by the sol-gel means and sintered at different temperatures. The effect of sintering temperature on the electrical properties and diffusion behavior of oxygen ions was systematically studied. The bulk conductivity of Na0.54Bi0.46TiO2.96 compound increases first and then decreases with the increase of sintering temperature. At the sintering temperature of 1273 K, the highest bulk conductivity of the Na0.54Bi0.46TiO2.96 sample can reach up to 3.35 × 10-4 S/cm. The total conductivity of the Na0.54Bi0.46TiO2.96 sample shows a monotonous upward trend with increasing sintering temperature. The internal friction spectrum results present that the oxygen vacancy concentration and the relaxation activation energy of the Na0.54Bi0.46TiO2.96 sample show an upward trend, but the mobility of the oxygen vacancy diffusion decreases with the sintering temperature. The Na0.54Bi0.46TiO2.96 sample sintered at 1273 K is characterized by a higher oxygen vacancy concentration and mobility, resulting in the high grain conductivity.

First-principles modeling of the highly dynamical surface structure of a MoS2 catalyst with S-vacancies.

Vacancy sites, e.g., S-vacancies, are essential for the performance of MoS2 catalysts. As earlier studies have revealed that the size and shape of the S-vacancies may affect the catalytic activity, we have studied the behavior and mobility of such vacancies on MoS2 using DFT calculations and kinetic Monte-Carlo (kMC) simulations. The diffusion barriers for the S-vacancies are highly dependent on the immediate environment: isolated single S-vacancies are found to be immobile. In contrast, small nS-vacancies formed from n = 2 to 5 neighboring S-vacancies are often highly dynamic systems that can move within a confined area. Large extended nS-vacancies are generally unstable and transform quickly into alternating patterns of S-atoms and vacancy sites. These results illustrate the importance of recognizing MoS2 (but also other catalysts) as dynamic structures when trying to tune their catalytic performances by introducing specific defect structures.

Autonomous self-optimizing defects by refining energy levels through hydrogenation in CeO2–x polymorphism: a walking mobility of oxygen vacancy with enhanced adsorption capabilities and photocatalytic stability

The proposed work on ceria photocatalyst shows facile reactivity of oxygen anions with hydrogen molecules, turning inefficient highly disordered CeO2−x into optimized disordered H-CeO2−x which sustains optimized oxygen vacancies and Ce3+ defects.

Electrical properties and relaxation behavior of Na-doped BaBiNb5O15 ceramics

The electrical performance and relaxation behavior of Na + doped BaBiNb 5 O 15 compound were investigated. The grain conductivity of the BaBi 0.96 Na 0.04 Nb 5 O 15-δ compound is 4.71 × 10 −5 S/cm at 763 K, which is an order of magnitude lower than the parent sample at the same measuring temperature. The highest total conductivity of the BaBi 0.96 Na 0.04 Nb 5 O 15-δ and BaBiNb 5 O 15 sample can reach 1.12 × 10 −4 S/cm and 4.0 × 10 −5 S/cm at 643 K, respectively, which indicates that the slight Na + doping can improve the total conductivity. A dielectric relaxation peak related to thermal activation was observed. The activation energy values are E = 0.47 eV for BaBiNb 5 O 15 sample, and E = 0.49 eV for BaBi 0.96 Na 0.04 Nb 5 O 15-δ sample, which indicates that the relaxation activation energy increases with the Bi 3+ ions partially replaced by Na + ions in the BaBiNb 5 O 15 compound. In addition, the oxygen relaxation peak position shifts to the low-frequency direction, which indicates that Na + ions doping causes the decrease of oxygen vacancy mobility. The lower oxygen vacancy mobility results in the lower grain conductivity in the BaBi 0.96 Na 0.04 Nb 5 O 15-δ sample. After aging for the different time at 773 K, the excellent thermal stability of the BaBi 0.96 Na 0.04 Nb 5 O 15-δ sample can be found.

Oxygen nonstoichiometry and magnetic properties of doped manganites La0.7Sr0.3Mn0.95Fe0.05O3-δ

In this work, solid solutions of La0.7Sr0.3Mn0.95Fe0.05O3-δ with different oxygen content were obtained by the solid-phase reactions technique. Based on the investigation of the dynamics of changes in the oxygen index (3 – δ) during heating of the samples, the formation of a stressed state in their grains as a result of annealing was established. This results in a decrease in the mobility of oxygen vacancies during the reduction of cations according to the Mn4+ + e– → Mn3+ scheme and explains the decrease of released oxygen amount with an increase of δ as well as the heating rate of the samples. When studying the magnetic properties of the obtained samples, it was found that the temperature dependence of the magnetization obeys the Curie–Weiss law and as the oxygen defficiency increases, the Curie temperature for solid solutions decreases. It was found that the particles are in a frozen ferromagnetic state when measured in the low-temperature region of the М (Т) dependence in “zero-field mode” at Т ˂ ТВ. The presence of ferromagnetism at Т ˃ ТВ leads to a magnetically ordered state, in which the resulting magnetic moment of the magnetic particle is influenced by thermal fluctuations. When considering the temperature values of the magnetization of lanthanum-strontium manganite samples, it was found that with an increase of temperature in the low-temperature region, magnetic ordering is disturbed due to the excitation of magnons with a quadratic dependence of the energy from the wave vector, the number of which increases in proportion to T3/2. This results in a decrease in the manganite magnetization. The observed temperature dependence of the magnetization measured in the “field-cooling mode” was approximated taking into account the quadratic and non-quadratic dispersion laws of the magnon spectrum.

Suppression of high-temperature dielectric loss by designed thermal annealing treatment in (Bi1/2Na1/2)TiO3 ceramics

Dielectrics for automobile applications generally require high temperature reliability. Bi1/2Na1/2TiO3-based materials are excellent high-temperature dielectric candidates with relatively large temperature-insensitive dielectric responses and a maximum dielectric permittivity temperature as high as 300 °C. However, they suffer from a high dielectric loss that increases exponentially above 200 °C due to ionic conduction from thermally activated oxygen vacancy migration. Here, we demonstrate that the impact of ionic conduction on dielectric loss can be effectively suppressed by introducing a sodium deficiency and thermal annealing An appropriate combination of both treatments elevated the temperature insensitivity of the dielectric loss up to ∼ 300 °C. A systematic investigation using impedance spectroscopy correlated with microstructure analysis revealed that the sodium deficiency and thermal annealing affected the mobile oxygen vacancy concentration differently.

Hydrothermal Synthesis of Nanocrystalline ZrO2-8Y2O3-xLn2O3 Powders (Ln = La, Gd, Nd, Sm): Crystalline Structure, Thermal and Dielectric Properties.

Zirconium dioxide (ZrO2) is one of the ceramic materials with high potential in many areas of modern technologies. ZrO2 doped with 8 wt.% (~4.5 mol%) Y2O3 is a commercial powder used for obtaining stabilized zirconia materials (8 wt.% YSZ) with high temperature resistance and good ionic conductivity. During recent years it was reported the co-doping with multiple rare earth elements has a significant influence on the thermal, mechanical and ionic conductivity of zirconia, due complex grain size segregation and enhanced oxygen vacancies mobility. Different methods have been proposed to synthesize these materials. Here, we present the hydrothermal synthesis of 8 wt.% (~4.5 mol%) YSZ co-doped with 4, 6 and 8 wt.% La2O3, Nd2O3, Sm2O3 and Gd2O3 respectively. The crystalline phases formed during their thermal treatment in a large temperature range were analyzed by X-ray diffraction. The evolution of phase composition vs. thermal treatment temperatures shows as a major trend the formation at temperatures >1000 °C of a cubic solid solutions enriched in the rare earth oxide used for co-doping as major phase. The first results on the thermal conductivities and impedance measurements on sintered pellets obtained from powders co-doped with 8 wt.% Y and 6% Ln (Ln = La, Nd, Sm and Gd) and the corresponding activation energies are presented and discussed. The lowest thermal conductivity was obtained for La co-doped 8 wt.% YSZ while the lowest activation energy for ionic conduction for Gd co-doped 8 wt.% YSZ materials.

On the implication of mobile hydrogen content on the surface reactivity of an austenitic stainless steel

The Influence of hydrogen on the corrosion behavior of an AISI 316 L austenitic stainless steel in borate buffer solution is studied after a cathodic pre-charging step. A high amount of hydrogen is absorbed and forms a concentration gradient from the surface to the bulk with a path length of about 80 µm. The surface reactivity in the passive regime after the cathodic charging and during the hydrogen desorption is investigated with a large set of electrochemical and physico-chemical technics: electrochemical polarization, EIS, Mott-Schottky analysis and XPS. This approach allows to demonstrate that surface reactivity increases drastically with hydrogen absorption. This effect is mostly due to mobile hydrogen and is mainly reversible with its desorption. Hydrogen absorption and desorption have a small impact on the passive layer composition and thickness. However, the hydrogen induces an increase in the dissolution rate, which is discussed based on the Point Defect Model. Present results suggest that the impact of hydrogen on the surface reactivity is a consequence of a strong increase in the mobility of anionic vacancy and/or hydrogen oxidation.

Effect of Ion Vacancy Migration on Open‐Circuit Voltage of Perovskite Solar Cells

Herein, the influence of ion vacancy migration on the open‐circuit voltage (Voc) of perovskite solar cells (PSCs) by computer simulation is explored. It is found that for PSCs, Voc is determined not only by the carrier recombination process, but also dominantly by the space charge effect of the ion vacancy stored within the Debye layers of the perovskite absorber layer. The migration of ion vacancy affects the ionic charge accumulation within the Debye layers, which affects the screen voltage via the space charge effect and thus determines Vocr and Vocf (open‐circuit voltage for reverse sweep and forward sweep, respectively). By tuning the parameters that determine ion vacancy mobility, the effects of ion vacancy mobility on Vocr and Vocf is investigated.

Radiation-resistant binary solid solutions via vacancy trapping on solute clusters

Additions of solute that trap vacancies slow down vacancy diffusion and promote point-defect recombination in alloys subjected to irradiation. Such selective alloying can thus help to minimize the detrimental consequences resulting from point defect fluxes. The current work investigates the effect of solute additions on the recombination rate using kinetic Monte Carlo simulations for a model alloy system, which was parametrized to Cu-Ag in the dilute limit, but with an increased solubility limit, ≈ 0.86 at.% at 300 K. As the solute concentration was increased above 0.1 at.%, solute clustering was observed and led to a strong increase in recombination rate. The beneficial effects of solute clustering on reducing vacancy mobility, and reducing solute drag, were analyzed by calculating relevant transport coefficients using the KineCluE code (Schuler et al., Computational Materials Science (2020) 172,109,191). Moreover, it was observed in the KMC simulations that large recombination rates resulted in a shift of steady-state distributions of solute cluster sizes to smaller clusters compared to equilibrium distributions in the solid solution. This shift is rationalized as resulting from the irreversible character of the interstitial-vacancy recombination reaction. These results suggest a novel irradiation effect on phase stability where a high recombination rate increases the solubility limit of a solute at steady state over its equilibrium value.

Nonlinear effects in memristors with mobile vacancies.

Because local concentration of vacancies in any material is bounded, their motion must be accompanied by nonlinear effects. Here, we look for such effects in a simple model for electric field-driven vacancy motion in a memristor, solving the corresponding nonlinear Burgers’ equation with impermeable nonlinear boundary conditions exactly. We find non-monotonous relaxation of the resistance while switching between the stable (‘on’ and ‘off’) states, and qualitatively different dependencies of switching time (under applied current) and relaxation time (under no current) on the memristor length. Our solution can serve as a useful benchmark for simulations of more complex memristor models.

Enhanced recombination suppresses the void swelling in bcc multi-component alloys

Multi-component alloys, e.g., high-entropy alloys, exhibit excellent mechanical and radiation tolerance properties, making them potential candidate materials in nuclear applications. In this work, the void swelling in a single-phase body-centered cubic (bcc) reduced activation multi-component alloy FeCrV under high dose Au-ion irradiation was investigated. It is found that FeCrV has a significantly suppressed void swelling compared to α-Fe, with voids of smaller size distributed in a narrower region. To understand the difference between these two alloys in the defect evolution and its possible effect on the void swelling, the diffusion of point defects in α-Fe and FeCrV was investigated. Combined Density Functional Theory (DFT) and kinetic Monto Carlo (kMC) method based on the environment-dependent vacancy migration energies was used to simulate the vacancy diffusion. It is shown that vacancy diffusion is enhanced, which is because vacancies prefer to migrate through the easy migration channels which have lower values of the wide migration energy distribution in multi-component alloys. Ab initio molecular dynamics (AIMD) was employed to simulate the interstitial diffusion. The results show the interstitial diffusion is sluggish in FeCrV due to the atomic traps existing that slow down interstitials’ migration. The migration process of point defects in bcc multi-component alloy shows clear chemical environment dependence. The closer mobilities of vacancies and interstitials enhance the local recombination of point defects and suppress the void swelling.

Room-Temperature Ferroelectricity in 2D Metal-Tellurium-Oxyhalide Cd7Te7Cl8O17 via Selenium-Induced Selective-Bonding Growth.

Two-dimensional (2D) ferroelectric materials have attracted increasing interest due to meeting the requirements of integration, miniaturization, and multifunction of devices. However, the exploration of intrinsic 2D ferroelectric materials is still in the early stage, for which the related reports are still limited, especially fewer ones prepared by chemical vapor deposition (CVD). Here, the ultrathin metal-tellurium-oxyhalide Cd7Te7Cl8O17 (CTCO) flakes as thin as 3.8 nm are realized via the selenium-induced selective-bonding CVD method. The growth mechanism has been confirmed by experiments and theoretical calculations, which can be ascribed to the induction of selective bonding of a hydrogen atom in H2O molecules by the introduction of selenium, leading to the generation of strong oxidants. Excitingly, switchable out-of-plane ferroelectric polarization was observed in CTCO flakes down to 6 nm at room temperature, which may be caused by mobile Cl vacancies. This work has implications for the synthesis and applications of 2D ferroelectric materials.

Modeling refractory high-entropy alloys with efficient machine-learned interatomic potentials: Defects and segregation

We develop a fast and accurate machine-learned interatomic potential for the Mo-Nb-Ta-V-W quinary system and use it to study segregation and defects in the body-centred cubic refractory high-entropy alloy MoNbTaVW. In the bulk alloy, we observe clear ordering of mainly Mo-Ta and V-W binaries at low temperatures. In damaged crystals, our simulations reveal clear segregation of vanadium, the smallest atom in the alloy, to compressed interstitial-rich regions like radiation-induced dislocation loops. Vanadium also dominates the population of single self-interstitial atoms. In contrast, due to its larger size and low surface energy, niobium segregates to spacious regions like the inner surfaces of voids. When annealing samples with supersaturated concentrations of defects, we find that in complete contrast to W, interstitial atoms in MoNbTaVW cluster to create only small ($\sim 1$ nm) experimentally invisible dislocation loops enriched by vanadium. By comparison to W, we explain this by the reduced but three-dimensional migration of interstitials, the immobility of dislocation loops, and the increased mobility of vacancies in the high-entropy alloy, which together promote defect recombination over clustering.