Cooperative mechanisms of oxygen vacancy stabilization and migration in the isolated tetrahedral anion Scheelite structure

Investigation and optimization of In-Vitro behaviour of Perovskite barium titanate as a scaffold and protective coatings

The piezoelectric effect is widely known to have a significant physiological function in bone development, remodeling, and fracture repair. As a well-known piezoelectric material, barium titanate is particularly appealing as a scaffold layer to improve bone tissue engineering applications. Currently, the chemical bath deposition method is used to prepare green synthesized barium titanate coatings to improve mechanical and biological characteristics. Molarity of the solutions, an essential parameter in chemical synthesis, is changed at room temperature (0.1-1.2 Molar) to prepare coatings. The XRD spectra for as deposited coatings indicate amorphous behavior, while polycrystalline nature of coatings is observed after annealing (300 °C). Coatings prepared with solutions of relatively low molarities, i.e. from 0.1 to 0.8 M, exhibit mixed tetragonal - cubic phases. However, the tetragonal phase of Perovskite barium titanate is observed using solution molarities of 1.0 M and 1.2 M. Relatively high value of transmission, i.e. ∼80%, is observed for the coatings prepared with high molarities. Band gap of annealed coatings varies between 3.47 and 3.70 eV. For 1.2 M sample, the maximum spontaneous polarization (Ps) is 0.327x10-3 (μC/cm2) and the residual polarization (Pr) is 0.072x10-3 (μC/cm2). For 1.2M solution, a high hardness value (1510 HV) is recorded, with a fracture toughness of 28.80 MPam-1/2. Low values of weight loss, after dipping the coatings in simulated body fluid, is observed. The antibacterial activity of BaTiO3 is tested against E. coli and Bacillus subtilis. Drug encapsulation capability is also tested for different time intervals. As a result, CBD-based coatings are a promising nominee for use as scaffold and protective coatings.

Tuning Oxygen Vacancies in Oxides by Configurational Entropy

Tuning surface oxygen vacancies is important for oxide catalysts. Doping elements with different chemical valence states or different atomic radii into host oxides is a common method to create oxygen vacancies. However, the concentration of oxygen vacancies in oxide catalysts is still limited to the amount of foreign dopants that can be tolerated (generally less than 10% atoms). Herein, a principle of engineering the configurational entropy to tune oxygen vacancies was proposed. First, the positive relationship between the configuration entropy and the formation energy of oxygen vacancies (Eov) in 16 model oxides was estimated by a DFT calculation. To verify this, single binary oxides and high-entropy quinary oxides (HEOs) were prepared. Indeed, the concentration of oxygen vacancies in HEOs (Oβ/α = 3.66) was higher compared to those of single or binary oxides (Oβ/α = 0.22-0.75) by O1s XPS, O2-TPD, and EPR. Interestingly, the reduction temperatures of transition metal ions in HEOs were generally lower than that in single-metal oxides by H2-TPR. The lower Eov of HEOs may contribute to this feature, which was confirmed by in situ XPS and in situ XRD. Moreover, with catalytic CO/C3H6 oxidation as a model, the high-entropy (MnCuCo3NiFe)xOy catalyst showed higher catalytic activity than single and binary oxides, which experimentally verified the hypothesis of the DFT calculation. This work may inspire more oxide catalysts with preferred oxygen vacancies.

Imperfect makes perfect: defect engineering of photoelectrodes towards efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting for hydrogen evolution has been considered as a promising technology to solve the energy and environmental issues. However, the solar-to-hydrogen (STH) conversion efficiencies of current PEC systems are far from meeting the commercial demand (10%) due to the lack of efficient photoelectrode materials. The recent rapid development of defect engineering of photoelectrodes has significantly improved the PEC performance, which is expected to break through the bottleneck of low STH efficiency. In this review, the category and the construction methods of different defects in photoelectrode materials are summarized. Based on the in-depth summary and analysis of existing reports, the PEC performance enhancement mechanism of defect engineering is critically discussed in terms of light absorption, carrier separation and transport, and surface redox reactions. Finally, the application prospects and challenges of defect engineering for PEC water splitting are presented, and the future research directions in this field are also proposed.

Oxide Ion-Conducting Materials Containing Tetrahedral Moieties: Structures and Conduction Mechanisms

This Review presents an overview from the perspective of tetrahedral chemistry on various oxide ion-conducting materials containing tetrahedral moieties which have received continuous growing attention as candidates for key components of various devices, including solid oxide fuel cells and oxygen sensors, due to the deformation and rotation flexibility of tetrahedral units facilitating oxide ion transport. Emphasis is placed on the structural and mechanistic features of various systems ranging from crystalline to amorphous materials, which include a variety of gallates, silicates, germanates, molybdates, tungstates, vanadates, aluminates, niobate, titanates, indium oxides, and the newly reported borates. They contain tetrahedral units in either isolated or linked manners forming different polyhedral dimensionality (0 to 3) with various defect properties and transport mechanisms. The development of oxide ion conductors containing tetrahedral moieties and the elucidation of the roles of tetrahedral units in oxide ion migration have demonstrated diverse opportunities for discovering superior electrolytes for solid oxide fuel cells and other related devices and provided useful clues for uncovering the key factors directing fast oxide ion conduction.

Rational design, crystal structure, and frustrated magnetism of the Ge-containing YbFe2O4-type layered oxides In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3)

YbFe₂O₄-type layered oxides have attracted tremendous interest because the unique crystal comprises two distinct geometrically frustrated triangular cation-sublattices. Herein, a series of YbFe₂O₄-type materials In₂Zn_3-xCo_xGeO₈ (0 ≤ x ≤ 3) were rationally designed and experimentally synthesized for the first time. The crystal structures of In₂Zn_3-xCo_xGeO₈ were investigated comprehensively by Rietveld refinements against high-resolution monochromatic Cu K_α1 XRD data. Zn²⁺, Co²⁺, and Ge⁴⁺ cations are distributed randomly on the [MO]₂ bilayer and possess a trigonal bipyramid (TBP) coordination geometry. Because Co²⁺ has an unpaired electron in the d_z2 orbital and a larger electronegativity than Zn²⁺, Co²⁺-to-Zn²⁺ equivalent substitution in In₂Zn_3-xCo_xGeO₈ results in more compact MO₅-TBPs, which is the origin of anisotropic lattice expansion and contraction along the a and c axes, respectively. The Co²⁺ moments in the [MO]₂ bilayer are strongly AFM coupled and geometrically frustrated, therefore resulting in a spin-glass magnetic transition at around T_g = 20 K for In₂ZnCo₂GeO₈, while a long-range AFM ordering is established for In₂Co₃GeO₈ with a Néel temperature of 53 K, attributed to the significantly enhanced AFM interactions and increased In³⁺/Co²⁺ anti-site disordering, as compared to those in In₂ZnCo₂GeO₈.

Hidden chemical order in disordered Ba7Nb4MoO20 revealed by resonant X-ray diffraction and solid-state NMR

The chemical order and disorder of solids have a decisive influence on the material properties. There are numerous materials exhibiting chemical order/disorder of atoms with similar X-ray atomic scattering factors and similar neutron scattering lengths. It is difficult to investigate such order/disorder hidden in the data obtained from conventional diffraction methods. Herein, we quantitatively determined the Mo/Nb order in the high ion conductor Ba₇Nb₄MoO₂₀ by a technique combining resonant X-ray diffraction, solid-state nuclear magnetic resonance (NMR) and first-principle calculations. NMR provided direct evidence that Mo atoms occupy only the M2 site near the intrinsically oxygen-deficient ion-conducting layer. Resonant X-ray diffraction determined the occupancy factors of Mo atoms at the M2 and other sites to be 0.50 and 0.00, respectively. These findings provide a basis for the development of ion conductors. This combined technique would open a new avenue for in-depth investigation of the hidden chemical order/disorder in materials.

Unconventional Substitution for BiVO4 to Enhance Photoelectrocatalytic Performance by Accelerating Polaron Hopping

Bismuth vanadate (BiVO4) as a fascinating semiconductor for photoelectrocatalytic (PEC) water oxidation with suitable band gap (Eg) has been limited by the issue of poor separation and transportation of charge carriers. Herein, we propose an unconventional substitution of V5+ sites by Ti4+ in BiVO4 (Ti:BiVO4) for the similar ionic radii and accelerated polaron hopping. Ti:BiVO4 increased the photocurrent density 1.90 times up to 2.51 mA cm-2 at 1.23 V vs RHE and increased the charge carrier density 1.81 times to 5.86 × 1018 cm-3. Compared with bare BiVO4, Ti:BiVO4 improves the bulk separation efficiency to 88.3% at 1.23 V vs RHE. The DFT calculations have illustrated that Ti-doping modification could decrease the polaron hopping energy barrier, narrow the Eg, and decrease the overpotential of the oxygen evolution reaction (OER) concurrently. With further spin-coated FeOOH cocatalyst, the photoanode has a photocurrent density of 3.99 mA cm-2 at 1.23 V vs RHE. The excellent PEC performance of FeOOH/Ti:BiVO4 is attributed to the synergistic effect of the FeOOH layer and Ti doping, which could promote charge carrier separation and transfer by expediting polaron migration.

Ni doped amorphous FeOOH layer as ultrafast hole transfer channel for enhanced PEC performance of BiVO4

Bismuth vanadate (BiVO₄), as the potential and prospective photocatalyst, has been limited by the issue of poor separation and transfer of charge carrier for photoelectrocatalytic (PEC) water oxidation. Here, a significant increase of surface injection efficiency for BiVO₄ is realized by the rationally designed Ni doped FeOOH (Ni:FeOOH) layer growing on BiVO₄ photoanode (Ni:FeOOH/BiVO₄), in which doped Ni²⁺ can induce partial-charge of FeOOH to serve as ultrafast transfer channel for hole transfer and transportation at the semiconductor/electrolyte interface. In addition, the Ni:FeOOH/BiVO₄ shows the η_surface value of 81.6 %, which is 3.28-fold and 1.47-fold of BiVO₄ and FeOOH/BiVO₄, respectively. The photocurrent density of Ni:FeOOH/BiVO₄ is 4.21 mA cm^-2 at 1.23 V vs. RHE, with the onset potential cathodically shifting 237 mV over BiVO₄ and a long-term stability for suppressing surface charge recombination. The UPS and UV-Vis spectra have confirmed the type-II band alignment between Ni:FeOOH and BiVO₄ for promoting carrier transfer. This facile and effective spin-coating method could deposit oxygen evolution catalysts (OECs) availably onto photoanodes with enhanced PEC water splitting.

Thermodynamics and Kinetics Accounting for Antithermal Quenching of Luminescence in Sc2(MoO4)3: Yb/Er: Perspective beyond Negative Thermal Expansion

Defects are common in inorganic materials and not static upon annealing of the heat effect. Antithermal quenching of luminescence in phosphors may be ascribed to the migration of defects and/or ions, which has not been well-studied. Herein, we investigate the antithermal quenching mechanism of upconversion luminescence in Sc₂(MoO₄)₃: 9%Yb1%Er with negative thermal expansion via a fresh perspective on thermodynamics and kinetics, concerning the thermally activated movement of defects and/or ions. Our results reveal a second-order phase transition taking place at ∼573 K induced by oxide-ion migration. The resulting variation of the thermodynamics and kinetics of the host lattice owing to the thermally induced oxide-ion movement contributes to a more suppressed nonradiative decay rate. The dynamic defects no longer act as quenching centers with regard to the time scale during which they stay nearby the Yb³⁺/Er³⁺ site in our proposed model. This research opens an avenue for understanding the antithermal quenching mechanism of luminescence via thermodynamics and kinetics.

Recent developments in oxide ion conductors: focusing on Dion-Jacobson phases

Oxide-ion conductors, also known as "oxygen ion conductors," have garnered significant attention in recent years due to their extensive applications in a variety of electrochemical devices, including oxygen concentrators, solid-oxide fuel cells (SOFCs), and solid oxide electrolysis cells. The key to improving the performance of these devices is the creation of novel oxide-ion conductors. In this feature article, we discuss the recent developments of new structural families of oxide-ion conductors and of the Dion-Jacobson-type layered oxide-ion conductors with a particular emphasis on CsM₂Ti₂NbO_10-δ (M = Bi and lanthanoids; δ represents oxygen-vacancy content) and their solid solutions. CsBi₂Ti₂NbO_10-δ is the first example of an oxide-ion conductor with a Dion-Jacobson-type layered perovskite structure, and the structural characteristics of these materials are extracted here. We have proposed an original concept that the large sized Cs⁺ cations and M³⁺ displacements yield the large bottlenecks for oxide-ion migration, which would facilitate the discovery of novel oxide-ion conductors. This article presents evidence that Dion-Jacobson-type layered perovskites are superior oxide-ion conductors. We also demonstrate how the information gleaned from these studies can be applied to the design of novel oxide-ion conductors.

Temperature-driven order-disorder structural transition in the oxygen sub-lattice and the complex superstructure of the high-temperature polymorph of CaSrZn2Ga2O7

Structural order-disorder plays a decisive role in the physical properties of materials, such as magnetism, second-order harmonic generation, and ionic conductivity, and it is thus widely utilized to manipulate the crystal structure and understand structure-property correlations. Herein, we report the structural polymorphism, complex crystal structure and temperature-driven irreversible order-disorder phase transition of the polar oxides (Sr_1-xCa_x)SrZn₂Ga₂O₇. The low-temperature (LT) structure crystallizes in Pna2₁ with partial Zn/Ga ordering. Upon heating, (Sr_1-xCa_x)SrZn₂Ga₂O₇ undergoes an irreversible phase transition from orthorhombic Pna2₁ to hexagonal P6₃. Interestingly, the high-temperature (HT) P6₃ structure possesses an unexpected 3/2-fold superstructure rather than a substructure of the low-temperature (LT) Pna2₁ structure, which is a rare structural phenomenon in solid-state chemistry. This new HT superstructure is the most complex one in this series of oxides with 21 crystallographically independent sites determined accurately by a combination of the maximum entropy method and Rietveld refinement against high-resolution neutron powder diffraction data. In terms of the mechanism, this is a temperature-driven order-to-disorder transition in the oxygen sublattice. A careful structural analysis revealed that the oxygen disordering mainly occurs in the [SrO₃] layers of the HT structure and it can be understood as respective clockwise and anticlockwise rotations of distinct GaO₄-tetrahedra along the c-axis. Alternating current electrochemical impedance spectroscopic analysis revealed that the oxygen disordering in the HT structure is incapable of giving rise to oxide ionic conductivity but does lead to increased electronic conduction compared to the LT structure. The optical properties of the CaSrZn₂Ga₂O₇ and Sr₂Zn₂Ga₂O₇ representatives are also investigated in-depth via diffuse reflectance spectroscopy and theoretic calculations.

Type-II Heterojunction CdIn2S4/BiVO4 Coupling with CQDs to Improve PEC Water Splitting Performance Synergistically

Bismuth vanadate (BiVO₄) has been considered as a promising photoelectrocatalytic (PEC) semiconductor, but suffers from severe hole recombination, attributed to the short hole-diffusion length and the low carrier mobility. Herein, a type-II heterojunction CdIn₂S₄/BiVO₄ is designed to improve the photocurrent density from 1.22 (pristine BiVO₄) to 2.68 mA cm^-2 at 1.23 V vs the reversible hydrogen electrode (RHE), accelerating the bulk separation of photogenerated carriers by the built-in field from the matched energy band. With the introduction of CQDs, CQDs/CdIn₂S₄/BiVO₄ increases the photocurrent density to 4.84 mA cm^-2, enhancing the light absorption and cathodically shifting its onset potential, due to the synergetic effect of the heterojunction and CQDs. Compared with BiVO₄, CQDs/CdIn₂S₄/BiVO₄ promotes the bulk separation efficiency to 94.6% and the surface injection efficiency to 72.2%. Additionally, spin-coating of FeOOH on CQDs/CdIn₂S₄/BiVO₄ could further improve the PEC performance and keep a long stability for water splitting. The density function theory (DFT) calculations illustrated that the type-II heterojunction CdIn₂S₄/BiVO₄ could decrease the oxygen evolution reaction (OER) overpotential and accelerate bulk charge separation for the built-in field of the aligned band structure.

Proton and Oxide Ion Conductivity in Palmierite Oxides

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy-related technologies. Here, we report on the discovery of significant proton and oxide ion conductivity in palmierite oxides A₃V₂O₈ (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO₄ units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (t _H ∼ 0.6-0.8) and high protonic mobility. In particular, the proton conductivity of Sr₃V₂O₈ is 1.0 × 10^-4 S cm^-1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the three-dimensional ionic transport is vacancy-driven and facilitated by rotational motion of the VO₄ units, which can stabilize oxygen defects via formation of V₂O₇ dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.

Oxide Ion Conductivity, Proton Conductivity, and Phase Transitions in Perovskite-Derived Ba3-x Sr x YGa2O7.5 0 ≤ x ≤ 3 Materials

We report the synthesis, structural characterization, and oxide ion and proton conductivities of the perovskite-related Ba_3-x Sr _x YGa₂O_7.5 family. Single-phase samples are prepared for 0 ≤ x ≤ 3 and show a complex structural evolution from P2/c to C2 space groups with an increase in x. For 1.0 ≲ x ≲ 2.4, average structures determined by X-ray and neutron powder diffraction show metrically orthorhombic unit cells, but HAADF-STEM imaging reveals this is caused by microstructural effects due to intergrowths of the Ba- and Sr-rich structure types. Variable-temperature powder diffraction studies suggest that 0 ≲ x ≲ 2.4 compositions undergo a phase transition upon being heated to space group Cmcm that involves disordering of the oxygen substructure. Thermal expansion coefficients are reported for the series. Complex impedance studies show that the Ba-rich samples are mixed proton and oxide ion conductors under moist atmospheres but are predominantly oxide ion conductors at high temperatures or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied.

Tetrahedral Tilting and Oxygen Vacancy Stabilization and Migration in La1-xSr2+x(GaO4)O1-0.5x Mixed Electronic/Oxide Ionic Conductors

The La₂O₃/SrO/Ga₂O₃ ternary system contains several compounds with remarkable oxide or proton ionic conduction. Among them, the layered LaSr₂(GaO₄)O compound is a less commonly studied material. Here, the crystal structure, electrical conduction properties, and ionic migration mechanism of the La_1-xSr_2+x(GaO₄) O_1-0.5x (0 ≤ x ≤ 0.3) system are thoroughly analyzed. Diffraction methods indicate that the system crystallizes in the tetragonal space group P4/ncc, which is compatible with the presence of a subtle GaO₄ tetrahedral tilting along the c axis, leading to a slight deviation of the body-centered tetragonal structure previously reported. This feature is essential to further understanding the mixed p electronic/oxide ion-conducting behavior of the system. Upon La³⁺ for Sr²⁺ substitution, oxygen vacancies arise at the loosely bound oxide sublattice, which at high temperature go through the GaO₄ tetrahedral layer, leading to the formation of intermediate corner-sharing Ga₂O₇ tetrahedral dimers, and migrate via the continuous breaking and re-formation of the dimers, assisted by the synergic rotation and deformation of neighboring GaO₄ tetrahedra. The unique structural and electrical features of La_1-xSr_2+x(GaO₄)O_1-0.5x materials within the La₂O₃/SrO/Ga₂O₃ ternary system emphasize their potential application as cathode materials in LaGaO₃-based fuel cells.

Structure, electrical properties, and conduction mechanism of new germanate mixed Zn-doped In2Ge2O7 conductors

A new germanate mixed electronic and oxide ionic conductor In1.8Zn0.2Ge2O6.9 was developed, which exhibits two-dimensional anisotropic transport nature by oxygen exchange between adjacent Ge2O7 units, with a conductivity of 1.62 × 10–2 S cm−1 at 1000 °C.

Oxide-ion Conductivity Optimization in BiVO4 Scheelite by Acceptor Doping Strategy

BiVO4 scheelite is one of the few tetrahedra-based structures able to display vacancy-mediated oxide ion conduction upon acceptor doping strategy, leading to oxide ionic migration. In order to modulate the...

Electrical conductivity and oxygen relaxation behavior in BaBi1−xSrxNb5O15−δ tungsten bronze ceramics

Sr2+ ion-doped BBN-based oxygen ion conductors were systematically investigated. When using the two-step sintering method, the bulk and total conductivities of the BBN-Sr2-TS sample are enhanced by one and two orders of magnitude, respectively.

Oxygen Vacancy Engineering Synergistic with Surface Hydrophilicity Modification of Hollow Ru Doped CoNi-LDH Nanotube Arrays for Boosting Hydrogen Evolution

With the development of clean hydrogen energy, the cost effective and high-performance hydrogen evolution reaction (HER) electrocatalysts are urgently required. Herein, a green, facile, and time-efficient Ru doping synergistic with air-plasma treatment strategy is reported to boost the HER performance of CoNi-layered double hydroxide (LDH) nanotube arrays (NTAs) derived from zeolitic imidazolate framework nanorods. The Ru doping and air-plasma treatment not only regulate the oxygen vacancy to optimize the electron structure but also increase the surface roughness to improve the hydrophilicity and hydrogen spillover efficiency. Therefore, the air plasma treated Ru doped CoNi-LDH (P-Ru-CoNi-LDH) nanotube arrays display superior HER performance with an overpotential of 29 mV at a current density of 10 mA cm^-2 . Furthermore, by assembling P-Ru-CoNi-LDH as both cathode and anode for two-electrode urea-assisted water electrolysis, a small cell voltage of 1.36 V is needed at 10 mA cm^-2 and can last for 100 h without any obvious activity attenuation that showing outstanding durability. In general, the P-Ru-CoNi-LDH can improve the HER performance from intrinsic electronic structure regulation cooperated with extrinsic surface wettability modification. These findings provide an effective intrinsic and extrinsic synergistic effect avenue to develop high performance HER electrocatalysts, which is potential to be applied to other research fields.

Oxide Ion and Proton Conductivity in a Family of Highly Oxygen-Deficient Perovskite Derivatives

Functional oxides showing high ionic conductivity have many important technological applications. We report oxide ion and proton conductivity in a family of perovskite-related compounds of the general formula A₃OhTd₂O_7.5, where Oh is an octahedrally coordinated metal ion and Td is a tetrahedrally coordinated metal ion. The high tetrahedral content in these ABO_2.5 compositions relative to that in the perovskite ABO₃ or brownmillerite A₂B₂O₅ structures leads to tetrahedra with only three of their four vertices connected in the polyhedral framework, imparting a potential low-energy mechanism for O^2- migration. The low- and high-temperature average and local structures of Ba₃YGa₂O₇ (P2/c, a = 7.94820(5) Å, b = 5.96986(4) Å, c = 18.4641(1) Å, and β = 91.2927(5) ° at 22 °C) were determined by Rietveld and neutron pair distribution function (PDF) analysis, and a phase transition to a high-temperature P112₁/a structure (a = 12.0602(1) Å, b = 9.8282(2) Å, c = 8.04982(6) Å, and γ = 107.844(3)° at 1000 °C) involving the migration of O^2- ions was identified. Ionic conductivities of Ba₃YGa₂O_7.5 and compositions substituted to introduce additional oxide vacancies and interstitials are reported. Most phases show proton conductivity at lower temperatures and oxide ion conductivity at high temperatures, with Ba₃YGa₂O_7.5 retaining proton conductivity at high temperatures. Ba_2.9La_0.1YGa₂O_7.55 and Ba₃YGa_1.9Ti_0.1O_7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.

Enhancing the oxide-ionic conductivity of Ba3Mo1+xNb1-2xGexO8.5 at intermediate temperatures: the effect of site-selective Ge4+-substitution

Significant oxide-ionic conductivity has been recently reported for a family of cation-deficient hexagonal perovskite derivatives Ba₃M₂O_8.5 (M = Mo/W⁶⁺ and Nb⁵⁺/V⁵⁺). Herein, strong 4-fold coordination geometry preferring Ge⁴⁺ ions are doped into Ba₃Mo_1+xNb_1-2xGe_xO_8.5 to manipulate the oxygen distribution within palmierite-like layers for the enhancement of oxide-ionic conductivity. Rietveld refinement of the neutron diffraction data of Ba₃Mo_1.2Nb_0.6Ge_0.2O_8.5 reveals that Ge⁴⁺-ions are selectively incorporated into the palmierite-like layers, owing to their strong 4-fold coordination environment preference. Such a site-selective doping behavior leads to an increase in the occupation proportion of the O3 site and a concomitant decrease in the occupancy factor for O2. Ionic conduction measurements show that the bulk conductivity of Ba₃Mo_1.2Nb_0.6Ge_0.2O_8.5 is about twice higher than that of the parent compound at intermediate temperatures (300-500 °C). Furthermore, bond-valence site energy (BVSE) landscape analysis reveals that the oxygen ionic conduction of Ba₃Mo_1+xNb_1-2xGe_xO_8.5 is dominated by the two-dimensional pathways along the palmierite-like layers, despite the three-dimensional (3D) oxygen diffusion pathways being present in the hybrid structure, which strongly confirms that the enhancement in ionic conductivity at intermediate temperatures is attributed to the site-selective Ge⁴⁺-substitution-induced redistribution of oxygen ions within the palmierite-like layers.

Synthesis and Ion-Transport Properties of EuKGe2O6-, Ca3Fe2Ge3O12-, and BaCu2Ge2O7-Type Oxide-Ion Conductors

EuKGe₂O₆-, Ca₃Fe₂Ge₃O₁₂-, and BaCu₂Ge₂O₇-type germanates are synthesized by a conventional solid-state method and characterized to reveal their oxide-ion-conducting properties. Materials of the EuKGe₂O₆ group exhibit oxide-ion conductivity (e.g., 4.6 × 10^-3 S/cm at 973 K for Eu_0.8Ca_0.2KGe₂O_6-δ) and transport numbers above 96%, whereas materials of the Ca₃Fe₂Ge₃O₁₂ and BaCu₂Ge₂O₇ groups exhibit mixed electron-/oxide-ion conduction. Conduction involves oxide-ion vacancies in the EuKGe₂O₆ group, interstitial oxide ions in the Ca₃Fe₂Ge₃O₁₂ group, and both oxide-ion vacancies and interstitial oxide ions in the BaCu₂Ge₂O₇ group. The doping-induced formation of impurity phases decreases the amount of oxide-ion carriers relative to the expected values.

Vacancy defect engineering of BiVO4 photoanodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting has been regarded as a promising technology for sustainable hydrogen production. The development of efficient photoelectrode materials is the key to improve the solar-to-hydrogen (STH) conversion efficiency towards practical application. Bismuth vanadate (BiVO₄) is one of the most promising photoanode materials with the advantages of visible light absorption, good chemical stability, nontoxic feature, and low cost. However, the PEC performance of BiVO₄ photoanodes is limited by the relatively short hole diffusion length and poor electron transport properties. The recent rapid development of vacancy defect engineering has significantly improved the PEC performance of BiVO₄. In this review article, the fundamental properties of BiVO₄ are presented, followed by an overview of the methods for creating different kinds of vacancy defects in BiVO₄ photoanodes. Then, the roles of vacancy defects in tuning the electronic structure, promoting charge separation, and increasing surface photoreaction kinetics of BiVO₄ photoanodes are critically discussed. Finally, the major challenges and some encouraging perspectives for future research on vacancy defect engineering of BiVO₄ photoanodes are presented, providing guidelines for the design of efficient BiVO₄ photoanodes for solar fuel production.

Experimental and Theoretical Solid-State 29Si NMR Studies on Defect Structures in La9.33+x(SiO4)6O2+1.5x Apatite Oxide Ion Conductors

Oxide ion conductors can be used as electrolytes in solid oxide fuel cells, a promising energy-conversion technology. Local structures around the defects in oxide ion conductors are key for understanding the defect stabilization and migration mechanisms. As the defect contents are generally low, it is rather difficult to characterize the defect structure and therefore elucidate how oxide ions migrate. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful technique for probing the local structures. However, the interpretation of NMR signals mainly based on the empirical knowledge could lead to unprecise local structures. There is still controversy regarding the defect structures in the apatite-type interstitial oxide ion conductors containing isolated tetrahedral units. Here, we combine the experimental solid-state ²⁹Si NMR spectroscopy with theoretical density functional theory calculations to investigate the defect structures in La_9.33+x(SiO₄)₆O_2+1.5x apatites. The results indicate that the ²⁹Si resonance signals on the high field side of the main peak corresponding to the Si atoms in the bulk structure are related to La vacancies and there is no steady-state SiO₅ in the defect structures. This finding provides new atomic-level understanding to the stabilization and migration of interstitial oxide ions in silicate apatites, which could guide the design and discovery of new solid oxide fuel cell electrolyte materials.

Neutron diffraction study of the monoclinic - tetragonal phase transition in NdNbO4 and NdTaO4

Phase transition and high-temperature properties of NdNbO₄ and NdTaO₄ were studied in situ using powder neutron diffraction methods. Both oxides undergo a reversible phase transition from a monoclinic I2/a phase at low temperatures to a tetragonal I4₁/a phase at high temperatures. The phase transition has been investigated through analysis of the spontaneous strains and symmetry distortion modes. Well below the transition temperature, T_c, the thermal evolution of the lattice parameters and symmetry modes suggest the transition is continuous, although a small discontinuity in both the spontaneous strains and symmetry distortion modes shows the transition is strictly first order. Analysis of the refined structures reveals that the Nb and Ta cations are best described as having a distorted 6-coordinate arrangement in the monoclinic structure, with four short and two long bonds. Breaking of the two long bonds at high temperatures, resulting in a transformation of the Nb(Ta) coordination to a regular tetrahedron, is believed to be responsible for the first order nature of the transition.

Sodium Site Exchange and Migration in a Polar Stuffed-Cristobalite Framework Structure

The study of ionic dynamics in solids is essential to understanding and developing modern energy technologies. Here we study the ionic dynamics of orthorhombic Na₂MgSiO₄, an interesting case of a polar stuffed-cristobalite-type structure that contains two inequivalent Na sites within the channels of the magnesium silicate tetrahedral framework. Its preparation by a solid-state reaction method favors the presence of ∼2% of Na vacancies, converting it into a pure Na ionic conductor with an optimized ionic conductivity of ∼10^-5 S cm^-1 at 200 °C. The macroscopic migration has been characterized through impedance spectroscopy and molecular dynamics simulation, which proves the pure Na ionic character of the compound through hopping between Na1 and Na2 sites, forming three-dimensional migration zigzag-shaped paths. High-resolution solid-state ²³Na magic-angle-spinning (MAS) NMR spectroscopy is employed to characterize the local structure and microscopic dynamics of Na-ion transport in Na₂MgSiO₄. Remarkably, variable-temperature ²³Na MAS NMR and two-dimensional exchange spectroscopy evidence for the first time a Na site exchange phenomenon at room temperature, which further triggers Na ionic conduction at elevated temperatures.

Phase Evolution, Electrical Properties, and Conduction Mechanism of Ca12Al14-xGaxO33 (0 ≤ x ≤ 14) Ceramics Synthesized by a Glass Crystallization Method

Mayenite Ca₁₂Al₁₄O₃₃, as an oxide-ion conductor, has the potential of being applied in many fields, such as solid-oxide fuel cells. However, its relatively low oxide-ion conductivity hinders its wide practical applications and thus needs to be further optimized. Herein, a new recently developed glass crystallization route was used to prepare a series of Ga-doped Ca₁₂Al_14-xGa_xO₃₃ (0 ≤ x ≤ 14) materials, which is not accessible by the traditional solid-state reaction method. Phase evolution with the content of gallium, the corresponding structures, and their electrical properties were studied in detail. The X-ray diffraction data revealed that a pure mayenite phase can be obtained for 0 ≤ x ≤ 7, whereas when x > 7, the samples crystallize into a melilite-like orthorhombic Ca₅Ga₆O₁₄-based phase. The electrical conduction studies evidence no apparent enhancement in the total conductivity for compositions 0 ≤ x ≤ 7 with the mayenite phase, and therefore, the rigidity of the framework cations and the width of the windows between cages are not key factors for oxide-ion conductivity in mayenite Ca₁₂Al₁₄O₃₃-based materials, and changing the free oxygen content through aliovalent cation substitution may be the right direction. For compositions with a pure melilite-like orthorhombic phase, the conductivities also mirrored each other and are all slightly higher than those of the mayenite phases. These melilite-like Ca₅Ga₆O₁₄-based materials show mixed Ca-ion, oxide-ion, and electron conduction. Furthermore, the conduction mechanisms of Ca ions and oxide ions in this composition were studied by a bond-valence-based method. The results suggested that Ca-ion conduction is mainly due to the severely underbonded Ca3 ions and that the oxide ions are most likely transported via oxygen vacancies.

Promising lanthanide-doped BiVO4 phosphors for highly efficient upconversion luminescence and temperature sensing

The semiconductor oxide BiVO₄ has been intensively studied as a highly efficient photocatalyst. Here we attempt to adopt trivalent lanthanide (Ln³⁺)-doped BiVO₄ as a novel upconversion luminescence (UCL) material for achieving high-efficiency UCL and temperature sensing under near-infrared (NIR) irradiation. Er³⁺/Tm³⁺, Yb³⁺/Er³⁺, and Yb³⁺/Tm³⁺ ions were selected to co-dope the BiVO₄ phosphors, achieving three primary colors of red, green, and blue (RGB) with high color-purity. At an optimal doping concentration, the upconversion quantum yield of the BiVO₄:8%Yb³⁺,18%Er³⁺ phosphor reaches as high as 2.9%. Furthermore, we, for the first time, demonstrate the non-contact temperature sensing properties of a BiVO₄:Er³⁺,Tm³⁺ phosphor via employing fluorescence intensity ratio technology. The results show that the maximum absolute thermal sensitivity is ≈70 × 10^-4 K^-1 at 473 K under 980 nm excitation, with high and stable sensitivity of more than 60 × 10^-4 K^-1 over a wide temperature range of 333-493 K. In addition, at a much safer wavelength of 1550 nm, this sample achieves maximum absolute sensitivity of 56 × 10^-4 K^-1 at 453 K. Moreover, the BiVO₄:Er³⁺,Tm³⁺ phosphor presents high relative sensitivity of about 1.1% K^-1 under both 980 and 1550 nm excitation at 293 K. These results indicate that the BiVO₄ semiconductor oxide can be used as a novel host to achieve high UCL efficiency and promising thermal sensing performance, suggesting potential applications in the new fields of anti-counterfeiting, displays, and non-contact temperature sensors.

Atomic-Level and Modulated Interfaces of Photocatalyst Heterostructure Constructed by External Defect-Induced Strategy: A Critical Review

Despite the existence of numerous photocatalyst heterostructures, their separation efficiency and charge flow precision remain low due to the poor study on interfacial properties. The photocatalysts with confined defects can effectively control the photogenerated carrier migration, but the metastability of such defects considerably decreases the photocatalyst stability. Meanwhile, the introduction of defective region can increase the coordinative unsaturation and delocalize local electrons to promote their interactions with the molecules/ions in that region. The selective growth of modulated heterogeneous interface by defect-induced strategy may not only increase the stability of defective structures, but also enhance the migration of interfacial charges. Using this method, photocatalytic heterostructures with low contact resistances and intimate interfaces are constructed to achieve the optimal charge migration in terms of efficiency and accuracy. In this work, the point, linear, and planar heterogeneous interfaces and related defect engineering techniques are discussed. Particularly, it is focused on the external, defect-induced interfacial heterogeneities with various spatial and dimensional configurations, which exhibit modulated and controllable interfacial properties. Furthermore, the main aspects of fabricating photocatalyst heterostructures by the defect-induced strategy, including the i) controllable generation of defects, ii) advanced characterization methods, and iii) elaborate construction of the minimal interface, are described.

Rare earth elements based oxide ion conductors

Rare-earth-elements-based oxide ion conductors with various structures and their structure-property relationships were systematically presented and summarized, which can provide new insight and guidance for the development of new oxide ion conductors.

Top-down fabrication of Ti doped BiVO4 nanosheets for efficient water oxidation under visible light

Ti dopant introduces surface oxygen vacancies for the reactive sites for the OER in a top-down fabricated 2D BiVO4 nanosheet.

Modulated structure determination and ion transport mechanism of oxide-ion conductor CeNbO4+δ

CeNbO_4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen content (CeNbO₄, CeNbO_4.08, CeNbO_4.25, CeNbO_4.33) that shows mixed electronic and oxide ionic conduction, has been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic structures of CeNbO_4.08 and CeNbO_4.33. Here, we report the complex (3 + 1)D incommensurately modulated structure of CeNbO_4.08, and the supercell structure of CeNbO_4.33 from single nanocrystals by using a three-dimensional electron diffraction technique. Two oxide ion migration events are identified in CeNbO_4.08 and CeNbO_4.25 by molecular dynamics simulations, which was a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb₂O₉ units. However, the excess oxygen in CeNbO_4.33 hardly migrates because of the high concentration and the ordered distribution of the excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO_4+δ compounds elucidated here provides a direction for the performance optimization of these compounds.

Quasi-degenerate states and their dynamics in oxygen deficient reducible metal oxides

The physical and chemical properties of oxides are defined by the presence of oxygen vacancies. Experimentally, non-defective structures are almost impossible to achieve due to synthetic constraints. Therefore, it is crucial to account for vacancies when evaluating the characteristics of these materials. The electronic structure of oxygen-depleted oxides deeply differs from that of the native forms, in particular, of reducible metal oxides, where excess electrons can localize in various distinct positions. In this perspective, we present recent developments from our group describing the complexity of these defective materials that highlight the need for an accurate description of (i) intrinsic vacancies in polar terminations, (ii) multiple geometries and complex electronic structures with several states attainable at typical working conditions, and (iii) the associated dynamics for both vacancy diffusion and the coexistence of more than one electronic structure. All these aspects widen our current understanding of defects in oxides and need to be adequately introduced in emerging high-throughput screening methodologies.

Electrical properties of Ca12Al14O33 synthesised through polymer assisted deposition and glass crystallization routes

In this paper, two new methods, polymer assisted deposition (PAD) and glass crystallization (GC) methods were reported for the first time to prepare oxide ion conductor Ca₁₂Al₁₄O₃₃, with their microstructures and electrical properties compared to that of a sample prepared by the traditional solid-state reaction (SSR) method. The results showed that these two new methods are effective for improving ceramic densities and oxide ion conductivities compared to the SSR method, and the ceramic prepared from the GC method in the present work possessed the highest level of ∼94% of the theoretical density and ∼1.0 × 10⁻³ S cm⁻¹ at 900 °C for the bulk conductivity. A new bond valence method was applied to study the oxide ion conducting mechanism, which revealed an exchange process between the free oxide ion and the framework ion, and the wide windows connecting the Ca–Al–O framework cages were shown to be the key factor limiting oxide ion transport.

Ca₁₂Al₁₄O₃₃ material was prepared by two new methods with optimized microstructures and improved electrical properties compared to the solid-reaction method.

Cobalt doped BiVO4 with rich oxygen vacancies for efficient photoelectrochemical water oxidation

A facile electrodeposition method was developed to prepare Co-BiVO₄ thin films with rich oxygen vacancies. The resulting photoanode exhibited a photocurrent density of 3.5 mA cm⁻² 1.23 V (vs. reversible hydrogen electrode (RHE), AM 1.5 G), which is over two times higher than that of undoped BiVO₄.

A facile electrodeposition method was developed to prepare Co-BiVO₄ thin films with rich oxygen vacancies.

Versatile Nature of Oxygen Vacancies in Bismuth Vanadate Bulk and (001) Surface

Bismuth vanadate (BiVO₄) has emerged as one of the most promising photoanode materials for solar fuel production. Oxygen vacancies play a pivotal role in the photoelectrochemical efficiency, yet their electronic nature and contribution to  n-type conductivity are still under debate. Using first-principles calculations, we show that oxygen vacancies in BiVO₄ have two distinguishable geometric configurations characterized by either undercoordinated, reduced V^IVO₃ and Bi^IIO₇ subunits or a V^IV-O-V^IV/V bridge (split vacancy), quenching the oxygen vacancy site. While both configurations have similar energies in the bulk, the (001) subsurface acts like an energetic sink that stabilizes the split oxygen vacancy by ∼1 eV. The barrierless creation of a bridging V₂O₇ unit allows for partial electron delocalization throughout the near-surface region, consistent with recent experimental observations indicating that BiVO₄(001) is an electron-rich surface.

Disorder in La1−x Ba1+x GaO4−x/2 ionic conductor: resolving the pair distribution function through insight from first-principles modeling

Ionic conduction in dry LaBaGaO4 occurs through the vacant oxygen sites formed by the substitution of Ba for La. The resulting La1−x Ba1+x GaO4−x/2 solid solution shows significant disorder characteristics. The local structure of compositions x = 0, 0.20 and 0.30 was studied using the pair distribution function (PDF). Unfortunately, increasing peak overlap and the number of independent structural parameters make PDF modeling challenging when dealing with low-symmetry phases. To overcome this problem, density functional theory (DFT) was employed to create different structural models, each one with a different relative position for the substitutional Ba ion with respect to the oxygen vacancy. The atomic distributions generated by DFT were used as a starting point to refine experimental PDF data. All models result in the formation of Ga2O7 dimers, with their major axis oriented along the c axis. At the local scale, the most stable DFT model also provides the best fit of the PDF. This accounts for the dopant as first and second neighbors of the vacancy and of the O bridge in the dimer, suggesting that substitutional barium ions act as pinning centers for oxygen vacancies. Above 6 Å the average orthorhombic structure fits the PDF better than the DFT models, thus indicating that Ga2O7 dimers are not correlated with each other to form extended ordered structures. The combination of DFT simulations and X-ray diffraction/PDF refinements was used successfully to model the local atomic structure in La1−x Ba1+x GaO4−x/2, thus suggesting that this approach could be positively applied in general to disordered systems.

Hexagonal perovskite derivatives: a new direction in the design of oxide ion conducting materials

Various structural families have been reported to support oxide ion conductivity; among these, perovskite conductors have received particular attention. The perovskite structure is generally composed of a framework of corner-sharing octahedral units. When the octahedral units share their faces, hexagonal perovskites are formed. Mixed combinations of corner-sharing and face-sharing octahedral units can give rise to a variety of hexagonal perovskite derivatives. However, the ionic conducting properties of these materials have not been well explored. In this feature article, we review the conducting properties of the most significant hexagonal perovskite derivatives, with special focus on Ba3MM'O8.5. Ba3MM'O8.5 is the first hexagonal perovskite derivative to exhibit substantial oxide ion conductivity, and here we outline the structural features that are key for the oxide ion conduction within this system. The results demonstrate that further investigation of hexagonal perovskite derivatives could open up new directions in the design of oxide ion conductors.