A-site Vacancies Research Articles

Mechanisms governing metal vacancy formation in BaTiO3 and SrTiO3

Barium titanate (BTO) and strontium titanate (STO) are often treated as close analogues, and models of defect behavior are freely transferred from one material to the other with only minor modifications. On the other hand, it is often reported that B-site vacancies (vB) are the dominant metal vacancy in BTO, while A-site vacancies (vA) dominate in STO. This difference precludes the use of analogous defect models for BTO and STO, begging the question: how similar are the defect chemistries of the two materials? Here, we address this question with density functional theory calculations using a state-of-the-art hybrid exchange correlation functional, which more accurately describes the electronic structure and charge localization than traditional functionals. We find that vA is the dominant metal vacancy in STO but that different combinations of vA, vB, and vB-vO complexes are present in BTO depending on processing and doping. Mechanistically, this occurs for two reasons: thermodynamic differences in the accessible processing conditions of the two materials and energy differences in the bonds broken when forming the vacancies. These differences can also lead to widely differing responses when impurity dopants are intentionally added. Therefore, the response of metal vacancy behavior in BTO and STO to the inclusion of niobium and iron, two typical dopants in these systems, is examined and compared.

Conductivity and stability of Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 superionic solid electrolytes

Oxide solid electrolytes Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 (LSLZT, x = 0, 0.025, 0.05) with different A-site vacancy were synthesized using conventional solid-state reaction procedure at 1300 °C. Approximately single-phase perovskite-type was obtained which was analyzed by X-ray diffraction. Moreover, scanning electron microscope, AC impedance spectroscopy and potentiostatic polarization measurement methods were adopted to study the microstructure, Li+ conductivities and electronic conductivities of the samples, respectively. Among these samples, the optimal composition of Li3/8Sr7/16-3x/2LaxZr1/4Ta3/4O3 (x = 0.025) was selected with bulk conductivity of 1.26 × 10−3 S cm−1, total conductivity of 3.30 × 10−4 S cm−1, electronic conductivity of 6.60 × 10−9 S cm−1 at 30 °C and activation energy of 0.28 eV. Furthermore, the cyclic voltammogram analysis indicated the stability of this solid electrolyte at voltages higher than 1.3 V against metallic lithium. The solid electrolyte as a separator in LiFePO4/Li half-cell showed good cycle performance that comprises 98.7% of original values at 0.2 C charge-discharge rates after 50 cycles.

Large strain with low hysteresis in Bi4Ti3O12 modified Bi1/2(Na0.82K0.18)1/2TiO3 lead-free piezoceramics

In order to obtain BNT-based ceramic system with excellent electric-field-induced strain performance for actuator applications, a novel solid solution (100-x)Bi1/2(Na0.82K0.12)1/2TiO3-xBi4Ti3O12 ((100-x)BNKT-xBiT, x = 0–12 wt%) was designed and fabricated by solid state synthesis. The microstructure and related electrical properties of this material were systematically investigated. It was found that BiT is dissolved into the lattice structure of the BNKT, leading to a greater increase in the volatilization of Na and K, thus produce more A-site vacancies compared with the undoped BNKT. The 9 wt%BiT doped sample not only has sufficient A-site vacancies to destroy the long-range ferroelectric order of the base composition but also favors the presence of extremely stable relaxor phase at room temperature. Further, the ferroelectric-to-relaxor phase transition temperature TF-R can be effectively tuned to about 0 °C, giving rise to a large signal piezoelectric coefficient d*33 of 485 pm/V with a small hysteresis η of 23%.

Ultrahigh energy density and improved discharged efficiency in bismuth sodium titanate based relaxor ferroelectrics with A-site vacancy

Novel A-site deficient (1-x-y)Bi0.5Na0.5TiO3-xBaTiO3-yBi0.2Sr0.7TiO3 lead-free relaxor ferroelectrics have been explored for energy storage property. Particularly slim polarization hysteresis (P-E) loops are observed in 0.655Bi0.5Na0.5TiO3-0.065BaTiO3-0.28Bi0.2Sr0.7□0.1TiO3 (6.5BNBT-BST) at ambient temperature resulting in a giant recoverable energy density (Wrec = 1.5 J cm−3) and extremely high efficiency (η = 90%) at 100 kV cm−1, which are closed related to the track of P-E loops. As the addition of Bi0.2Sr0.7TiO3 (BST) content, the ergodic relaxor phase becomes dominant with dynamic polar nano-regions attributed to the absence of ferroelectric domain in the relaxor phase. Furthermore, the recoverable energy density exhibits small variation in elevated temperature where the depressed polarization is compensated by almost hysteresis free loops (η up to 97%). The achievement of these characteristics in P-E loops provides that Bi0.2Sr0.7TiO3 tailoring by A-site vacancies is a potential route when designing new relaxor ferroelectrics for energy-storage applications.

Structural, morphological and optical properties of new Eu-doped and vacancied lead molybdato-tungstates

A new tetragonal, scheelite-type Pb1–3x▯xEu2x(MoO4)1–3x(WO4)3x (0 < x ≤ 0.1970 and ▯ denotes A-site vacancies) solid solution was synthesized via solid state reaction method. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results confirm the formation of single, tetragonal scheelite-type phases (space group I41/a) with the average crystallite size in the range of ∼20–100 μm. Substitution of Pb2+ with Eu3+ is relatively easy despite the large difference in ionic radii, and the formation of vacancies is necessary to compensate the excess positive charge in PbMoO4 framework. A change in lattice parameters (both a and c as well as lattice parameter ratio c/a) and progressive deformation of MoO4 and WO4 tetrahedra with increasing Eu concentration are observed. Thermal stability of Eu-doped materials strongly depends on the concentration of Eu3+ ions. The Pb1–3x▯xEu2x(MoO4)1–3x(WO4)3x solid solution for x = 0.0098 shows the highest melting point (1057 °C) which is slightly higher than that of pure PbMoO4 (1040 °C). The UV-vis diffuse reflectance spectroscopy (DRS) and the Tauc plots were used to extrapolate the optical indirect band gap (Eg) of doped materials. Eu-doped ceramics are insulators (Eg > 3 eV) and their band gap nonlinearly decreases with increasing dopant concentration.

GEOCHEMISTRY AND SECONDARY ALTERATIONS OF MICROLITE FROM ELUVIAL DEPOSITS IN THE NUMBI MINING AREA, SOUTH KIVU, DEMOCRATIC REPUBLIC OF THE CONGO

Microlite and U-enriched microlite were found in eluvium close to highly weathered granitic pegmatites actively mined as a source of tantalum ore and situated within the ~1000–900 Ma old Kibara belt unit, Numbi mining area, South Kivu province of the Democratic Republic of the Congo. Two examined pegmatites differ in their mineral assemblages and in the degree of fractionation: in the Mungwe pegmatite (lepidolite subtype) cassiterite, microlite-group minerals (microlite, U-enriched microlite), columbite-group minerals (columbite–tantalite), wodginite, ferrotapiolite; at the Misumari II pegmatite (beryl-columbite subtype) microlite, cassiterite, columbite–tantalite, wodginite–ferrowodginite. Both pegmatites underwent deep tropical weathering; however, the degree of hydrothermal alteration of the (Nb,Ta)-oxide minerals is rather low. Fragments of primary microlite and U-enriched microlite (the latter present only at the Mungwe pegmatite) are mostly homogeneous and compositionally uniform; locally U-enriched microlite forms outer zones of the grains. Primary microlite from the lessfractionated Misumari II pegmatite is rather homogeneous and has Ta/(Ta+Nb)=0.74–0.94 and higher Ti=0.07–0.23 apfu at the B-site compared to the Mungwe pegmatite with similar Ta/(Ta+Nb)=0.82–0.88 and low Ti. Microlite from Misumari II has higher contents of almost all trace elements (including REE) except for Li, Sb, and Pb. Primary microlite from both sites has Ca/Na close to 1, low A-site vacancy, and high F. Uranium-enriched microlite shows 0.29 apfu U, moderate A-site vacancy, and lower concentrations of F. Alteration of the U-poor microlite is very limited compared to the U-enriched microlite where radiation damage disrupted the mineral structure prior to the alteration. The altered microlite is depleted in Na and F and has high vacancy at the A-site, but the B-site population is almost identical to that of the primary microlite, as are the U concentrations. Such a composition is typical for secondary low-T alterations related to weathering. We demonstrate that the combined results from Automated Mineralogy, Electron Microprobe Analysis, and LA-ICP-MS techniques (along with the specific alteration features) can be successfully used to prove the provenance of tantalum ore by comparison with other localities where microlite is a major Ta-bearing mineral.

96Zr Tracer Diffusion in AZrO3 (A = Ca, Sr, Ba)

Cation tracer diffusion in polycrystalline AZrO3 (A = Ca, Sr, Ba) perovskites was studied at 1300–1500 °C in air using the stable isotope 96Zr. Thin films of 96ZrO2 were deposited on polished ceramic pellets by drop casting of an aqueous precursor solution containing the tracer. The pellets were subjected to thermal annealing, and the isotope depth profiles were measured by secondary ion mass spectrometry. Two distinct regions with different slopes in the profiles enabled to assess separately the lattice and grain boundary diffusion coefficients using Fick’s second law and Whipple–Le Clair’s equation. The cation diffusion along grain boundaries was 4–5 orders of magnitude faster than the corresponding lattice diffusion. The magnitude of the diffusivity of Zr4+ was observed to increase with decreasing size of the A-cation in AZrO3, while the activation energy for the diffusion was comparable 435 ± 67, 505 ± 56, and 445 ± 45 and kJ·mol−1 for BaZrO3, SrZrO3, and CaZrO3, respectively. Several diffusion mechanisms for Zr4+ were considered, including paths via Zr- and A-site vacancies. The Zr4+ diffusion coefficients reported here were compared to previous data reported on B-site diffusion in perovskites, and Zr4+ diffusion in fluorite-type compounds.

Superior energy storage properties and excellent stability of novel NaNbO3-based lead-free ceramics with A-site vacancy obtained via a Bi2O3 substitution strategy

Novel environment-friendly NaNbO3-based lead-free ceramics with ultrahigh energy storage density and power density for multilayer ceramic capacitor applications.

Atomistic simulations of ion migration in sodium bismuth titanate (NBT) materials: towards superior oxide-ion conductors

Fabricating (111)-ordered NBT and avoid any A-site vacancies will yield even higher oxide-ion conductivities.

Concurrent La and A-Site Vacancy Doping Modulates the Thermoelectric Response of SrTiO3: Experimental and Computational Evidence.

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1-xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration.

(La,Sr)(Fe,Co)O3-based cathode contact materials for intermediate-temperature solid oxide fuel cells

Development of the cathode contact materials (CCMs) for intermediate-temperature solid oxide fuel cells (IT-SOFCs) is a relatively new task promoted by the appearance of full-sized cells, which may operate below 700 °C with sufficient performance, on the market. CCM must ensure a low-resistive interface between interconnect and cathode and is supposed to be sintered at temperatures below 800 °C, as imposed by the specifications of IT-SOFCs and corresponding sealants. The present work is focused on the elaboration of CCMs derived from perovskite-like La0.6Sr0.4Co0.2Fe0.8O3-δ employing two approaches: introduction of the A-site cation vacancies and partial substitution by copper in B sublattice. Both approaches were found to result in a higher electrical conductivity below 800 °C if compared to the parent material. All studied materials exhibit acceptable coefficients of thermal expansion, 13.5–14.8 ppm K−1 at 25–700 °C. Area-specific resistance (ASR) of CCM/chromium barrier (Mn1.5Co1.5O4)/interconnect (Crofer 22APU) assemblies prepared by the screen-printing was measured in air at 660–750 °C. The studies revealed that morphology of CCM powder should be considered as a key parameter in the formation of interfaces with a low resistivity. The best ASR values, below 4 mΩ cm2 at 660–700 °C, were obtained for La0.6Sr0.4Co0.15Cu0.10Fe0.75O3-δ, La0.6Sr0.4Co0.15Cu0.05Fe0.80O3-δ and (La0.60Sr0.40)0.995Co0.20Fe0.80O3-δ as CCMs.

Large signal electrical property of CuO-doped of a Bi0.5Na0.5TiO3–SrTiO3

This report describes the sintering and ferroelectric characteristic of 0.76 Bi0.5Na0.5TiO3 (BNT)–0.26SrTiO3 (ST) with CuO addition, synthesized by using a solid-state reaction. The XRD, microstructure, and density results reveal that Cu element reduces the sintering temperature from 1150 to 1000 °C. In dielectric study, CuO addition induces the increase of relaxation behavior for 26ST ceramic in the high temperature regime. In the low temperature regime (< 150 °C), the Cu doping to 26ST accelerates the degree in freezing of dipole, presumably due to the interaction between A site vacancy and nanodomain boundaries. The electromechanical properties of 26ST ceramics with CuO doping show a significant decay in frequency dependence of polarization and strain, which might be related to the formation of A-site cation vacancy.

Columbitization of fluorcalciopyrochlore by hydrothermalism at the Saint-Honoré alkaline complex, Québec (Canada): New insights on halite in carbonatites

Niobium (Nb) in carbonatite is mainly hosted in fluorcalciopyrochlore and columbite-(Fe). Information related to Nb petrogenesis is useful for understanding the processes related to Nb mineralization and carbonatite evolution. The Saint-Honoré, Quebec, alkaline complex offers a rare opportunity for studying these processes as the complex is not affected by post-emplacement deformation, metamorphism nor weathering. Columbite-(Fe) is shown to be an alteration product of fluorcalciopyrochlore (columbitization). Columbitization is characterized by the leaching of Na and F from the A- and Y-sites of the pyrochlore crystal structure. As alteration increases, Fe and Mn are slowly introduced while Ca is simultaneously leached. Leached Ca and F then crystallize as inclusions of calcite and fluorite within the columbite-(Fe). A-site cations and vacancies in the crystal structure of fresh and altered pyrochlores demonstrate that pyrochlore alteration is hydrothermal in origin. Moreover, halite is a ubiquitous mineral in the Saint-Honoré alkaline complex. Petrographic evidence shows that halite forms in weakly altered pyrochlores, suggesting halite has a secondary origin. As alteration increases, halite is expelled by the hydrothermal fluid and is carried farther into the complex, filling factures throughout the carbonatite. The hydrothermal hypothesis is strengthened by significant enrichments in Cl and HREEs in columbite-(Fe). Chlorine is most likely introduced by a hydrothermal fluid that increases the solubility of REEs.

Density Functional Theory Modeling of A-site Cation Diffusion in Bulk LaMnO3 ±δ for Solid Oxide Fuel Cell Cathodes

In this work, the diffusion barriers of A-site cation and dopants in bulk LaMnO3±δ (LMO) are determined based on density functional theory (DFT) simulations. These properties are highly relevant for solid oxide fuel cell (SOFC) cathode applications. Due to the weak repulsive interactions (0.2~0.3 eV) between the nearest neighbor A-site and B-site cation vacancies, the A-site and B-site cation vacancy clusters can readily form in LMO. The DFT modeling unveils a facile A-site cation diffusion pathway via a cation vacancy cluster mechanism with a migration barrier reduction of about 1.3 eV versus the direct A-site vacancy migration. By combining these results with the bulk LMO defect modeling, the temperature and oxygen partial pressure dependences of the A-site cation tracer diffusion coefficients are assessed based on the concentration of transport carriers and the migration barriers associated with the A-site cation transport in bulk LMO. The trends in the migration barriers vs. various types of metal cations relevant to SOFC applications i.e. La3+, Sr2+, Zr4+, Y3+, are also investigated, and the results obtained suggest that both the ionic charge and the ionic radius correlate with the calculated cation migration barriers

A-Site Cation Bulk and Surface Diffusion in A-Site-Deficient BaZrO3 and SrZrO3 Perovskites

Diffusion of A-site cations in A-site-deficient BaZrO3 and SrZrO3 perovskite oxides was investigated by first-principles nudged elastic band calculations. We found an energy barrier for surface diffusion that was significantly lower than that for bulk diffusion (3.41 eV vs 2.41 eV for Ba diffusion in Ba-deficient BaZrO3 and 3.07 eV vs 1.44 eV for Sr diffusion in Sr-deficient SrZrO3), which suggests unusually rapid cation diffusion at low temperatures in nanomaterials with a large proportion of surface-exposed atoms. The lower mobility of Ba in BaZrO3 compared to that of Sr in SrZrO3 explains why the proportion of A-site vacancies in BaZrO3 nanoparticles is much greater than that in SrZrO3 nanoparticles, as previously observed during nanoparticle formation in which an A-site-deficient nucleus gradually incorporated A-site cations. Finally, thermal and nanoscale effects on the lattice parameter were studied by carrying out high-temperature X-ray diffraction measurements on BaZrO3 nanoparticles synthesized i...

Giant strain with low hysteresis in A-site-deficient (Bi0.5Na0.5)TiO3-based lead-free piezoceramics

We report a giant strain (0.72%) with a low degree of hysteresis (ca. 36.2%) and a giant Smax/Emax ratio (916 pm V−1, Smax and Emax denote the maximum strain and the corresponding electric field, respectively) for lead-free (1−x)(0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3)-xSr0.8Bi0.1□0.1Ti0.8Zr0.2O2.95 piezoceramics with x = 0.06. The giant strain originates from a reversible transition between the ergodic relaxor and ferroelectric states under applied electric fields. A-site vacancies (VA) and oxygen vacancies (VO), deliberately introduced to the system, induce a randomly distributed local polarization field. The local field induces embryonic polarization domains that have a broad distribution of maturity and thus smears the transition between the ferroelectric and relaxor states. This leads to a narrow hysteresis loop. The poling field required for the relaxor-to-ferroelectric transition is reduced significantly, due to the remanent ferroelectric phase at zero field acting as the seed, and the point defects synergistically facilitating the nucleation and growth of the ferroelectric phase. Our work provides a novel route for designing piezoelectric materials with both a giant strain and a narrow hysteresis for practical actuator applications.

Thermoelectric properties of EuZn2Sb2 Zintl compounds: zT enhancement through Yb substitution for Eu

AZn2Sb2 (A: Ca, Sr, Eu, Yb) Zintl compounds have shown excellent thermoelectric performance peaking around 700 K. Among these materials, EuZn2Sb2 and YbZn2Sb2 compounds have the highest carrier mobility, making them particularly interesting. We present the thermoelectric properties of EuδZn2Sb2 (δ = 0.9, 1.0, 1.1) and Eu1-xYbxZn2Sb2 compounds (x = 0, 0.1, 0.25, 0.5, 0.75, 1) from 373 to 773 K. Our interest is in studying whether introducing A-site vacancies increases the hole carrier concentration with decreasing Eu content or with increasing Yb substitution, which could lead to optimized power factor at high temperatures. The electrical conductivity showed a non-monotonic change caused by carrier mobility scattering. The thermal conductivity decreased through deviation of the Eu content and through Eu-Yb substitution; in particular, the phonon contribution at 773 K reduced gradually from 1.1 W m−1 K−1 (x = 0) to 0.42 W m−1 K−1 (x = 1.0) with increasing Yb fraction and reached the predicted minimum lattice thermal conductivity. The dimensionless figure of merit increased from 0.58 to 0.68 at 773 K through a 10% Yb substitution, i.e. Eu0.9Yb0.1Zn2Sb2, mainly due to the reduction of phonon thermal conductivity without lowering the power factor.

Donor doping of K0.5Na0.5NbO3 ceramics with strontium and its implications to grain size, phase composition and crystal structure

In this study, the particular effects of A-site donor doping, such as crystal-structure change, the secondary-phase formation and the grain-size decrease, in a lead-free piezoceramic material K0.5Na0.5NbO3 (KNN) doped with Sr2+, were investigated. Extended X-ray absorption fine structure (EXAFS) analyses proved that Sr occupies the perovskite A-sublattice, and locally modifies the KNN monoclinic structure to cubic. Introducing Sr into the A-sublattice, as well as accounting for the charge-compensating A-site vacancies in the starting composition, causes increasing lattice disorder and microstrain, as determined from a Rietveld refinement of the synchrotron X-ray diffraction data. Above 2% Sr the system segregates the A-site vacancies in a secondary phase in order to release the chemical pressure, as revealed by Raman spectroscopy. All these effects result in an increasing number of low-angle grain boundaries that limit the grain growth and finally lead to a significant grain-size decrease.

Microstructure and Properties of KSr2Nb5O15 Ceramics with Excess K+

KSr2Nb5O15-xK (KSN-xK, x = 0 mol.%, 4 mol.%, 8 mol.%, 12 mol.%, 16 mol.%, and 20 mol.%) lead-free ferroelectric ceramics have been prepared by a buried sintering method using needle-like KSN particles synthesized by molten salt synthesis, and their microstructure, dielectric properties, and infrared transmittance investigated. The results suggest that the KSN-xK ceramics had simplex tungsten bronze structure for x ≤ 12 mol.%, but K2Nb8O21 secondary phase appeared at higher x. Excess K+ compensated the shortage of A-site ions in KSN crystallites, alleviated lattice distortion, and drove the KSN component closer to stoichiometric ratio, all of which increased the Curie temperature. The dielectric relaxor behavior of the ceramics was enhanced as the excess K+ content was increased. The dielectric constant, dielectric tunability, and infrared transmittance initially increased then decreased with increasing x. The specimen with 12 mol.% excess K+ showed optimum electrical properties, including maximum infrared transmittance of ∼60%. This work confirms that A-site vacancies in KSN can be compensated by excess K+, and that this effect can be used to adjust the local composition, alleviate structural distortion of the oxygen octahedron, enhance the Curie temperature, etc.

Structure refinements and the influences of A-site vacancies on properties of Na0.5Bi2.5Nb2O9-based high temperature piezoceramics

Na0.5Bi2.5Nb2O9 (NBN) based high temperature piezoelectric ceramics with A-site vacancies have been synthesized by the conventional solid state reaction method. Rietveld refinements and in-situ X-ray diffraction were performed to explore the influence of A-site vacancies on the crystalline structure and the phase evolution of NBN based ceramics with temperature, respectively. Pseudo-tetragonal distortion induced by A-site vacancies or temperature was found for the first time in the Aurivillius family, and this structural distortion might be responsible for the improvement of piezoelectric activity and the stability of piezoelectric property against thermal depolarization temperature (Td). The composition with 12 mol. % A-site vacancies exhibited good integrated performance, piezoelectric coefficient d33 and Tc were about 24.8 pC/N and 770 °C, respectively, together with excellent resistance to the thermal depolarization behavior (d33 decreased by 3% even if Td reached up to Curie temperature TC). This high temperature piezoelectric ceramics show potential applications in high temperature environments.