Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Structure and photocatalytic activity studies of TiO2-supported over Ce-modified Al-MCM-41

Ce-Al-MCM-41, TiO₂/Al-MCM-41 and TiO₂/Ce-Al-MCM-41 materials with varying contents of Ce (by impregnation) and TiO₂ loaded (by solid-state dispersion) on Al-MCM-41 support are prepared. The Ce modified and TiO₂ loaded composite systems are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectroscopy (XPS) techniques. The DRS and XPS of low Ce content (0.2-0.5 wt.%) modified Al-MCM-41 samples are showing more characteristic of Ce³⁺ species wherein cerium in interaction with Al-MCM-41 and that of high Ce (0.8, 3.0 wt.%) content modified samples are showing the characteristic of both Ce⁴⁺and Ce³⁺species. A series of Ce-modified Al-MCM-41 and TiO₂ loaded composite catalysts are evaluated for photocatalytic degradation of phenol under UV irradiation. Low Ce content in Ce³⁺ state on Al-MCM-41 is showing good photoactivity in comparison with high Ce content samples and pure ceria. The composite TiO₂/Ce-Al-MCM-41 is showing enhanced degradation activity due decreased rate of electron-hole recombination on TiO₂ surface by the redox properties of cerium. The photocatalyst TiO₂/Ce-Al-MCM-41 with an optimum of 10 wt.% TiO₂ and 0.3 wt.% Ce is showing maximum phenol degradation activity. The possible mechanism of phenol degradation on the composite photocatalyst is proposed.     

X-ray Rietveld analysis and Fourier transform infrared spectra of the solid solutions [Eu2−xMx][Sn2−xMx]O7−3x/2 (M = Mg or Zn)

Two series of mixed oxides with formula [Eu_2−xM_x][Sn_2−xM_x]O_7−3x/2 (M = Mg or Zn) have been synthesized. The study by X-ray diffraction and Fourier transform infrared spectroscopy shows that the solids obtained are new non-stoichiometric solid solutions with the pyrochlore type structure. For both series a decrease of the cell parameter is observed when the degree of substitution, x, increases. The structural refinements (X-ray studies) were achieved in the space group Fd-3m, no. 227 (origin at center -3m) by using the Fullprof software. The Rietveld refinements show that the divalent cations M²⁺ (Mg²⁺, Zn²⁺) substitute isomorphically for Eu³⁺ and Sn⁴⁺ ions producing vacancies in the anionic sublattice.     

Synthesis and structural characterization of Ce-doped bismuth titanate

Ce-modified bismuth titanate nanopowders Bi_4−xCe_xTi₃O₁₂ (x ≤ 1) have been synthesized using a coprecipitation method. DTA/TG, FTIR, XRD, SEM/EDS and BET methods were used in order to investigate the effect of Ce-substitution on the structure, morphology and sinterability of the obtained powders. The phase structure investigation revealed that after calcinations at 600 °C powder without Ce addition exhibited pure bismuth titanate phase; however, powders with Ce (x = 0.25, 0.5 and 0.75) had bismuth titanate pyrochlore phase as the second phase. The strongest effect of Ce addition on the structure was noted for the powder with the highest amount of Ce (x = 1) having a cubic pyrochlore structure. The presence of pure pyrochlore phase was explained by its stabilization due to the incorporation of cerium ions in titanate structure. Ce-modified bismuth titanate ceramic had a density over 95% of theoretical density and the fracture in transgranular manner most probably due to preferable distribution of Ce in boundary region.     

Protonic conduction in Ca-doped La2M2O7 (M = Ce, Zr) with its application to ammonia synthesis electrochemically

Complex oxides La_1.95Ca_0.05M₂O_7−δ (M = Ce, Zr) were prepared by sol-gel method and characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD). On the sintered complex oxides as solid electrolyte, the conductivity was measured in various atmospheres, and ammonia was synthesized from nitrogen and hydrogen at atmospheric pressure in the solid states proton conducting cell reactor by electrochemical methods. The rates of ammonia formation were up to 2.0 × 10⁻⁹ mol s⁻¹ cm⁻² for La_1.95Ca_0.05Zr₂O_7−δ and 1.3 × 10⁻⁹ mol s⁻¹ cm⁻² for La_1.95Ca_0.05Ce₂O_7−δ, respectively, at 520 °C.     

Novel composite interconnecting ceramics La0.7Ca0.3CrO3−δ/Ce0.8Sm0.2O1.9 for solid oxide fuel cells

In this paper, an interconnecting ceramic for solid oxide fuel cells was developed, based on the modification from La_0.7Ca_0.3CrO_3−δ by addition of Ce_0.8Sm_0.2O_1.9. It is found that addition of small amount Ce_0.8Sm_0.2O_1.9 into La_0.7Ca_0.3CrO_3−δ dramatically increased the electrical conductivity. For the best system, La_0.7Ca_0.3CrO_3−δ + 5 wt.% Ce_0.8Sm_0.2O_1.9, the electrical conductivity reached 687.8 S cm⁻¹ at 800 °C in air. In H₂ at 800 °C, the specimen with 3 wt.% Ce_0.8Sm_0.2O_1.9 had the maximal electrical conductivity of 7.1 S cm⁻¹. With the increase of Ce_0.8Sm_0.2O_1.9 content the relative density increased, reaching 98.7% when the Ce_0.8Sm_0.2O_1.9 content was 10 wt.%. The average coefficient of thermal expansion at 30-1000 °C in air increased with Ce_0.8Sm_0.2O_1.9 content, ranging from 11.12 × 10⁻⁶ to 12.46 × 10⁻⁶ K⁻¹. The oxygen permeation measurement illustrated a negligible oxygen ionic conduction, indicating it is still an electronically conducting ceramic. Therefore, this material system will be a very promising interconnect for solid oxide fuel cells.     

Luminescence response and CL degradation of combustion synthesized spherical SiO2:Ce nanophosphor

This study was aimed to systematically investigate the luminescence response of SiO₂:Ce³⁺ nanophosphors with different excitation sources. The powders were synthesized by using an urea assisted combustion method. SiO₂:Ce_1m% samples were also annealed at 1000 °C for 1 h in a charcoal environment to reduce incidental Ce⁴⁺ to partial Ce³⁺ ions. High resolution transmission electron microscopy (HRTEM) images of the as synthesized and annealed powder samples confirmed that the particles were spherical and in the size range of 3-8 nm in diameter. X-ray diffraction (XRD) and electron dispersion spectroscopy (EDS) results showed that the SiO₂ was crystalline and pure. Diffused reflectance, photoluminescence (PL) and cathodoluminescence (CL) results of the SiO₂:Ce³⁺ samples were obtained and compared with each other. The CL degradation and the surface reactions on the surface of the SiO₂:Ce³⁺ were studied with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). A clear improvement in the chemical stability of the SiO₂:Ce³⁺ annealed at 1000 °C were obtained.     

Synthesis, characterization and electrical properties of quaternary selenodiphosphates: AMP2SE6 with A = Cu, Ag and M = Bi, Sb

The new quaternary selenophosphate phases AMP₂Se₆ (A=Cu, Ag and M=Bi, Sb) were synthesized by ceramic methods at 1023 K. These phases were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDX) and a.c. and d.c. electrical conductivity measurements. The phases all show values of electrical conductivity, σ, of about 10⁻⁴ Ω⁻¹ cm⁻¹ at 303 K and photoconductive effect. The conductivity is nearly five orders of magnitude larger than that of related phases.     

Study of Ln0.6Sr0.4Co0.8Mn0.2O3-δ (Ln=La, Gd, Sm or Nd) as the cathode materials for intermediate temperature SOFC

Perovskite type oxides Ln_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ (Ln=La, Gd, Sm, or Nd) have been prepared by the solid state reaction of corresponding oxides. The crystal parameters of the compositions were determined by XRD powder diffraction, which revealed that all the compositions have orthorhombic structure. The reaction test of all samples with Ce_0.8Gd_0.2O_1.9 was carried out at 1200 °C for 48 h, and no reaction product was detected by XRD. The change in mass of La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ as a function of temperature was determined by thermogravimetric analysis (TGA). The electrical conductivity of the sintered samples were measured as a function of temperature from 200 to 1000 °C. The highest conductivity of about 1400 S cm⁻¹ was found in La_0.6Sr_0.4Co_0.8Mn_0.2O_3−δ. The cathodic polarization of these oxides electrodes deposited on Ce_0.8Gd_0.2O_1.9 tablet was studied at 500-800 °C in air.     

Compositionally controlled rhombohedral to cubic phase transition in CsTe2−xWxO6 related to pyrochlore structure

CsTe₂O₆ adopts a rhombohedrally distorted pyrochlore related structure due to the 1:3 ordering of Te⁴⁺ and Te⁶⁺ in the octahedral sites respectively. Phases of the type CsTe_2−xW_xO₆ were found to have the cubic pyrochlore structure from x = 0.2 to 0.5. These phases all contain Te⁴⁺ and Te⁶⁺ (mixed with W⁶⁺) and are disordered in octahedral sites of the pyrochlore structure. This mixed valence situation results in strong optical absorption in the visible region of the spectrum but does not produce a measurable electrical conductivity.     

Synthesis and characterization of photoluminescent cerium-doped yttrium aluminum garnet

Powder phosphor yttrium aluminum garnet (YAG), doped with trivalent cerium (Ce³⁺) is synthesized by sol-gel method. The formation of YAG and YAG:Ce (cerium-doped yttrium aluminum garnet) was investigated by means of X-ray diffraction (XRD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also used. The purified crystalline phases of YAG and YAG:Ce were obtained at 1000 °C. The maximum average grain size is about 20-23 nm for undoped samples and 28-34 nm for doped samples. The crystalline YAG:Ce emission shows one peak in the range 480-535 nm with the maximum near 520 nm. Photoluminescence (PL) intensity of 5d → 4f transition of Ce³⁺ increased with increasing annealing temperature. With increasing the concentration of Ce³⁺, the photoluminescence peak shifts towards the red region.     

Preparation and electrical properties of xCaRuO3/(1 − x)CaTiO3 perovskite composites

CaRuO₃-CaTiO₃ ceramic composites were prepared by sintering for short times for potential applications in the areas of electronic ceramics. Scanning electron microscopy and energy dispersive X-ray analysis showed two separate phases, CaRuO₃ and CaTiO₃ in the composite. Conductivity data, measured by the four-probe method, showed that the composites have high electrical conductivity when x ≥ 0.19 in xCaRuO₃-(1 − x)CaTiO₃ composites. Furthermore, the nanoparticle of calcium ruthenate prepared by reverse micelle synthesis was used to be conductive agent for the composite. The result shows that the use of nano-sized calcium ruthenate enabled higher electrical conductivity to be maintained down to x = 0.09.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

High performance Gd and Y co-doped ceria-based electrolytes for intermediate temperature solid oxide fuel cells

Co-doped ceria of Ce_1−xGd_x−yY_yO_2−0.5x, wherein x = 0.15 and 0.2, 0 ≤ y ≤ x, were prepared by glycine-nitrate method. Their structures and ionic conductivities were characterized by X-ray diffraction (XRD) and AC impedance spectroscopy (IS). All the electrolytes were found to be ceria-based solid solutions of fluorite type structures. Co-doping was found to effectively enhance the conductivity. In comparison to the singly doped ceria, the co-doped ceria showed much higher ionic conductivities at 673-973 K. At 773 K, the ionic conductivity of Ce_0.8Gd_0.05Y_0.15O_1.9 is 0.013 S cm⁻¹ which is three times as high as that of Ce_0.8Gd_0.2O_1.9. These Gd³⁺and Y³⁺ co-doped ceria are ideal electrolyte materials of intermediate temperature solid oxide fuel cells (SOFCs).     

Oxide-ion conductivity in CuxCe1−xO2−δ (0 ≤ x ≤ 0.10)

Up to 10 at.% of copper readily substitutes for cerium in ceria. It is found that at oxygen partial pressures between 0.21 atm and 10⁻⁵ atm, Cu_xCe_1−xO_2−δ (0 ≤ x ≤ 0.10) solid solution behave as an oxide-ion electrolyte. Interestingly, Cu_0.10Ce_0.90O_2−δ exhibits the oxide-ion conductivity of ca. 10⁻⁴ Ω⁻¹ cm⁻¹ at 600 °C at an oxygen partial pressure of 10⁻⁵ atm.     

Luminescence properties of LaPO4:Tb,Me (Me = Gd, Bi, Ce) under VUV excitation

The luminescence properties of LaPO₄:Tb³⁺,Me³⁺ (Me = Gd, Bi, Ce) were investigated under VUV excitation. The results indicate that only Gd³⁺ plays an intermediate role in energy transfer from the host absorption band to Tb³⁺ under 147 nm excitation, Bi³⁺ and Ce³⁺ have no contribution to improving the emission intensity of La_0.95PO₄:Tb_0.05³⁺ because the charge transfer band of Bi³⁺ is mismatching for the excitation wavelength (147 nm) and Ce³⁺ can be oxidized easily. A new band at 135 nm is observed in the excitation spectrum of La_0.92PO₄:Tb_0.05³⁺,Bi_0.03³⁺, which may correlate with the absorption of Bi²⁺.     

Luminescence and afterglow in Sr2SiO4:Eu, RE [RE = Ce, Nd, Sm and Dy] phosphors—Role of co-dopants in search for afterglow

Luminescence of Eu²⁺ in Sr₂SiO₄:Eu²⁺, RE³⁺ [RE = Ce, Nd, Sm and Dy] phosphors was studied with a view to obtain an afterglow phosphor. The synthesized phosphors were characterized by powder X-ray diffraction (XRD), diffuse reflectance, photo- and thermoluminescence spectroscopic techniques. Afterglow was observed only with Dy³⁺ co-doped phosphor. The observed afterglow with Dy³⁺ co-doping originated from the formation of suitable traps which was supported by thermoluminescence results.     

The synthesis and optical property of solid-state-prepared YAG:Ce phosphor by a spray-drying method

Ce³⁺-activated yttrium aluminum garnet (Y₃Al₅O₁₂:Ce, YAG:Ce) powder as luminescent phosphor was synthesized by the solid-state reaction method. The phase identification, microstructure and photoluminescent properties of the products were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), absorption spectrum and photoluminescence (PL) analysis. Spherical phosphor particle is considered better than irregular-shaped particle to improve PL property and application, so this phosphor was granulated into a sphere-like shape by a spray-drying device. After calcinating at 1500 °C for 0, 4, and 8 h, the product was identified as YAG and CeO₂ phases. The CeO₂ phase content is decreased by increasing the calcination time or decreasing the Ce³⁺ doping content. The product showed higher emission intensity resulted from more Ce³⁺ content and larger grain size. The product with CeO₂ was found to have lower emission intensity. This paper presents the crystal structures of Rietveld refinement results of powder XRD data.     

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.     

The electronic properties of metal complexed poly(3-alkylthiophene) films

Dielectric spectroscopy has been used to extract the AC and DC electrical conductivities of P3MT complexed with the paramagnetic ion Fe²⁺, and these were compared to data for uncomplexed P3MT in the frequency range of 1 Hz-10 MHz. The polymers were nominally undoped; however, the presence of residual Fe³⁺ ions from the polymerisation reaction renders the materials p-type semiconductors. Measurements were carried out over the temperature range 123-323 K. An analysis of the frequency-dependent complex conductivity together with the DC conductivity can be used to elucidate whether free charge conduction, charge hopping or quantum tunnelling is the dominant conduction mechanism. The results were compared to the predictions of a variety of theoretical conduction models. It was found that the presence of the Fe²⁺ ions produced greater long-range order at low temperatures by complexing between the sulphur heteroatoms of four 3-methylthiophene monomer units. This provided an additional barrier hopping conduction mechanism at low temperatures (183-243 K), and the hopping occurred over a barrier height within the range 0.45-0.6 eV.