Effect Of B-site Substitution Research Articles

The influence of processing methods on the dielectric properties of BaTi1-xGdxO3-x/2 - Based materials

In this study, the intermediate rare-earth oxide Gd2O3 (Gd) was substituted in different amounts (x = 0.2–2 mol%) for the formulation of BaTi1-xGdxO3-x/2 (BTGx) dielectric materials. The effect of B-site substitution was confirmed by the additional Raman active A1g octahedral peak at ~835cm-1 strengthened at x ≥ 0.4 mol%. Additionally, properties of 0.9BTG0.007-0.1BA dielectric ceramics were analysed based on the influence of various processing methods as a function of sintering temperature. The focal samples were labelled Method-A (direct-mix) and Method-B (indirect-mix). As the sintering temperature (1075–1200 °C) increased, the 1 kHz response of the ε–T curves of Method-A samples transformed from a single peak to broad-narrow double peaks of high dielectric loss tangent (tan δ). Nonetheless, samples of Method-B possessed a clearly defined transmission electron microscopy (TEM) core-shell structure, flattened double-peak ε-T curves, optimised dielectric properties (ε = ~1563–1851 and tan δ < 1.5% at room temperature), and a wide-ranging temperature behaviour that meets the X8R dielectric standards (ΔC/C25°C < ±15%). The maximum dielectric breakdown strength of Method-B samples reached ~131 kVcm, while the energy storage density was ~0.726 J/cm3 at a maximum efficiency of ~80% at 1100 °C. Thus, exhibiting good potentials for balancing temperature stability with energy storage applications.

Structural, magnetic and magnetocaloric effect studies of Nd0.6Sr0.4AxMn1-xO3 (A=Co, Ni, Zn) perovskite manganites

The effect of B-site substitution by other transition elements on the structural, magnetic, and magnetocaloric effect properties of Nd0.6Sr0.4AxMn1-xO3 (x = 0.1 and x = 0.2, A=Co, Ni, and Zn) nanopowders has been reported in this study. Auto-combustion sol-gel method was used to synthesize all the nanopowder samples. Room temperature X-ray diffraction shows that all the nanopowders have orthorhombic structure of Pnma space group crystallography, confirming the purity of the single phase. Also, the average nanosized scale of these powder samples is ~ 43 nm, revealing a spherical shape with a packed and homogenous structure. The Mn4+/Mn3+ ratio in these nanopowder compounds is highly dependent on the dopants concentration (x), which shows a significant diminishes as the dopants introduce into the parent compound (Nd0.6Sr0.4MnO3). The decrease in the Mn4+/Mn3+ ratio enhances in the antiferromagnetic (AFM) super exchange interaction (SE) due to the lack of Mn3+— O—Mn4+ ferromagnetic (FM) double exchange (DE). The magnetization measurements indicate that all samples exhibit a ferromagnetic (FM) to paramagnetic (PM) transition with increasing temperature. The Curie temperature (TC) is also affected by the dopants concentrations where considerable decrease has been noticed for all compounds compared to the parent compound. The M-H curves for all compounds reveal PM behavior at 300 K, whereas FM behavior characterized the magnetic hysteresis at low temperature (2 K) which also shows the presence of exchange bias effect. The magnetic entropy change |−∆SMMax| has shown bigger effect for Co dopant (5.92 J/kg.K) compared to Ni and Zn dopants with values of 5.19 J/kg.K, and 4.28 J/kg.K respectively at an applied magnetic field of 9 T. Despite substituting at Mn-site by other transition elements lower the Tc, |−∆SMMax| and RCP, the obtained results are promising for magnetic cooling materials and further investigation.

Effect of B-site substitution on the crystal structure, electrical conductivity and oxygen transport properties of La0.5Sr0.5M0.2Fe0.8O3-δ (M = Co, Al, and Zn) perovskite

The crystal structure, electrical conductivity, and oxygen transport coefficients, namely chemical bulk diffusion coefficient (Dchem), and surface exchange coefficient (Kchem) of La0.5Sr0.5M0.2Fe0.8O3-δ (M ​= ​Co, Al, Zn) perovskite oxides systematically investigated. B-site substituted oxides exhibited a distorted rhombohedral crystal structure. The lattice volume and oxygen nonstoichiometry in La0.5Sr0.5Al0.2Fe0.8O3-δ (LSAF) or La0.5Sr0.5Zn0.2Fe0.8O3-δ (LSZF) surrogates increases as compared to La0.5Sr0.5Co0.2Fe0.8O3-δ (LSCF). X-ray photoelectron spectroscopic (XPS) study indicates the variation of B-site cations oxidation state and oxygen vacancy concentration of the synthesized perovskite oxides as a function of substituents. The microstructural study on samples sintered under identical conditions revealed grain size refinement in the LSAF sample as compared to others. Studies on electrical conductivity and oxygen transport properties of LSAF samples showed an increase in conductivity and a decrease in Dchem and Kchem parameters with increasing the grain size. LSAF samples exhibited enhanced electrical conductivity and Dchem and Kchem values as compared to LSZF samples when the grain size kept identical and correlated with the defects present in the samples. The study further suggests that variation in the functional properties of these perovskite oxides when sintered under identical conditions is a combined effect of grain size and composition.

Effect of B-site substitution on structural, magnetic and optical properties of Ho2Ti2O7 pyrochlore oxide

Ho2Ti2O7 is a prominent quantum material, known for its exotic spin ice state, where structural and electronic properties are coupled. We report the systematic examination of the effects of Fe substitution at the non-magnetic Ti site (x = 0, 0.05, 0.075, 0.1, 0.125 and 0.15) in Ho2Ti2O7 on the structural, optical and magnetic properties. It is found that the optical properties of polycrystalline Ho2Ti2-xFexO7-δ (x = 0, 0.05, 0.075, 0.1, 0.125 and 0.15) can be tuned through partial substitution of the trivalent Fe3+ ion for the tetravalent Ti4+ ion. This Fe substitution not only creates charge-compensating oxygen vacancies but also modifies the optical band gap. A systematic structural, X-ray photoelectron spectroscopy, magnetic and optical analysis reveals that above a critical content of x = 0.1, a possible reduction in the oxygen coordination number (octahedral to tetrahedral) occurs, which appears in the form of high spin to low spin crossover for a fraction of the substituted Fe3+ ions. These results provide a new route to tune the optical band gap and absorption coefficient of Ho2Ti2O7 for magneto-optical applications.

Electrochemical oxygen reduction on layered mixed metal oxides: Effect of B-site substitution

Oxygen reduction reaction (ORR) is critical in advancing the frontiers of electrochemical energy conversion and storage technologies. To date, Pt-based electrocatalysts exhibit state-of-the-art performance for low temperature ORR, but are hindered by their high cost. Therefore, it is imperative to find cost-effective alternatives for the practical realization of these technologies. Herein, we investigate the effect of layered, mixed metal oxides belonging to first-series Ruddlesden-Popper (R-P) phases with a general composition of A2BO4 (A=rare earth/alkaline earth metals; B=3d transition metal) for ORR in alkaline enviroment of anion exchange membrane fuel cells. We show that the nature of the B-site 3d transition metal in these oxides is critical in tuning the ORR activity. Using a combination of well-controlled synthesis, chemical, as well as electrochemical studies, we find that substitution of the Ni B-site in these oxides with nominal amounts of Mn and Co (La2Ni0.88Mn0.12O4 and La2Ni0.88Co0.12O4, respectively) leads to enhanced overall ORR activity by selective promotion of the electrochemical reduction of hydrogen peroxide, an intermediate in the pseudo 4e− ORR path. The reported findings can be used in designing nonprecious metal-based oxide electrocatalysts for oxygen reduction in alkaline anion exchange memebrane fuel cells.

Sn promoted BaFeO3−δ catalysts for N2O decomposition: Optimization of Fe active centers

A series of BaFe1−xSnxO3−δ catalysts were prepared by sol-gel method and tested for N2O decomposition to shed light on the effect of B-site substitution on the catalytic behavior of perovskite catalysts. 119Sn and 57Fe Mössbauer results confirmed that the 5-fold coordinated Fe3+ cations with one adjacent oxygen vacancy (Fe3+O5) were the main active centers for N2O decomposition. Doping of Sn cations can significantly improve the percentage of Fe3+O5 from 30% (x=0) to 68% (x=0.8). More importantly, the valence state of Fe could be gradually reduced due to weakening of FeO bond with increasing the Sn content, which was attributed to the stronger force of Sn than Fe in FeOSn structure to draw the oxygen anion and expansion of unit cell volume. Such change of Fe chemical state favored the oxygen mobility of the catalyst, leading to reduction in activation energy for N2O decomposition from ca. 241 (x=0) to 178kJmol−1 (x=0.8). BaFe0.2Sn0.8O3−δ catalyst exhibited the highest intrinsic rate of 1.49s−1 (550°C), nearly 4 times larger than that of BaFeO3−δ (0.43s−1).

The effect of B-site substitution on the structural evolution and electrical properties of lead-free (K,Na)NbO3 ceramics

Lead-free (K0.49Na0.51)(Nb1−xSbx)O3 piezoelectric ceramics were prepared via the conventional sintering method and the effect of the substitution of Nb with Sb on the phase structure, microstructure and electrical properties of the prepared (K0.49Na0.51)(Nb1−xSbx)O3 ceramics was systematically investigated. The prepared ceramics exhibited a single-phase perovskite structure which changed from a standard orthorhombic structure to a pseudocubic structure as x was increased from 0 to 0.1. X-ray diffraction patterns and Raman spectra obtained for the prepared ceramics clearly revealed that the degree of structural symmetry increased with x. Substituting an appropriate amount of Sb5+ for Nb5+ was found to improve the microstructure and thereby enhance the piezoelectric/ferroelectric properties. Further increasing the Sb content resulted in a decrease of the average grain size and a deterioration of the performance. The peak values of the piezoelectric constant d33 (182 pC/N) and the electromechanical coupling coefficient kp (41%) were obtained for the ceramic with x=0.06.

The effect of B-site substitution on structural transformation and ionic conductivity in Ho2(ZryTi1−y)2O7

Compounds in the pyrochlore systemHo2(ZryTi1−y)2O7 exhibit an order-disorder transition from pyrochlore to a defect-fluorite type structure. Compositions in this system were prepared via mechanical milling, followed by a two-step sintering process. Structural characterization was carried out via Rietveld refinements using neutron powder diffraction data, supported by X-ray diffraction to determine the phase and location of the pyrochlore-fluorite transformation. Unit-cell parameters were determined for the whole series using Rietveld refinements as well as the Nelson–Riley function. The neutron refinement results confirmed that the cation disorder was independent of the anion Frenkel disorder. The relation between the x-parameter in the oxygen 48f position and anion Frenkel disorder was found to be linear for the pyrochlore structure. The ionic conductivity studies were undertaken via AC impedance analysis to determine the electronic behaviour and its relation to the structural change in the temperature range 300°C–700 °C. The trends in ionic conductivity and activation energy were explained structurally via neutron powder diffraction and X-ray diffraction data. The pyrochlore-fluorite boundary composition (at y = 0.5) exhibited the lowest activation energy and highest ionic conductivity.

The Effect of Cation Substitution on Chemical Stability of Ba0.5Sr0.5Co0.8Fe0.2O3-δ -Based Mixed Conductors

Cubic perovskite Ba0.5Sr0.5Co0. 8Fe0.1R0.1O3–δ (R=Fe, Nb, Mo, Sb) was synthesized, and the chemical stability under high oxygen partial pressure (p(O2)) was studied by using HT-XRD in the temperature range of 773-1373 K. It was found that Ba0.5Sr0.5Co0. 8Fe0.1Sb0.1O3–δ was remarkably stable than Ba0.5Sr0.5Co0. 8Fe0.2O3–δ and Ba0.5Sr0.5Co0. 8Fe0.1Mo0.1O3–δ in high p(O2) condition. The effect of B-site substitution on the CO2-tolerance of Ba0.5Sr0.5Co0. 8Fe0.1R0.1O3–δ was also investigated by TG-DTA analysis at 300-1373 K. It was demonstrated that the chemical stability of Ba0.5Sr0.5Co0. 8Fe0.2O3–δ against CO2 could be significantly improved when Fe were partially substituted by Nb or Sb ions.

Effect of B-site substitution on the stability of La0.2Sr0.8Fe0.8B0.2O3 − δ, B = Al, Ga, Cr, Ti, Ta, Nb

Single phase, cubic perovskites of composition La0.2Sr0.8Fe0.8B0.2O3−δ, B=Al, Ga, Cr, Ti, Ta and Nb, were prepared by spray pyrolysis from aqueous precursor solutions. The effect of B-site substitution on the stability in a H2 containing atmosphere was investigated using temperature programmed reduction (TPR). High temperature X-ray diffraction (HT-XRD) was performed under conditions similar to those in the TPR experiments. La0.2Sr0.8FeO3−δ forms a brownmillerite-type anion-deficient ordered perovskite upon reduction at T≈500. B-site substitution suppresses oxygen vacancy ordering, maintaining a perovskite structure under the above reducing conditions up to T≈700°C. Trends in the kinetics of decomposition of B-site substituted La0.2Sr0.8FeO3−δ are discussed.

Effect of B-site substitution by (Ni1/3Nb2/3)4+ for Ti4+ on microstructure and piezoelectric properties in (Bi1/2Na1/2)TiO3 piezoelectric ceramics

To develop new (Bi 1/2 Na 1/2 )TiO 3 -based ceramics with optimum piezoelectric properties, lead-free piezoelectric ceramics of (Bi 1/2 Na 1/2 )Ti 1− x (Ni 1/3 Nb 2/3 ) x O 3 with x = 0, 0.01, 0.02, 0.03 and 0.04 have been synthesized by using a two-step calcination approach. The effect of B-site substitution by the complex (Ni 1/3 Nb 2/3 ) 4+ for Ti 4+ on microstructure and piezoelectric properties was investigated. X-ray diffraction analysis shows that the materials possess a pure perovskite structure. The grain size decreases and the diffuse phase transition behavior becomes more evident with increasing x . The piezoelectric constant d 33 increases and the planar electromechanical coupling factor k p exhibits no obvious enhancement with the increasing content of (Ni 1/3 Nb 2/3 ) 4+ substituting Ti 4+ in (Bi 1/2 Na 1/2 )TiO 3 ceramics. A maximum d 33 value of 101 pC/N is obtained at x = 0.03. The mechanism of piezoelectric properties changes was also discussed according to the domain-wall contributions to the piezoelectric response.