A-and B-site Research Articles

(Invited) Multi-Element-Doped Ceria-Based Metal Oxides for Advanced Proton Conducting SOFCs

The ability of solid oxide fuel cells (SOFC) to convert the chemical energy of various kinds of fuels into electricity at high efficiency, and with reduced environmental impacts makes them an attractive technology for current and future plans for clean power generation. State-of-the-art yttria-stabilised ZrO2 (YSZ) electrolyte shows high conductivity (10-2 S/cm) at about 1000 °C [1]. However, the high operating temperature leads to durability and cost issues and hinders its full market implementation. Operating temperature of SOFCs can be lowered by employing ceramic proton conducting electrolytes based on doped- BaCeO3 which exhibits high proton conductivity (10-2 S/cm) in the intermediate temperature (IT, 400-700 °C) range [2]. However, the poor chemical stability of doped-BaCeO3 under SOFC by-products CO2, and H2O limits its use as stable electrolyte [2]. Here we report, perovskite–type Ba0.5Sr0.5Ce1-x-y-z Zr x Gd y Y z O3-δ as proton conductors for IT-SOFCs [3]. A-and B-site of BaCeO3 were doped by more electronegative elements to improve its chemical stability under H2O and CO2 at elevated temperature. In terms of chemical stability and conductivity, Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ seems to be the optimal composition with conductivity of 10-3 S/cm at 700 °C in 3% H2O/H2. Open circuit voltage of 1.15 V at 700 °C for H2-air cell suggests pure ionic (proton) conduction in Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3-δ [3]. The effect of sintering temperature on bulk and grain boundary conductivity of these oxides was investigated using dielectric loss spectroscopy [4]. Moreover, the difference in the relaxation times in the current study suggests that short-range and long-range proton dynamics seems to be differing to previous studies on Y-doped BaZrO3 systems [5]. Additionally, layered perovskite-type Gd0.5Pr0.5BaCo2O5+δ were characterised as cathode for H-SOFCs [6]. Symmetrical cell measurements under air and wet air gave an area specific resistance of 2.4 Ω cm2 and 1.9 Ω cm2 for oxygen reduction reaction at 700 °C [6]. The effect of phase purity and synthesis methods on the electrochemical performance of Ni+Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3 -δ anode composites was investigated through symmetrical cell measurements in 3% H2O/H2. References 1) J. W. Fergus, R. Hui, X. Li, D. P. Wilkinson, J. Zhang, Solid Oxide Fuel Cells: Materials Properties and Performance, CRC Press, New York (2009). 2) K. D. Kreuer, Annu. Rev. Mater. Res. 33 (2003) 333. 3) R. Kannan, K. Singh, S. Gill, T. Fürstenhaupt, V. Thangadurai, Sci. Reports. 3 (2013) 2138. 4) K. Singh, A. Baral, V. Thangadurai, J. Am. Ceram. Soc. 99 (2016) 866. 5) Y. Yamazaki, F. Blanc, Y. Okuyama, L. Buannic L, J. C. Lucio-Vega, C. P. Grey, S. M. Haile, Nat. Mater. 12 (2013) 647. 6) K. Singh, A. K. Baral, V. Thangadurai, Solid State Ionics (2016) DOI:10.1016/j.ssi.2015.12.010.

Microstructural And Dielectric Investigations Of Vanadium Substituted Barium Titanate Ceramics

In this investigation, the microstructural and dielectric properties of pure BaTiO3 and vanadium (V 5+ ) substituted on Ba 2+ site (A-site) and Ti 4+ site (B-site) in BaTiO3 ceramic have been studied. The three compositions of BaTiO3 (BT), Ba0.9V0.1TiO3 (BTA) and BaTi0.9V0.1O3 (BTB) were synthesized using solid-state reaction route. The XRD analysis of all three compositions has been carried out at room temperature and proper phase formation for BT, BTA and BTB are confirmed. However, compositions BTA and BTB indicate the presence of secondary phases, and it may be due to higher amount of vanadium substitution at A and B sites. Addition of vanadium inhibited the grain growth of BaTiO3 ceramic. Vanadium substitution on Aand B-site have resulted in decrease of Curie temperature as well as dielectric loss compared to pure BT. A more diffused behavior is observed in vanadium substituted samples as compare to pure BT which shows a sharp transition and lower value of diffuseness parameter. Impedance study shows that substitution of vanadium on A- as well as B-site results in decrease of AC conductivity. These properties of vanadium substituted samples can be utilized to reduce the dielectric loss in capacitors and in radio frequency applications. Copyright © 2016 VBRI Press.

Synthesis and characterisation of ceramic proton conducting perovskite-type multi-element-doped Ba0.5Sr0.5Ce1−x−y−zZrxGdyYzO3−δ (0 < x < 0.5; y = 0, 0.1, 0.15; z = 0.1, 0.2)

This study addresses the influence of A-and B-site co-doping on the chemical and electrical properties of perovskite-type Ba0.5Sr0.5Ce1−x−y−zZrxGdyYzO3−δ (0 < x < 0.5; y = 0, 0.1, 0.15; z = 0.1, 0.2). The powders synthesised at 1450 °C through solid state (ceramic) reaction showed relative density greater than 94% by Archimedes method, and is supported by dense microstructure using scanning electron microscopy images. The effect of doping electronegative elements on the A-and B-site on chemical stability is exemplified by dopant dependent chemical stability under CO2 and water vapour. Thermogravimetric analysis (TGA) of the powdered samples under CO2: N2 (1:1) showed variation in weight gain with respect to doping element concentration in Ba0.5Sr0.5Ce1−x−y−zZrxGdyYzO3−δ, revealing the improved chemical stability of Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3−δ and Ba0.5Sr0.5Ce0.5Zr0.3Gd0.1Y0.1O3−δ. Excellent chemical stability under water vapour at 90 °C for 24 h was observed for all the investigated compositions. However, extended exposure time of 168 h lead to the appearance of a small amount of Ba(OH)2, as observed from the powder X-ray diffraction patterns and TGA analysis after stability tests. The electrical conductivity measurements by ac impedance spectroscopy under dry and humid atmospheres revealed the proton conductivity. Among the samples investigated in this work, Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3−δ (BSCZGY3) showed the highest electrical conductivity of ∼10−3 S/cm at 700 °C in air/3% H2O and H2/3% H2O.

Investigation of Hydrothermally Synthesized Greigite Magnetic Properties

Greigite (Fe3S4) is an iron sulfide with inverse spinel structure, same as the structure of magnetite. Half-metallic properties in combination with ferrimagnetic nature of greigite puts this mineral in a fairly narrow group of materials that are potentially suitable for spintronics [1]. In this paper, we have conducted a comparative study of magnetic properties of two crystalline greigite samples obtained by hydrothermal synthesis with different parameters.Conducted Mossbauer spectroscopy study has shown that there are two magnetically non-equivalent B-site iron positions. The interpretation of this result was given using a model for magnetite from [2]. Unusual temperature dependencies of effective magnetic fields for A-and B-site iron atoms were obtained. It was found that coercive magnetic parameters as well as greigite stability depend on the synthesis procedure. Stepwise thermomagnetic analysis in saturating field allowed us to obtain the synthesized greigite critical temperatures of structural stability (T ~ 240oC), after which the sequential process of greigite transformation to pyrite, metastable maghemite (or magnetite) and finally to hematite begins. All of the found greigite peculiarities not only expand the knowledge about this complicated ferrimagnetic phase but can be used as guidance for optimal synthesis conditions.

Effects of Sm and Mn Co-Doping on Structural and Optical Properties of BiFeO&lt;sub&gt;3&lt;/sub&gt; Thin Films Prepared by Sol-Gel Technique

(Bi1-xSmx)(Fe0.95Mn0.05)O3(x=0.00, 0.03 and 0.06) thin films were deposited on the quartz substrates by sol-gel technique. The results of X-ray diffraction patterns indicated all thin films had rhombohedral perovskite structure. Moreover, the Sm and Mn co-doping at A-and B-site of BiFeO3resulted in the structural distortion. Scanning electron microscope measurements exhibited that the uniform surface morphology could be obtained by co-doping and the average grain size of the films decreased with increasing Sm content. Furthermore, the fundamental absorption edges ofxBSFMO films showed a blue shift with the increase of Sm content which can be observed in transmittance spectra. The optical band gap of the thin films forx= 0.00, 0.03 and 0.06 can be expressed by (0.84x+2.62) eV, which is due to the Burstein-Moss effect.

Defect Equilibria Of (Pr0.6Sr0.4)(Co0.2Fe0.8)O3-δ

The nonstoichiometry of single phase perovskite Pr0.6Sr0.4Co0.2Fe0.8O3-δ (PSCF) was measured in the temperature range 800 ⁰C-600 ⁰C. The nonstoichiometry for PSCF was found to be greater than LSCF for all measured temperatures and oxygen partial pressures (Po2) tested. A defect equilibria model was developed based on small polaron formation on both A-and B-sites of the perovskite Good fits of the model to the experimental data were obtained enabling the relative concentrations of the point defect species in PSCF to be determined.

Variable Energy Positron Annihilation Spectroscopy of Perovskite Oxides

The application of variable energy positron annihilation spectroscopy (VE-PAS) methods to the study of perovskite oxide, ABO3, material thin films and near-surface regions is reviewed. The primary focus is on ferroic perovskite titanate oxide materials SrTiO3and Pb (ZrxTi1-x)O3, but studies of BaTiO3, LaxSr1-xCoO3, La0.67Sr0.33MnO3and YBa2CuO7-δare also included. Characterization of single layer and multilayer structures is discussed. The methods, in particular positron annihilation lifetime spectroscopy, allow the identification of cation vacancy defects at both the A-and B-sites with parts per million sensitivity. Varying oxygen deficiency is often observed to result in marked changes in PAS spectra; these effects are reviewed and discussed.

Cation Distribution Dependence on Thermoelectric Properties of Doped Spinel M&lt;sub&gt;0.6&lt;/sub&gt;Fe&lt;sub&gt;2.4&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;

The electrical conductivity, Seebeck coefficient, and thermal conductivity of polycrystalline M0.6Fe2.4O4 (M = Ni, Ni0.5Mg0.5, Ni0.5Zn0.5, Zn) were measured to elucidate cation distribution-dependent changes. Preferential occupation by the doped cation in the iron spinel has been noted: Zn2+ ions prefer to occupy the tetrahedral A-site, while Ni2+ and Mg2+ prefer to occupy the octahedral B-site. While the electrical conductivity and Seebeck coefficient are almost cation distribution-independent, the thermal conductivity at room temperature is sensitive to the cation distributions. The lowest thermal conductivity of 2.0Wm11 K11 at room temperature is observed for Zn0.6Fe2.4O4. The value is about one third of that of Ni0.6Fe2.4O4. The thermal transport of MxFe31xO4 is mainly affected by cation distribution at the A-site, while the electrical transport is affected by the B-site, which is discussed in terms of the point defects at the Aand B-sites. Due to the disordering at the Aand Bsites, the thermal conductivity of MxFe31xO4 could be reduced without decreasing the electrical conductivity. Doped spinel-ferrite MxFe31xO4 would be a kind of “phonon-glass electron-crystal” material. [doi:10.2320/matertrans.M2012023]

Synthesis and Magnetic Properties of $\hbox{BiFeO}_{3}$, ${(\hbox{BiFeO}_{3})}_{0.95}{(\hbox{BaTiO}_{3})}_{0.05}$, and $(\hbox{Bi}_{0.95}\hbox{Ba}_{0.05})(\hbox{Fe}_{0.95}\hbox{Ti}_{0.05})\hbox{O}_{3}$ Ceramics

Magnetization, grain size, structure, and temperature-dependent dielectric permittivity of BiFeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , (BiFeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95</sub> (BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.05</sub> -(BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> )0.05(5%BTO-BFO), and (Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.05</sub> )(Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95</sub> Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.05</sub> )O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> [5%(Ba, Ti)-BFO] ceramics have been carried out. BFO and 5%BTO-BFO exhibit an typical antiferromagnetic behavior, whose magnetization curve is linear with magnetic field at room temperature. The magnetic hysteresis loop of 5%(Ba,Ti)-BFO ceramic shows a weak ferromagnetic behavior with a small remanent magnetization of about 0.01 emu/g possibly due to a higher density in the grain matrix. This result also implies that the A-and B-site substitutions with Ba <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> and Ti <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> could efficiently suppress the spiral magnetic modulation. The magnetic susceptibilities of BFO, 5%BTO-BFO, and 5%(Ba,Ti)-BFO ceramics are about 1.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , 5.43 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> , and 8.67 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> emu/gOe, respectively. Frequency-dependent dielectric maxima were observed in BFO, 5%BTO-BFO, and 5%(Ba,Ti)-BFO ceramics upon heating. This dielectric response is likely triggered by the antiferromagnetic-paramagnetic transition near the Neel temperature.

A-and B-site solubilities of trivalent rare-earth cations and B-site cation ordering in lead magnesium niobate

The solubility limits of trivalent rare-earth cations of widely different ionic radii on the A and B sites of lead magnesium niobate (PMN) have been determined. The yttrium, magnesium, and niobium cations in yttrium-doped PMN have been shown to be 1: 1 ordered on the B site. The ordering has been studied using computer modeling, both with and without allowance for Pb vacancies on the A site. The dielectric properties of the trivalently doped PMN ceramics have been investigated.

Selective Protein Adsorption Property and Structure of Nano-Crystalline Hydroxy-Carbonate Apatite

The selective protein adsorption property and the local structure around carbonate ions of nanocrystalline hydroxy-carbonate apatite were examined in this study. Considerable change in the selectivity in the adsorption of BSA and β2-MG was observed due to the incorporation of thecarbonate ions in hydroxyapatite lattice. Since the protein adsorption property seems to be related to the surface charge density of hydroxyapatite due to the carbonation, the chemical states of the incorporated carbonate ions were examined by the 31C CP-MAS NMR spectroscopy. At least four peaks assignable to carbonate ions in A-site(OH-) and B-site(PO4 3-) were observed in 13C CP-MAS NMR spectrum. Thus, we must take into consideration that the surface charge distribution and the decrement of polar groups such as OH- groups due to the distribution of carbonate ions in both Aand B-sites of the hydroxyapatite lattice are particularly favorable for β2-MG adsorption rather than for BSA adsorption.

Effects of site substitution and metal ion addition on doped manganites

We report transport, magnetization and transmission electron microscopy studiesof the effects of A-and B-site substitution, and the addition of metal ions such asPt, Ag and Sr, on doped ABO3 perovskites, where A = La, Pr etc and B = Mn. Disorder induced by such substitution changes the behaviour ofthe charge-ordered (CO) state significantly. A-and B-site substitutionsuppresses the CO phase due to size mismatch and disorder produced byinhomogeneity. On the other hand, addition of metal ions such as Pt and Agimproves several colossal-magnetoresistance properties significantly due tomicrostructural effects and enhanced current percolation through grainboundaries.

Temperature dependence of the hyperfine parameters of maghemite and al-substituted maghemites

Synthetic aluminum-substituted maghemite samples, γ-(AlyFe1-y)2O3 with y=0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by Mossbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: δA=δB-0.12 mm/s. From the temperature dependence of δB it was possible to determine the characteristic Mossbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing Al content, at least up to 10 mole%. The temperature dependence of the A-and B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: JAB=-25 K; JAA=-18 K and JBB= -3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% Al. The gradual decrease of the saturation value of the A-site hyperfine field with increasing Al substitution and the constancy of this quantity for the B sites, suggest that the Al cations occupy the B sites.

Fe-Fe coupling in iron-doped lithium aluminate phosphors

For very low iron concentration the ordered irondoped lithium aluminate, LiAls_xFexOs, exhibits a luminescence ascribed to the electronic transitions of the Fe 3÷ impurity ions occupying the tetrahedrally co-ordinated sites (A-sites) of the host crystal [1, 2]. This material has an inverse spinel structure, space group P4332 in the ordered phase, and forms homogeneous solid solutions when the concentration of iron ions lies within the limits 1/> x i> 4 [3, 4]. For higher iron concentrations this phosphor becomes a dilute magnetic lithium ferrite with interesting magnetic properties [5, 6]. Luminescence of Fe 3+ ions as a diluted impurity in many host materials has been reported in the literature [1,2, 7-9], but there are only few reports on exchange effects observed in the optical properties of these ions [10, 11]. It is difficult to study concentration effects in the optical properties of Fe 3+ because this ion has strong charge transfer bands superimposed on the lower-energy crystal field transitions which rapidly covers all the visible region and reaches the near-infrared region when the iron concentration increases. For MN 2+, of the same electronic configuration, this charge transfer band occurs in the far-ultraviolet region [11], which makes it easier to use for these studies. However, in the LiA15Os:Fe 3+ phosphor with low iron concentrations most of the iron ions are occupying A-sites for which the charge transfer bands are shifted less to the visible region than this band for the B-sites, allowing the observation of the d-d electronic transitions within Fe 3+ ions, both in the excitation and in the absorption spectra [1, 2, 9, 12]. This letter presents luminescence results of LiA15_xFexOs, showing that in addition to processes involving single Fe 3÷ ions concentration-dependent features are also observed due to exchange coupled iron ions. The iron content of the LiA15_xFexO8 powder samples whose luminescences were measured lay in the range 0.005-0.500 for stoichiometric nominal x-values. The samples were prepared by the usual ceramic method [3, 4] from high-purity iron and aluminium oxides and lithium carbonate. X-ray diffraction spectra of all samples ensured the proper ordered crystallographic phase with the lattice parameter showing a linear dependence on the iron concentration in a good Vegard's law behaviour, and the iron distribution among the Aand B-sites was derived by correlating the intensity of the (1 1 1) diffraction line with the iron B-sites occupation [13]. The equipment used for the optical measurements was reported in [2]. Fig. 1 shows the luminescence of samples with iron concentrations up to x = 0.125. For x -0.005 (Fig. la) a zero phonon line at 658 nm is observed and a vibronic band at lower energy due to the 4Tl(4G ) ~ 6At(6S ) electronic transition of the Fe 3+ ions occupying the A-sites as reported in [1, 2]. For higher concentration (Fig. lb) the intensity of the zero phonon line decreases relative to the integrated intensity of the emission as a whole, and for x > 0.050 new features are observed. For these higher concentrations the lines at 665, 670, 675 and 687 nm dominate the emission as shown in Fig. lc and d. A structure at about 710 nm due to the presence of Fe 3+ ions in the B-sites was also observed for the higher-concentration samples, because as the concentration of iron is raised the

Effect of rare-earth oxide on the properties of piezoelectric ceramics

Abstract The influence of rare-earth oxide on the Sr0.02Pb0.98(Li0.008W0.012Ti0.46Zr0.52)O3 composition is studied. The ion size of rare-earth oxide are between those of Pb2+ and Zr4+, so both A-and B-sites are occupied for the same kind of doping. By the weight change method and lattice parameter variation method, the percent substitution of each kind doping can be calculated. For the lanthanum series, La3+, Nd3+ favor to the production of A-site vacancy, and Er3+ favor to the production of O-site vacancy. The piezoelectric properties of each kind of doping were measured too.

MAGNETIC DIPOLAR AND ELECTRIC QUADRUPOLE EFFECTS ON THE MÖSSBAUER SPECTRA OF MAGNETITE ABOVE THE VERWEY TRANSITION

Mossbauer spectroscopic studies (57Fe) of powdered magnetite have been undertaken between 120 K and 880 K. Below the magnetic transition temperature (TC=839.5 K) three six-line patterns have been fitted to our experimental spectra. The broadening of the B-pattern is explained by two magnetically non-equivalent B-site irons, suggesting broadening due to electron hopping to be negligible. In the paramagnetic state the electric quadrupole splittings of iron at A-and B-sites are found to be constant, independent of temperature, having the values zero and 0.16 mm/s, respectively. The centroid shifts, on the other hand, show above 700 K large deviations from the calculated second order Doppler shift. It is proposed that the deviations arise from a variation in band overlap. The temperature variation of the magnetic fields is found to be proportional to the sublattice magnetization. The difference in the magnetic fields at the two non-equivalent B-sites is measured to be 1.1 T at 310 K.

Magnetic dipolar and electric quadrupolar effects on the M�ssbauer spectra of magnetite above the Verwey transition

Mossbauer spectroscopic studies (57Fe) of powdered magnetite have been undertaken between 120 K and 880 K. Below the magnetic transition temperature (T C=839.5 K) three six-line patterns have been fitted to our experimental spectra. The broadening of the B-pattern is explained by two magnetically non-equivalent B-site irons, suggesting broadening due to electron hopping to be negligible. In the paramagnetic state the electric quadrupole splittings of iron at A-and B-sites are found to be constant, independent of temperature, having the values zero and 0.16 mm/s, respectively. The centroid shifts, on the other hand, show above 700 K large deviations from the calculated second order Doppler shift. It is proposed that the deviations arise from a variation in band overlap. The temperature variation of the magnetic fields is found to be proportional to the sublattice magnetization. The difference in the magnetic fields at the two non-equivalent B-sites is measured to be 1.1 T at 310 K.