Study on the electrical conductivity and oxygen diffusion of oxide-ion conductors La2Mo2−xTxO9−δ (T = Al, Fe, Mn, Nb, V)

In this paper, the influence of cationic substitutions at Mo site with Al³⁺, Fe³⁺, Mn⁴⁺, Nb⁵⁺ and V⁵⁺ ions on the structure, oxygen ion diffusion and electrical properties in La₂Mo₂O₉ oxide-ion conductors have been investigated by X-ray diffraction method, dielectric relaxation technique and direct current conductivity measurement. Except for V⁵⁺ substitution all of these substitutions up to 5% cannot suppress the phase transition in La₂Mo₂O₉. In the dielectric measurement, one prominent relaxation peak is observed in temperature spectrum as well as in frequency spectrum, which is associated with the short-distance diffusion of oxygen vacancies. The activation energy for oxygen ion diffusion is deduced as in the range of 1-1.1 eV for Al, Fe, Mn and Nb doped samples and 1.4-1.5 eV for V doped samples. All substituted samples have a higher conductivity than the un-doped compound. In the Al, Fe, Mn and Nb substituted materials the phase transition is not suppressed; however, K substitution at the La site can completely suppress the transition and maintains high conductivity at low temperature.     

Locating the normal to relaxor phase boundary in Ba(Ti1−xHfx)O3 ceramics

In this article, we report our studies on the relaxor behavior of Ba(Ti_1−xHf_x)O₃ ceramics, made with close compositions between 0.20 ≤ x ≤ 0.30, to locate the hafnium concentration boundary for the normal to relaxor crossover. X-ray diffraction followed by Rietveld refinement shows the occurrence of single-phase cubic structure for the synthesized Ba(Ti_1−xHf_x)O₃ ceramics. Temperature and frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric permittivity has been studied in the temperature range of 90-350 K at frequencies of 0.1, 1, 10, and 100 kHz. A diffuse phase transition accompanying frequency dispersion is observed in the permittivity versus temperature plots revealing the occurrence of relaxor ferroelectric behavior. The T_m verses Hf concentration plot shows a discontinuous jump and change in the slope at x = 0.23. Quantitative characterization based on phenomenological models has also been presented. The plausible mechanism of the relaxor behavior has been discussed. Substitution of Hf⁴⁺ for Ti⁴⁺ in BaTiO₃ reduces the long-range polar ordering yielding a diffuse ferroelectric phase transition.     

Dielectric and pyroelectric anisotropy in melt-quenched BaBi2(Nb1 − xVx)2O9 ceramics

The (0 0 l) textured BaBi₂(Nb_1 − xV_x)₂O₉ (where x = 0, 0.03, 0.07, 0.1 and 0.13) ceramics were fabricated via the conventional melt-quenching technique followed by high temperature heat-treatment (800-1000 °C range). The influence of vanadium content and sintering temperature on the texture development and relative density were investigated. The samples corresponding to the composition x = 0.1 sintered at 1000 °C for 10 h exhibited the maximum orientation of about 67%. The Scanning electron microscopic studies revealed the presence of platy grains having the a-b planes perpendicular the pressing axis. The dielectric constant and the pyroelectric co-efficient values in the direction perpendicular to the pressing axis were higher. The anisotropy in the dielectric constant is about 100 (at 100 kHz) at the dielectric maximum temperature and anisotropy in the pyroelectric co-efficient is about 50 μC cm⁻² °C⁻¹ in the vicinity of pyroelectric anomaly for the sample corresponding to the composition x = 0.1 sintered at 1000 °C. Higher values of the dielectric loss and electrical conductivity were observed in the direction perpendicular to the pressing axis which is attributed to the high oxygen ion conduction in the a-b planes.     

Electrochemical reduction of nitrous oxide on La1−xSrxFeO3 perovskites

The electrochemical reduction of nitrous oxide and oxygen has been studied on cone-shaped electrodes of La_1−xSr_xFeO_3−δ perovskites in an all solid state cell, using cyclic voltammetry. It was shown that the activity of the La_1−xSr_xFeO_3−δ perovskites for the electrochemical reduction of nitrous oxide mainly depends on the amount of Fe(III) and oxide ion vacancies. The activity of the La_1−xSr_xFeO_3−δ perovskites towards the electrochemical reduction of nitrous oxide is much lower than the activity of the La_1−xSr_xFeO_3−δ perovskites towards the electrochemical reduction of oxygen, making the possibility of electrochemically reducing nitrous oxide selectively in an exhaust gas containing excess oxygen on this type of materials very doubtful.     

Grain size-dependent electrical transport properties in La0.75Sr0.25Mn1−xMgxO3±δ ceramics

The role of GB in Mg-substituted lanthanum-strontium manganite ceramics is studied with microstructural details. At higher concentrations of Mg (x>0.05), where the average grain size is ∼1 μm, the M-I transition is shifted from 348 to 110 K. Annealing in lower p_O2 (10⁻⁶ atm) at 1375 K for 1 h obliterates the M-I transition and brings in insulating behaviour throughout the temperature of measurement. Re-annealing in oxygen atmosphere for 10-25 min reintroduces the M-I transition, indicating that the electrical transport properties depend on the chemical inhomogeneity introduced by the in- or out-diffusion of oxygen through the GB regions. Samples with larger grain size (∼35 μm) do not exhibit major modifications in electrical resistivity on annealing in different p_O2. The insulating manganites display non-linear J-E characteristics below the magnetic transition temperature at electric field strengths <50 V/cm. The non-linear behaviour is explained on the basis of the inelastic tunnelling through the multiple localised states in the insulating GB regions. The external magnetic field lowers the voltage at which the non-linearity sets in. The tunnelling therefore may be not only through independent defect centres of oxygen vacancies (V_O), but possibly from defect complexes such as Mn³⁺-V_O or Mn²⁺-V_O, where spin-dependent tunnelling can take place.     

Synthesis of low loss, thermally stable CexY1−xTiTaO6 microwave ceramics

Ce_xY_1−xTiTaO₆ ceramics were prepared through the solid-state ceramic route. The materials were sintered in the range 1520-1580 °C. The structure of the system was analyzed by X-ray diffraction and Raman spectroscopic methods. The cell parameters of solid solutions were calculated using the least square method. The microstructure was analyzed using scanning electron microscopy. The dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) are measured in the microwave frequency region using cavity resonator method. The dielectric constant increases with higher concentrations of Ce in the solid solutions. Nearly zero temperature coefficient of resonant frequency (τ_f) was obtained for Ce_0.24Y_0.76TiTaO₆. The samples are of high quality factor and are useful electronic materials for microwave applications.     

The mechanism of long phosphorescence of SrAl2−xBxO4 (0<x<0.2) and Sr4Al14−xBxO25 (0.1<x<0.4) co-doped with Eu and Dy

The role of B₂O₃ in realizing the long phosphorescence of Eu(II)+Dy(III) doped strontium aluminates has been investigated. IR and solid state MAS NMR spectra show the incorporation of boron as BO₄ in the AlO₄ framework of SrAl₂O₄ and Sr₄Al₁₄O₂₅. Phosphor, made free of glassy phases by washing with hot acetic acid+glycerol, did not show any photoconductivity under UV irradiation, indicating that the mechanism involving hole conduction in valence band is untenable for long phosphorescence. EPR studies confirm the presence of both electron and hole trap centers. Dy³⁺ forms substitutional defect complex with borate; [Dy-BO₄-V_Sr]²⁻, and acts as a hole trap center. The electron centers are formed by the oxygen vacancies associated with BO₃³⁻, i.e. [BO₃-V_O]³⁻. Under indigo light or near UV irradiation, the photoinduced electron centers are formed as [BO₃-V_O(e′)]⁴⁻. The holes are released from [Dy-BO₄-V_Sr(h)]¹⁻ under thermal excitation at room temperature. The recombination of electrons with holes releases energy which is expended to excite Eu²⁺ to induce long phosphorescence.     

Low loss Sr1−xCaxLa4Ti5O17 microwave dielectric ceramics

Microwave dielectric ceramics in the Sr_1−xCa_xLa₄Ti₅O₁₇ (0 ≤ x ≤ 1) composition series were prepared through a solid state mixed oxide route. All the compositions formed single phase ceramics within the detection limit of in-house X-ray diffraction when sintered in the temperature range 1450-1580 °C. Theoretical density and molar volume decreased due to the substitution of Ca²⁺ for Sr²⁺ which was associated with a decrease in the dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) but an increase in quality factor, Qf_o. Optimum properties were achieved for Sr_0.4Ca_0.6La₄Ti₅O₁₇ which exhibited, ?_r ∼ 53.7, Qf_o ∼ 11,532 GHz and τ_f ∼ −1.4 ppm/°C.     

Tantalum influence on physical properties of (K0.5Na0.5)(Nb1−xTax)O3 ceramics

Lead-free (K_0.5Na_0.5)(Nb_1−xTa_x)O₃ ceramics with x = 0.00-0.30 were prepared by the solid-state reaction technique. The effects of Ta on microstructure, crystallographic structure, phase transition and piezoelectric properties have been investigated. It has been shown that the substitution of Ta decreases Curie temperature T_C and orthorhombic-tetragonal phase transition temperature T_O-T, while increasing the rhombohedral-orthorhombic phase transition temperature T_R-O. In addition, piezoelectric activity is enhanced with the increase of Ta content. The ceramics with x = 0.30 have the high value of piezoelectric coefficient d₃₃ = 205 pC/N. Moreover, k_p shows little temperature dependence between −75° C and 75 °C, and d₃₃ exhibits very good thermal stability over the range from −196 °C to 75 °C in the aging test.     

Photoluminescence properties of V-doped La0.67Ca0.33Mn1−xVxO3

The optical spectra have been investigated for a prototypical double exchange ferromagnetic La_0.67Ca_0.33MnO₃ with Mn substituted by V above the paramagnetic-magnetic transition temperature T_C. The excitation spectra under the probe wavelength of λ_em = 473 nm for all samples exhibit two activation bands around 360 and 294 nm, involving an electron transfer from oxygen 2p states to the Mn d states in MnO₆ octahedra. The photoluminescence spectra at λ_ex = 290 nm have the similar spectral features for all samples. The photoluminescence spectral peaks located at 400, 473, 534, 670, 738 and 770 nm, and the corresponding photon energy is in a broad range of 3.1-1.6 eV, indicating that the PL bands could have the different origin: the self-trapped excitons localized on MnO₆ octahedra; the interband transition between the O 2p and Mn 3d bands; the transition between the 3d electron states of Mn ions. So, it can be clearly seen that the electronic behavior above T_C is very complicated. Our results suggest that the charge transfer from O 2p to Mn 3d has the important effects on the electronic structure, and it not only contributes to the optical transition but is helpful and even important to understand the electric, magnetic and thermal properties etc. due to the strong correlation among charge, spin and lattice in perovskite manganites.     

High-performance yellow ceramic pigments Zr(Ti1−x−ySnx−yVyMy)O4 (M = Al, In, Y): Crystal structure, colouring mechanism and technological properties

Zirconium titanate-stannate doped with V with co-dopants Al, In or Y was synthesised by solid state reaction and its structural (XRD, SEM), optical (DRS) and technological properties were determined to assess its potential use as ceramic pigment. These compounds have a srilankite-type, disordered orthorhombic structure, implying a random distribution of Zr, Ti, Sn and dopants in a single, strongly distorted octahedral site. Doping caused an increase of unit-cell dimensions, metal-oxygen distances and octahedron distortion. Optical spectra show crystal field electronic transitions of V⁴⁺ as well as intense bands in the blue-UV range due to V⁴⁺-V⁵⁺ intervalence charge transfer and/or to V-O charge transfer. The formation of oxygen vacancies is supposed to compensate the occurrence of V⁴⁺ ensuring the lattice charge neutrality. These srilankite-type oxides develop a deep and brilliant yellow shade with colourimetric parameters close to those of industrial ceramic pigments. Technological tests in several ceramic applications proved that zirconium titanate-stannate is very stable at high temperature, exhibiting an excellent performance in the 1200-1250 °C range, even better than praseodymium-doped zircon.     

Structure analysis on the Ba3Mg(Ta1−xNbx)2O9 ceramics: Coexistence of order and disorder

The Ba₃ZnTa₂O₉ (BZT) and Ba₃MgTa₂O₉ (BMT) ceramics, a family of A₃B²⁺B⁵⁺₂O₉ complex perovskites, are extensively utilized in mobile based technologies due to their intrinsic high unloaded quality factor, high dielectric constant and a low (near-zero) resonant frequency temperature coefficient at microwave frequencies. The preparation conditions as well as size and nature of B cations have a profound effect on the final dielectric properties. In this article, we report the effect of Nb⁵⁺ at the Ta⁵⁺ site on the BMT structure prepared at four synthesis temperatures (1300, 1400, 1500 and 1600 °C). The analysis has been carried out using the Rietveld technique on the X-ray powder diffraction data. Results suggest that both the preparation temperatures and Nb⁵⁺ content have significant effect on the ordering of B cations in the Ba₃Mg(Ta_1−xNb_x)₂O₉ solid solution. A disordered (cubic) structure is preferred by the 1300 °C compounds. The weight percentage of the ordered (trigonal) phase escalates, for a given composition, with increasing calcination temperature. A fully ordered trigonal arrangement exists only for x = 0.0 and 0.2 compounds calcined at 1600 °C, and the rest are biphasic (cubic and trigonal). The increase in the cubic fraction upon Nb⁵⁺ augmentation suggests that the solid solution leans more toward the disordered structural arrangement of B²⁺ and B⁵⁺ cations.     

Oxygen nonstoichiometry of NdNiO3−δ and SmNiO3−δ

The rare-earth nickelates are among few perovskite oxides showing very sharp temperature-induced metal/insulator transition. The synthesis and study of oxygen-deficient NdNiO_3−δ and SmNiO_3−δ are described in this paper. The results obtained indicate that the oxygen nonstoichiometry greatly influences transport properties of these compounds. The sharpness of metal/insulator transition strongly rises with decrease of oxygen deficiency without any noticeable change of transition temperature. The boundaries of oxygen and cation homogeneity ranges of Nd_1−xNiO_3−δ were determined for the first time. The thermogravimetric study was used to explain certain difficulties in oxidation of as-synthesized oxygen-deficient NdNiO_3−δ samples.     

Ba3ZnTa2−xNbxO9 and Ba3MgTa2−xNbxO9 (0≤x≤1): synthesis, structure and dielectric properties

Oxides belonging to the families Ba₃ZnTa_2−xNb_xO₉ and Ba₃MgTa_2−xNb_xO₉ were synthesized by the solid state reaction route. Sintering temperatures of 1300°C led to oxides with disordered (cubic) perovskite structure. However, on sintering at 1425°C hexagonally ordered structures were obtained for Ba₃MgTa_2−xNb_xO₉ over the entire range (0≤x≤1) of composition, while for Ba₃ZnTa_2−xNb_xO₉ the ordered structure exists in a limited range (0≤x≤0.5). The dielectric constant is close to 30 for the Ba₃ZnTa_2−xNb_xO₉ family of oxides while the Mg analogues have lower dielectric constant of ∼18 in the range 50 Hz to 500 kHz. At microwave frequencies (5-7 GHz) dielectric constant increases with increase in niobium concentration (22-26) for Ba₃ZnTa_2−xNb_xO₉; for Ba₃MgTa_2−xNb_xO₉ it varies between 12 and 14. The “Zn” compounds have much higher quality factors and lower temperature coefficient of resonant frequency compared to the “Mg” analogues.     

Structural, spectroscopic and dielectric investigations on Ba8Zn(Nb6−xSbx)O24 microwave ceramics

Ba₈Zn(Nb_6−xSb_x)O₂₄ (x = 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8 and 2.4) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered in the range 1400-1425 °C. The structure of the system was analyzed by X-ray diffraction, Fourier transform infrared and Raman spectroscopic methods. The theoretical and experimental densities were calculated. The microstructure of the sintered pellets was analyzed using scanning electron microscopy. The low frequency dielectric properties were studied in the frequency range 50 Hz-2 MHz. The dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) are measured in the microwave frequency region using cavity resonator method. The τ_f values of the samples reduced considerably with the increase in Sb concentration. The materials have intense emission lines in the visible region. The compositions have good microwave dielectric properties and photoluminescence and hence are suitable for dielectric resonator and ceramic laser applications.     

La1−xSrxCuO2.5−δ as new cathode materials for intermediate temperature solid oxide fuel cells

La_1−xSr_xCuO_2.5−δ (LSCu), which exhibit excellent electrical conductivity and oxygen vacancies were investigated as potential cathode materials for solid oxide fuel cell (SOFC) applications. The structure stability, electrical conductivity, cathodic overpotential, and the reactivity with yttria-stabilized zirconia (YSZ) were examined in this study. It was found that the LSCu perovskite was obtained only when the addition of strontium fell in the range between 15 and 30%. With more than 20% of strontium addition, this material showed excellent electrical property and immunity to the reaction with YSZ at 800 °C. The conductivities of LSCu were as high as 900 S/cm at 600 °C, and 800 S/cm at 800 °C. The cathodic overpotential of this material was approximately 3.8 and 10.6 mV at a current density of 100 mA/cm² at 850 and 750 °C, respectively. These properties are superior to Sr-doped lanthanum manganite (LSM), which is the state-of-the-art cathode material of SOFCs.     

Microwave driven hydrothermal synthesis of Ba1−xSrxTiO3 nanoparticles

Ba_1−xSr_xTiO₃ (BST) nanopowders with composition x = 0.1-0.4 have been prepared using microwave driven hydrothermal synthesis (MDHS). A low temperature process has been chosen in order to avoid high temperature heat treatment leading to particle growth and agglomeration. MDHS method allows obtaining the nanocrystalline powder samples during relatively short period of time (10 min) and therefore MDHS was optimized. In case of the phase evolution studies the XRD measurements were performed. The average size of crystallites was estimated using Scherrer equation. TEM and SEM images were taken for the detailed analysis of the grain size, surface and morphology.     

Synthesis and transport properties study of Nd1−xSmxNiO3−δ solid solutions

Nd_1−xSm_xNiO_3−δ ceramics were studied by means of XRD, iodometric titration, DSC and resistivity measurements. The metal-insulator transition for all compositions was studied by two independent procedures: resistivity and DSC measurements. The reason for differences in transition temperatures, defined by these two techniques is discussed. It was shown that these materials are promising for practical applications in bolometric devices due to high values of temperature coefficient of resistivity at room temperature.     

Microwave dielectric properties of (Pb1−3x/2Lax)(Mg1/2W1/2)O3

La modified Pb(Mg_1/2W_1/2)O₃ were prepared by solid-state reaction process, and the sintering behavior, microstructure and microwave dielectric properties were investigated by X-ray powder diffraction (XRD), Raman scattering and HP network analyzer in this paper. A series of single phase perovskite type solid solutions with A-site vacancies (Pb_1−3x/2La_x(Mg_1/2W_1/2)O₃ (0 ≤ x ≤ 2/3)) were formed. The solid solution took cubic perovskite type structure (Fm3m) with random distribution of A-site vacancies when 0 < x < 0.5, and tetragonal or orthorhombic structure with the ordering of A-site vacancies when 0.5 ≤ x ≤ 2/3. The dielectric constant and temperature coefficient of resonant frequency decrease with increasing La content. Relatively good combination microwave dielectric properties were obtained for x = 0.56: ?_r = 28.7; Q × f = 18098; and τ_f = −5.8 ppm/°C.     

Ferroelectric and dielectric properties of sol-gel derived BaxSr1−xTiO3 thin films

Ferroelectric thin films of Ba_0.7Sr_0.3TiO₃, Ba_0.8Sr_0.2TiO₃, Ba_0.9Sr_0.1TiO₃ and BaTiO₃ were fabricated by a modified sol-gel technique on Pt/Ti/SiO₂/Si substrates. All the compositions crystallized in perovskite structure and consist of well-defined grains. As the value of x increases grain size of the Ba_xSr_1−xTiO₃ thin films increases while the dielectric permittivity decreases. Ba_0.7Sr_0.3TiO₃ composition possesses the highest dielectric permittivity of 748 (at 100 kHz). Hysteresis loops measured at room temperature for all compositions showed that BaTiO₃ has the largest remnant polarization of 4.8 μC/cm². The dielectric and ferroelectric properties of the sol-gel derived Ba_xSr_1−xTiO₃ thin films are strongly dependent on the Sr content and the grain size.