B-site Cations Research Articles

Stabilizing ferroelectricity in alkaline-earth-metal-based perovskites (ABO3) via A- (Ca2+/Sr2+/Ba2+) and B-site (Ti4+) cationic radius ratio (RA /RB )

Various distortion parameters for alkaline-earth-metal-based perovskites (A 2+ B 4+O3) have been analyzed as a function of A- and B-site cationic radii R A and R B . The observed octahedral rotations and their associated mode amplitudes have shown an increasing trend with larger B-site cations, while a decreasing trend has been observed with larger A-site cations. Moreover, the analysis demonstrates that for incipient ferroelectrics like CaTiO3 and SrTiO3, having respective space groups Pnma (a − 0 b + 0 a − 0) and Pm 3 m (a 0 0 a 0 0 a 0 0), ferroelectric displacements are achieved via cation manipulation, which is governed by the R A /R B parameter. The increase in R A /R B through substitutions on the A site may suppress octahedral rotations as well as A-site anti-polar displacements in CaTiO3 and can consequently lead to a ferroelectrically distorted BaTiO3-like P4mm (a 0 0 a 0 0 c 0 +) phase via a cubic phase of SrTiO3, which has an intermediate R A /R B parameter. These results have been further corroborated by the calculated amplitudes of various frozen phonon modes associated with the cubic Pm 3 m Brillouin zone, responsible for symmetry breaking to tilt-oriented non-ferroelectric Pnma and ferroelectric P4mm phases.

High-pressure stabilisation of R = Y member of R2CuTiO6 double perovskite series

Double perovskite oxides of the A2B’B″O6 type with Jahn-Teller active Cu2+ as the B′ constituent have gained considerable research interest in recent years. For fundamental studies, the rare earth element (R) based systems such as R2CuTiO6 form an intriguing research platform as they allow systematic chemical-pressure studies by simply controlling the size of the R constituent. However, for the R2CuTiO6 compounds conventional ambient-pressure high-temperature synthesis yields an orthorhombic (Pnma) double perovskite structure for the largest R constituents (La–Gd) only, while the compounds with the smaller R:s adopt a hexagonal structure. Here we demonstrate a hexagonal-to-perovskite structure conversion for the R ​= ​Y compound achieved through a high-pressure (HP) high-temperature treatment at 4 ​GPa and 1000 ​°C. Structural details of the thus stabilized new double perovskite phase of Y2CuTiO6 are addressed through a combined DFT simulation and Rietveld refinement study, revealing signs towards the rare layered-type ordering of the B-site (Cu and Ti) cations. Similar to the previously reported R2CuTiO6 perovskite phases with R ​= ​La, Pr, and Nd, the R ​= ​Y member is found paramagnetic throughout the measured temperature range of 5–300 ​K. From UV–vis absorption measurements the optical bandgap is estimated to be ca 3.4 ​eV.

Complete B-site cation regulation accomplish UV-excited red phosphors in double perovskite oxides

As a widely explored red luminescent center, Eu3+ can reflect the features of the occupied lattice structure. Eu3+ ions energy levels and splitting in different hosts are mainly constrained by spin-orbit coupling and crystal field effects. This work reports an all-B-substituted perovskite-type red phosphor, and the role of the above two effects in this system is investigated by replacing Li+ (B1) and W6+ (B2) in BaLaLiWO6 with Na+ and Mo6+, respectively. Each sample's XRD and refinement results show that the all-B-site substitution does not affect the space group type; it only has a certain change in the unit cell parameters. The introduction of sodium ion makes the lattice expansion significantly enhance the intensity of the charge transfer band in the UV region. The introduction of Mo6+ significantly undermines the luminescence performance due to the influence of spin-orbit coupling. Subsequently, the symmetry changes of the position occupied by Eu3+ were verified by the red-orange ratio of Eu3+. The quantum yield of BaLaLi0·4Na0·6WO6: 0.30Eu3+ red phosphor under near ultraviolet excitation reaches 49.6%. Warm white light emission of color coordinates (0.363, 0.365) can be achieved by packaging with near UV chips, and the color rendering index can reach 83.3, which means that the phosphor has certain application prospects in WLED.

Realizing simultaneously excellent energy storage and discharge properties in AgNbO3 based antiferroelectric ceramics via La3+ and Ta5+ co-substitution strategy

AgNbO3 based antiferroelectric (AFE) ceramics have large maximum polarization and low remanent polarization, and thus are important candidates for fabricating dielectric capacitors. However, their energy storage performances have been still large difference with those of lead-based AFEs because of their room-temperature ferrielectric (FIE) behavior. In this study, novel La3+ and Ta5+ co-substituted AgNbO3 ceramics are designed and developed. The introduction of La3+ and Ta5+ decreases the tolerance factor, reduces the polarizability of B-site cations and increases local structure heterogeneity of AgNbO3, which enhance AFE phase stability and refine polarization-electric field (P–E) loops. Besides, adding La3+ and Ta5+ into AgNbO3 ceramics causes the decrease of the grain sizes and the increase of the band gap, which contribute to increased Eb. As a consequence, a high recoverable energy density of 6.79 J/cm3 and large efficiency of 82.1%, which exceed those of many recently reported AgNbO3 based ceramics in terms of overall energy storage properties, are obtained in (Ag0.88La0.04)(Nb0.96Ta0.04)O3 ceramics. Furthermore, the discharge properties of the ceramic with discharge time of 16 ns and power density of 145.03 MW/cm3 outperform those of many lead-free dielectric ceramics.

Synthesis and Properties of the Gallium-Containing Ruddlesden-Popper Oxides with High-Entropy B-Site Arrangement.

For the first time, the possibility of obtaining B-site disordered, Ruddlesden–Popper type, high-entropy oxides has been proven, using as an example the LnSr(Co,Fe,Ga,Mn,Ni)O4 series (Ln = La, Pr, Nd, Sm, or Gd). The materials were synthesized using the Pechini method, followed by sintering at a temperature of 1200 °C. The XRD analysis indicated the single-phase, I4/mmm structure of the Pr-, Nd-, and Sm-based materials, with a minor content of secondary phase precipitates in La- and Gd-based materials. The SEM + EDX analysis confirms the homogeneity of the studied samples. Based on the oxygen non-stoichiometry measurements, the general formula of LnSr(Co,Fe,Ga,Mn,Ni)O4+δ, is established, with the content of oxygen interstitials being surprisingly similar across the series. The temperature dependence of the total conductivity is similar for all materials, with the highest conductivity value of 4.28 S/cm being reported for the Sm-based composition. The thermal expansion coefficient is, again, almost identical across the series, with the values varying between 14.6 and 15.2 × 10−6 K−1. The temperature stability of the selected materials is verified using the in situ high-temperature XRD. The results indicate a smaller impact of the lanthanide cation type on the properties than has typically been reported for conventional Ruddlesden–Popper type oxides, which may result from the high-entropy arrangement of the B-site cations.

Accelerated kinetics of hydrogen oxidation reaction on the Ni anode coupled with BaZr0.9Y0.1O3-δ proton-conducting ceramic electrolyte via tuning the electrolyte surface chemistry

Owing to the promise of proton mobility in solids, proton ceramic fuel cells (PCFCs) have demonstrated their great potential for operating at the low temperature range of 400-600˚C. For speeding up electrode reactions in PCFCs, the kinetics of the hydrogen oxidation reaction (HOR) on the Ni anode coupled with BaZr0.9Y0.1O3-δ (BZY10) proton-conducting ceramic electrolyte (abbreviated as Ni/BZY10) was investigated in correlation to the electrolyte surface chemistry. It was found that thermal annealing treatments are an effective approach to tune the surface chemistry of BZY10 (ABO3 perovskite structure) electrolytes, resulting in progressively Ba segregation toward the outer surface and enriched Y cations in B-site at subsurface. As the post-annealing temperature of BZY10 electrolytes increase from 600˚C to 1500˚C, the polarization resistance for the corresponding BZY10-based half cells decreases by a factor of 3 and 6 at the testing condition of open circuit potential and anodic overpotential, respectively. Considering a core-space-charge layer model at the Ni/BZY10 interface, the segregation of net charged Y cations (YZr') at the subsurface can partially counterbalance the positive electric potential induced by accumulation of protons in the outer surface, led to a reduced electrical potential and thus the accelerated HOR kinetics on the Ni/BZY10.

Nickel Doping Manipulation towards Developing High-Performance Cathode for Proton Ceramic Fuel Cells

An ideal cathode for proton ceramic fuel cell (PCFC) should have superior oxygen reduction reaction activity, high proton conductivity, good chemical compatibility with electrolyte and sufficient stability, thus rational design of the electrode material is needed. Here, by taking advantage of the limited solubility of nickel in perovskite lattice, we propose a new dual phase cathode developed based on nickel doping manipulation strategy. We rationally design a perovskite precursor with the nominal composition of Ba(Co0.4Fe0.4Zr0.1Y0.1)0.8Ni0.2O3−δ (BCFZYN0.2). During high temperature calcination, a nanocomposite, composed of a B-site cation deficient and nickel-doped BCFZY perovskite main phase and nanosized NiO minor phase, is formed. The NiO nanoparticles effectively improve the surface oxygen exchange kinetics and the B-site cation deficiency structure enhances proton conductivity, thus leading to superior ORR activity of BCFZYN0.2. Furthermore, a low thermal expansion coefficient (15.3 × 10–6 K−1) is achieved, ensuring good thermomechanical compatibility the electrolyte. A peak power density of 860 mW cm−2 at 600 °C is obtained from the corresponding PCFC, and the cell operates stably for 200 h without any significant degradation. The proposing strategy, by providing a new opportunity for the development of highly active and durable PCFC cathodes, may accelerate the practical use of this technology.

The critical role of A, B-site cations and oxygen vacancies on the OER electrocatalytic performances of Bi0.15Sr0.85Co1−xFexO3−δ (0.2 ≤ x ≤ 1) perovskites in alkaline media

Development of perovskite-type electrocatalysts with high activity, excellent durability and affordable cost provides a promising solution to promote commercialization of clean energy technologies. Here we prepared Bi0.15Sr0.85Co1−xFexO3−δ (x = 0.2, 0.4, 0.6, 0.8, 1) perovskites by the conventional solid-state reaction route, and investigated their OER electrocatalytic activity and durability in 0.1 M KOH solution to reveal the critical role of A, B-site cations and oxygen vacancies. Results show the OER activity decreases with increasing x. The optimum composition, BiSC0.8F0.2, presents impressive electrocatalytic performances with low overpotential (330 mV on rotation disk electrode, and 283 mV on carbon paper electrode at 10 mA/cm2) and excellent long-time durability showing a small increase in the overpotential of 17 mV in 167.5 h. Comprehensive analysis and discussion on the bulk and surface properties reveal the critical role of A, B-site cations and oxygen vacancies on OER, and interrelations of Co2+ fraction, oxygen non-stoichiometry and the OER overpotential have been established. Most importantly, a highly A-site deficient surface layer is found on the as-synthesized perovskite, which facilitate OER by exposing more active B-site cations. This work not only reports a promising OER electrocatalyst with high activity and superior durability, but also provides perspectives on relational design for highly efficient and stable perovskite electrocatalysts for OER using solid-state reaction method.

Key role of tetragonality in the enhancement of ferroic response of modified Ba0.85Sr0.15Zr0.1Ti0.9O3

We examine the correlation of tetragonality factor and underlying ferroelectric response via performing B-site doping, more competent in comparison to A-site. B-site doping opens a route toward optimization of various physical parameters via promoting the off-centering of central ions. Here, we provide detailed investigation of the effect of doping in Ba0.85Sr0.15Zr0.1Ti0.9O3 (BSZT) modified with x wt. % CdO (x = 0, 1, 2) ceramics. Room temperature structural analysis emphasizes that tetragonality of the BSZT system increases with doping, which results in a realization of large spontaneous polarization and pinched PE loop by providing additional octahedral voids to move B-site cations. Additionally, Cd doping strengthens the anomaly in the Pyroelectric coefficient. This results in significant improvement in electrocaloric response and pyroelectric figure of merits. The electrocaloric response at ambient temperature enhances from 0.45 (x = 0%) to 0.69 J/Kg-K (x = 2%) ceramic. Cd doping improves energy storage efficiency and is found to be 61% (x = 0%) and 83% (x = 2%). A considerable change in the pyroelectric figure of merit is observed. Our study reveals that manipulation of tetragonality seems a suitable strategic route for developing effective pyroelectric and energy storage materials.

Boosting the stability of perovskites with exsolved nanoparticles by B-site supplement mechanism

Perovskites with exsolved nanoparticles (P-eNs) have immense potentials for carbon dioxide (CO2) reduction in solid oxide electrolysis cell. Despite the recent achievements in promoting the B-site cation exsolution for enhanced catalytic activities, the unsatisfactory stability of P-eNs at high voltages greatly impedes their practical applications and this issue has not been elucidated. In this study, we reveal that the formation of B-site vacancies in perovskite scaffold is the major contributor to the degradation of P-eNs; we then address this issue by fine-regulating the B-site supplement of the reduced Sr2Fe1.3Ni0.2Mo0.5O6-δ using foreign Fe sources, achieving a robust perovskite scaffold and prolonged stability performance. Furthermore, the degradation mechanism from the perspective of structure stability of perovskite has also been proposed to understand the origins of performance deterioration. The B-site supplement endows P-eNs with the capability to become appealing electrocatalysts for CO2 reduction and more broadly, for other energy storage and conversion systems.

Enhanced thermoelectric power factor led by isovalent substitution in Sr2CrMoO6 double perovskite

The strategy of using aliovalent substitution in A2B2O6 double perovskites remained the popular choice to enhance the charge carrier concentration in order to increase their electrical conductivity. In the present investigation, we have shown that the isovalent substitution in A-site can facilitate in manipulating the oxidation states of B-site transition metal cations in double perovskites, which in turn helps in increasing the carrier concentration. Further, using the strategy of manipulating valence states of B-site cations, we could enhance the thermoelectric (TE) power factor of double perovskites. Ceramic samples of Ba x Sr2−x CrMoO6 (x = 0.0, 0.1, 0.2, and 0.3) double perovskites have been synthesized via solid-state reaction route. The phase constitution and morphological study have been carried out via x-ray diffraction (XRD) and field scanning electron microscope (FESEM). Rietveld refinement of XRD data confirms the polycrystalline cubic structure with Pmm space group. Negative values of Seebeck coefficient have been observed for these oxides in the temperature range from room temperature to 1100 K, confirming electrons as the majority charge carriers. The electrical conductivity of Sr2CrMoO6 double perovskite is found to be increased by more than an order of magnitude due to isovalent Ba2+ doping in place of Sr2+. As a result, 5 times enhancement of TE power factor has been attained in Ba x Sr2−x CrMoO6. Charge transport mechanism of these double perovskites is found to be governed by the small polaron hopping conduction model. x-ray photoelectron spectroscopy spectra validate the presence of multivalent cations of Mo5+, Mo6+, Cr3+, and Cr6+ in these double perovskites. Furthermore, the detailed defect chemistry analysis suggests that owing to Ba substitution, Cr is oxidized from Cr3+ to Cr6+ oxidation states, which enhances the electron concentration and reduces the low mobility oxygen vacancies leading to dramatically improved electrical conductivity.

Cobalt doping in LaMnO3 perovskite catalysts – B site optimization by solution combustion for oxygen evolution reaction

The development of a highly active, stable, and cost-effective electrocatalyst for oxygen electrocatalysis has created a revolution in the field of sustainable energy research. Among them, noble metal free Lanthanum based perovskite got much attention as a catalyst for Oxygen Evolution Reaction (OER) due to their flexibility in the cation arrangement and tailorable electronic and ionic properties. In this study a rapid and cost effective solution combustion technique was used for the preparation of LaMn1-xCoxO3 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1). Following this method, Co doped LaMnO3 has been successfully prepared. From the results of X-ray photoelectron spectroscopy (XPS), it was found that the high Co content levels activate the surface enhancement in B-site cation in their respective higher oxidation state (Mn4+ and Co3+). It was observed that, compared with other catalysts, LaMn0.2Co0.8O3 had good stability and shows acceptable OER activity with a lower overpotential of 430 mV at 10 mA/cm2 and a lower Tafel slope of value 57.95 mV/dec. This study provides a simple cost effective synthesis route for the preparation of perovskite material and also offers a path for enhancing OER activity by B-site cation substitution.

High-entropy Sm2B2O7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides with highly disordered B-site cations for ultralow thermal conductivity

• A family of high-entropy Sm2B2O7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides with highly disordered cations on the B-sites has been synthesized by introducing large atomic-size mismatch, and mass and charge disorder at B-sites. • The high-entropy Sm2(Nb0.2Sn0.2Ti0.2Y0.2Zr0.2)2O7 and Sm2(Nb0.2Ta0.2Y0.2Yb0.2Zr0.2)2O7 exhibit ultralow thermal conductivity of 1.35 W•m−1•K−1 and 1.23 W•m−1•K−1, respectively. • The ultralow thermal conductivity of Sm2B2O7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides can be attributed to the highly disorder on the B-sites including atomic-size mismatch, mass fluctuations and strain field fluctuations together with oxygen vacancies . • The high-entropy fluorite Sm2(Nb0.2Ta0.2Y0.2Yb0.2Zr0.2)2O7 also shows average thermal expansion coefficient of 10.2 °C−1 and high-temperature stability up to 1600 °C in air. Materials with ultralow thermal conductivity and good thermal stability are of great interest in numerous applications such as energy storage and conversion devices, and thermal insulation components. In this work, a family of high-entropy Sm 2 B 2 O 7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) oxides with highly disordered cations on the B-site has been synthesized by introducing large atomic-size mismatch, mass and charge disorder. Through tuning the composition, the high-entropy Sm 2 B 2 O 7 oxides can be engineered from pyrochlore to fluorite structure, accompanied with an order-disorder transition. The pyrochlore Sm 2 (Nb 0.2 Sn 0.2 Ti 0.2 Y 0.2 Zr 0.2 ) 2 O 7 and fluorite Sm 2 (Nb 0.2 Ta 0.2 Y 0.2 Yb 0.2 Zr 0.2 ) 2 O 7 exhibit low thermal conductivities of 1.35 W·m −1 ·K −1 and 1.23 W·m −1 ·K −1 , respectively, indicating their good thermal insulation. In addition, the high-entropy fluorite Sm 2 (Nb 0.2 Ta 0.2 Y 0.2 Yb 0.2 Zr 0.2 ) 2 O 7 also shows average thermal expansion coefficient of 10.2 × 10 −6 °C −1 and high-temperature stability even after thermal exposure at 1600 °C in air for 30 h. These results indicate that the high-entropy Sm 2 B 2 O 7 (B=Ti, Zr, Sn, Hf, Y, Yb, Nb, and Ta) can be promising candidates for thermal barrier coatings and thermally insulators.

Enhancing Comprehensive Energy Storage Properties in Tungsten Bronze Sr0.53Ba0.47Nb2O6-Based Lead-free Ceramics by B-Site Doping and Relaxor Tuning.

Dielectric ceramics with relaxor characteristics are promising candidates to meet the demand for capacitors of next-generation pulse devices. Herein, a lead-free Sb-modified (Sr0.515Ba0.47Gd0.01) (Nb1.9-xTa0.1Sbx)O6 (SBGNT-based) tungsten bronze ceramic is designed and fabricated for high-density energy storage capacitors. Using a B-site engineering strategy to enhance the relaxor characteristics, Sb incorporation could induce the structural distortion of the polar unit BO6 and order-disorder distribution of B-site cations as well as the modulation of polarization in the SBGNT-based tungsten bronze ceramic. More importantly, benefiting from the effective inhibition of abnormal growth of non-equiaxed grains, Sb introduction into SBGNT-based ceramics could effectively suppress the conductivity and leakage current density, enhancing the breakdown strength, as proved by the electrical impedance spectra. Consequently, a remarkable comprehensive performance via balancing recoverable energy density (∼3.26 J/cm3) and efficiency (91.95%) is realized simultaneously at 380 kV/cm, which surpasses that of the pristine sample without the Sb dopant (2.75 J/cm3 and 80.5%, respectively). The corresponding ceramics display superior stability in terms of fatigue (105 cycles), frequency (1∼200 Hz), and temperature (20∼140 °C). Further charge-discharge analysis indicates that a high power density (89.57 MW/cm3) and an impressive current density (1194.27 A/cm2) at 150 kV/cm are achieved simultaneously. All of the results demonstrate that the tungsten bronze relaxors are indeed gratifying lead-free candidate materials for dielectric energy storage applications.

Structural evolution and ferroelectric properties of relaxor ferroelectric single crystal Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 under high pressure

Relaxor ferroelectric crystals of lead magnesium niobate–lead titanate (PMN-xPT) have attracted great attention due to their extraordinary dielectric, piezoelectric, and electromechanical properties. PMN-xPT shows different relaxor behavior and structural phase transitions in a wide temperature and chemical component range. Here, we studied spectroscopy and ferroelectricity of PMN-0.28PT under high pressure. The appearance of a new Raman band and the sudden redshift of UV-vis absorption edge imply a structural phase transition at about 8 GPa. More importantly, the ferroelectricity of the sample is suppressed above a pressure of 5 GPa, and there is no ferroelectricity under further compression. We suggest that the disappearance of ferroelectricity may be related to the polar nanoregions being suppressed by pressure. Our observations of ferroelectricity disappearance above 5 GPa indicate the B-site cation rearrangement in the several nanometers region.

Compositional-induced structural transformation and relaxor ferroelectric behavior in Sr/Nb-modified Bi4Ti3O12 Aurivillius ceramics

This study reports the synthesis of Sr/Nb-modified Bi4Ti3O12 Aurivillius phase, SrxBi4-xTi3-xNbxO12 (x = 0, 0.5, 1, 1.5, and 2) for lead-free relaxor ferroelectric materials using the molten-salt method. The effects of cation substitution (x content) on the structure, morphology, electrical and optical properties was investigated. X-ray diffraction and dielectric measurements revealed that the phase structures of the parent compound Bi4Ti3O12 (x = 0) experienced a gradual transformation from the orthorhombic B2cb phase to the tetragonal I4/mmm phase, for x = 1.5 and 2. Upon increasing the x content, the Curie-Weiss temperature (Tc) and dielectric constant decreased, which are explained in terms of reducing the structural distortion of orthorhombic structure. The cation substitution induced the A- and B-site cation disorder and intensified diffusion and relaxor-ferroelectric behavior. These findings will provide significant impetus for further research on relaxor ferroelectric Aurivillius phases.

Structure and properties of the Sr2In1-xSnxSbO6 double perovskite

A series of n-type oxide double perovskite semiconductors, Sr2In1-xSnxSbO6 (0 ​≤ ​x ​≤ ​0.3) has been synthesized; Sn4+ partially substitutes for In3+. 121Sb and 119Sn Mössbauer spectroscopy are employed to investigate the B-site cation ordering because this issue cannot be resolved by conventional diffraction techniques alone. Rigid ordering between In3+/Sn4+ and Sb5+ sites is revealed by the spectroscopic method, and hence in combination with the structural parameters extracted from the XRD structural refinements, the crystallographic structure of this series of compounds is depicted. The temperature dependent magnetic susceptibilities, band gaps, and carrier type are characterized, and the calculated band structure is presented.

Near-ideal electromechanical coupling in textured piezoelectric ceramics

Electromechanical coupling factor, k, of piezoelectric materials determines the conversion efficiency of mechanical to electrical energy or electrical to mechanical energy. Here, we provide an fundamental approach to design piezoelectric materials that provide near-ideal magnitude of k, via exploiting the electrocrystalline anisotropy through fabrication of grain-oriented or textured ceramics. Coupled phase field simulation and experimental investigation on <001> textured Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 ceramics illustrate that k can reach same magnitude as that for a single crystal, far beyond the average value of traditional ceramics. To provide atomistic-scale understanding of our approach, we employ a theoretical model to determine the physical origin of k in perovskite ferroelectrics and find that strong covalent bonding between B-site cation and oxygen via d-p hybridization contributes most towards the magnitude of k. This demonstration of near-ideal k value in textured ceramics will have tremendous impact on design of ultra-wide bandwidth, high efficiency, high power density, and high stability piezoelectric devices.

Structure characteristics and enhanced microwave dielectric properties of Ba(Mg1/3Nb2/3)O3–Mg2SiO4 composite ceramics with a tunable τf

The performance of microwave communication devices widely used in 5G network is strongly determined by the properties of dielectric ceramics used in the fabrication, which are required to have a suitable dielectric constant (εr), high Q × f value and near-zero temperature coefficient of resonant frequency (τf). In this work, (1-x)Ba(Mg1/3Nb2/3)O3–xMg2SiO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5) composite ceramics meeting the application requirements have been developed and investigated. Ba(Mg1/3Nb2/3)O3 and Mg2SiO4 could coexist without any unexpected phases due to the different crystal structures and coordination relationship. The sintering behavior and the structural changes of Ba(Mg1/3Nb2/3)O3 were specifically relevant to the microwave dielectric properties. Introducing appropriate Mg2SiO4 content promoted the sintering of Ba(Mg1/3Nb2/3)O3 and the maximum Q × f value of 110,000 GHz was obtained at the sample with x = 0.3 sintered at the optimized temperature of 1420 °C. Due to the low dielectric loss and the opposite sign τf value of Mg2SiO4, the composite ceramic successfully adjusted the τf value to near zero while maintaining a high Q × f value. However, the experimentally measured values of εr and τf were significantly different from the theoretically calculated values due to the “rattling” effect of B-site cations and the enhanced symmetry of oxygen octahedra in Ba(Mg1/3Nb2/3)O3. These structural changes were confirmed by the results of bond valence analysis and Raman vibration spectra. The 0.6Ba(Mg1/3Nb2/3)O3–0.4Mg2SiO4 ceramic sintered at 1420 °C exhibits competitive microwave dielectric properties: εr = 18.9, Q × f = 91,000 GHz (at 8.016 GHz) and τf = 2.6 ppm/°C, which indicates it a promising application prospect of microwave communication.

The Perfect Imperfections in Electrocatalysts.

Modern day electrochemical devices find applications in a wide range of industrial sectors, from consumer electronics, renewable energy management to pollution control by electric vehicles and reduction of greenhouse gas. There has been a surge of diverse electrochemical systems which are to be scaled up from the lab-scale to industry sectors. To achieve the targets, the electrocatalysts are continuously upgraded to meet the required device efficiency at a low cost, increased lifetime and performance. An atomic scale understanding is however important for meeting the objectives. Transitioning from the bulk to the nanoscale regime of the electrocatalysts, the existence of defects and interfaces is almost inevitable, significantly impacting (augmenting) the material properties and the catalytic performance. The intrinsic defects alter the electronic structure of the nanostructured catalysts, thereby boosting the performance of metal-ion batteries, metal-air batteries, supercapacitors, fuel cells, water electrolyzers etc. This account presents our findings on the methods to introduce measured imperfections in the nanomaterials and the impact of these atomic-scale irregularities on the activity for three major reactions, oxygen evolution reaction (OER), oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Grain boundary (GB) modulation of the (ABO3 )n type perovskite oxide by noble metal doping is a propitious route to enhance the OER/ORR bifunctionality for zinc-air battery (ZAB). The perovskite oxides can be tuned by calcination at different temperatures to alter the oxygen vacancy, GB fraction and overall reactivity. The oxygen defects, unsaturated coordination environment and GBs can turn a relatively less active nanostructure into an efficient redox active catalyst by imbibing plenty of electrochemically active sites. Obviously, the crystalline GB interface is a prerequisite for effective electron flow, which is also applicable for the crystalline surface oxide shell on metal alloy core of the nanoparticles (NPs). The oxygen vacancy of two-dimensional (2D) perovskite oxide can be made reversible by the A-site termination of the nanosheets, facilitating the reversible entry and exit of a secondary phase during the redox processes. In several instances, the secondary phases have been observed to introduce the right proportion of structural defects and orbital occupancies for adsorption and desorption of reaction intermediates. Also, heterogeneous interfaces can be created by wrapping the perovskite oxide with negatively charged surface by layered double hydroxide (LDH) can promote the OER process. In another approach, ion intercalation at the 2D heterointerfaces steers the interlayer spacing that can influence the mass diffusion. Similar to anion vacancy, controlled formation of the cation vacancies can be achieved by exsolving the B-site cations of perovskite oxides to surface anchored catalytically active metal/alloy NPs. In case of the alloy electrocatalysts, incomplete solid solution by two or more mutually immiscible metals results in heterogeneous alloys having differently exposed facets with complementary functionalities. From the future perspective, new categories of defect structures including the 2D empty spaces or voids leading to undercoordinated sites, the multiple interfaces in heterogeneous alloys, antisite defects between anions and cations, and the defect induced inverse charge transfer should bring new dimensionalities to this riveting area of research.