Room-Temperature Ferromagnetism in Chemically Synthesized Ce0.97Cr0.03O2−δ Nanopowders

In this paper, we study structural and magnetic properties of undoped (CeO_2?δ ) and Cr-doped (Ce_0.97Cr_0.03O_2?δ ) cerium oxide nanopowders synthesized by a sol-gel-based method. We estimate the crystallite sizes calculated from X-ray diffraction measurements to be around 9.4±0.3 nm for both CeO_2?δ and Ce_0.97Cr_0.03O_2?δ samples. Raman measurements indicate that microstructural defects are generated when Cr substitutes the Ce in the CeO₂ crystal lattice. Magnetic measurements of the Ce_0.97Cr_0.03O_2?δ sample at 300 K indicate ferromagnetic behavior, with a coercive field of 34.27 Oe and a saturation magnetization of 5.8×10^?4 emu/g. We interpret the nature of room-temperature ferromagnetism by taking into account the exchange interaction between Cr³⁺ ions and oxygen vacancies in CeO₂.     

Highly (100)-oriented Ce1 − xFexO2 − δ solid solution films prepared by laser chemical vapor deposition

Ce_1 − xFe_xO_2 − δ solid solution films were prepared on amorphous silica substrates by laser chemical vapor deposition using metal dipivaloylmethanate precursors and a semiconductor InGaAlAs (808 nm in wavelength) laser. X-ray diffraction revealed the formation of single Ce_1 − xFe_xO_2 − δ phase at x ≤ 0.15, while CeO₂ and Fe₂O₃ phases were found for higher Fe content. Highly (100)-oriented Ce_1 − xFe_xO_2 − δ (x = 0.02) films were obtained at laser power, P_L = 50-200 W and deposition temperature, T_dep = 800-1063 K. Lotgering factor (200) was calculated to be above 0.8 for films prepared at P_L = 50-150 W. X-ray photoelectron spectroscopy revealed the presence of Fe³⁺, Ce⁴⁺ and Ce³⁺ on solid solution films. Cross-sectional transmission electron microscope images disclosed a film columnar feather-like structure with a large number of nano-scale interspaces. Deposition rates were 2 or 3 orders of magnitude higher than those reported for conventional metal organic chemical vapor deposition of CeO₂.     

Hydrothermal synthesis of Sr–Ce–Sn–Mn–O mixed oxidic/stannate pyrochlore and its catalytic performance for NO reduction

Mixed oxides and pyrochlore-type materials based on the Sr, Ce, Sn, and Mn elements have been prepared by hydrothermal method using a mixture of nitrate salts at 200 °C for 40 h under N₂ atmosphere. Two groups of solids were synthesized: (i) (Sr_xCe_1?x)₂Sn₂O_7±δ (0.345 ≤ x ≤ 0.365) and (ii) (Sr_xCe_1?x)₂(Sn_yMn_1?y)₂O_7±δ (0.345 ≤ x ≤ 0.365, 0.385 ≤ y ≤ 0.395). The structure of the solids were studied by X-ray diffraction and the main crystal phases determined were SnO₂, (SrCe)₂Sn₂O₇ and a trace amount of CeO₂ in group (i) and only SnO₂ and CeO₂ were detected in group (ii). The mixed oxides/stannate pyrochlore have been tested as catalyst for NO reduction with hydrocarbons (CH₄, C₂H₄, and C₃H₆) in the presence of O₂. The CeO₂ containing stannate Sr_2xCe_2?2xSn₂O_7±δ pyrochlore coexist with SnO₂ (group (i)) was found to be the best for NO + C₃H₆ reaction giving very high N₂ production at 350 °C in the presence of O₂ and water vapor. SnO₂ as well as CeO₂ alone were also synthesized by the hydrothermal method and their NO reduction properties have been compared with those of the groups (i) and (ii).     

Electrochemical performance of (La,Sr)(Co,Fe)O3−δ–(Ce,Sm)O2−θ–CuO composite cathodes for intermediate temperature solid oxide fuel cells

La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ–CuO composite cathodes were studied for the potential application in intermediate temperature solid oxide fuel cells. Ce_0.8Sm_0.2O_2?θ electrolyte with porous Ce_0.8Sm_0.2O_2?θ interlayer was successfully prepared by one-step sintering process. The effect of interlayer between cathode and electrolyte and CuO on the electrochemical performance of the composite cathodes was investigated by AC impedance spectroscopy. The application of interlayer decreased the area specific resistance of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ cathode. The addition of CuO to La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ reduced the phase formation temperature of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ by 150 °C and the addition of CuO to La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ cathode reduced the optimal calcination temperature of the cathode to 800 °C. The composite cathode with 2 mol% CuO calcined at 800 °C exhibited the lowest area specific resistance of 0.05 Ω cm² at 700 °C in air, which was reduced by 67% compared with that of La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ–Ce_0.8Sm_0.2O_2?θ cathode. The studies of the corresponding single cell performance, thermal expansion and thermal cycling behaviors further indicated that the composite cathode with 2 mol% CuO could be a promising cathode material.     

Single step synthesis of nanosized CeO2–MxOy mixed oxides (MxOy?=?SiO2, TiO2, ZrO2, and Al2O3) by microwave induced solution combustion synthesis: characterization and CO oxidation

Various CeO₂ –M _x O _y (M _x O _y  = SiO₂, TiO₂, ZrO₂, and Al₂O₃) mixed oxides were prepared by microwave induced solution combustion method and analyzed by different complimentary techniques, namely, X-ray diffraction (XRD), Raman spectroscopic (RS), UV–Vis diffuse reflectance spectroscopy (UV-DRS), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG-DTA), and BET surface area. XRD analyses revealed that CeO₂ –SiO₂ and CeO₂ –TiO₂ mixed oxides are in slightly amorphous form and exhibit only broad diffraction lines due to cubic fluorite structure of ceria. XRD lines due to the formation of cubic Ce_0.5Zr_0.5O₂ were observed in the case of CeO₂ –ZrO₂ sample. RS results suggested defective structure of the mixed oxides resulting in the formation of oxygen vacancies. The UV-DRS measurements provided valid information about Ce⁴⁺ ← O²⁻ and Ce³⁺ ← O²⁻ charge transfer transitions. XPS studies revealed the presence of cerium in both Ce³⁺ and Ce⁴⁺ oxidation states. The ceria–zirconia combination exhibited better oxygen storage capacity (OSC) and CO oxidation activity when compared to other samples. The significance of present synthesis method lays mostly on its simplicity, flexibility, and the easy control of different experimental factors.     

Properties and performance of La1.6Sr0.4NiO4+δ-Ce0.8Sm0.2O1.9 composite cathodes for intermediate temperature solid oxide fuel cells

La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 composite cathodes were prepared successfully using combustion synthesis method for intermediate temperature solid oxide fuel cells. The chemical compatibility, thermal expansion behavior, electrical conductivity and electrode performance were studied. The X-ray diffraction of La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 composite result proved a slight reaction between La_1.6Sr_0.4NiO_4+δ and Ce_0.8Sm_0.2O_1.9. Both the thermal expansion coefficient and the electrical conductivity of La_1.6Sr_0.4NiO_4+δ-Ce_0.8Sm_0.2O_1.9 decreased with increasing Ce_0.8Sm_0.2O_1.9 content. AC impedance spectroscopy measurements indicated that the addition of 30 wt% Ce_0.8Sm_0.2O_1.9 to La_1.6Sr_0.4NiO_4+δ exhibited the lowest polarization resistance (0.238 Ωcm²) at 800 °C in air, which was only one fourth of the La_1.6Sr_0.4NiO_4+δ electrode measured at the same temperature.     

Structure and Magnetic Properties of Ce-Substituted Yttrium Iron Garnet Prepared by Conventional Sintering Techniques

Y_3?xCe _x Fe₅ O ₁₂ (CeYIG) ceramics, with x = 0, 0.15, 0.25, 0.35, 0.45, and 0.5, were fabricated by a conventional ceramic sintering technique. We studied the structures and magnetic fields of a series of CeYIG ceramics using X-ray powder diffraction, a scanning electron microscope, and a superconducting quantum interference device magnetometer. Findings showed that the substitution limit of the concentration of Ce³⁺ ions in the yttrium iron garnet structure was approximately x = 0.25. An extra CeO₂ phase was detected in the ceramic when the addition of CeO₂ content overtook the limit. The lattice constants and relative densities increased by increasing the Ce³⁺ contents in the ceramics. First, the saturation magnetization increased gradually with increases in the substitute concentration of Ce³⁺ ions and then decreased gradually when x = 0.35, 0.45, and 0.5. Overall, this study showed that the Y_3?xCe _x Fe₅ O ₁₂ material with x ≤ 0.15 exhibited excellent magnetic properties. Hence, the material show promise for magneto-optical and microwave communication applications.     

Room-temperature multiferroic properties and magnetoelectric coupling in Bi4−x Sm x Ti3−x Co x O12−δ ceramics

We present the structural, dielectric, ferroelectric, magnetic and magnetoelectric studies of lead free; single phase Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (0 ≤ x ≤ 0.07) ceramics, synthesized using a standard solid-state reaction technique. Raman spectroscopy analysis reveals the relaxation of distortion in TiO₆ octahedron. Field emission scanning electron microscopy confirmed the growth of plate-like grains. It is observed that with the substitution of Sm³⁺ and Co³⁺ ions the dielectric constant, loss tangent and ferroelectric transition temperature decreases. Electrical dc resistivity, remnant polarization and magnetization increases with increasing Sm³⁺ and Co³⁺ contents. The magnetoelectric coupling co-efficient, α = 0.65 mV cm^?1 Oe^?1 is realized for Bi_4?x Sm _x Ti_3?x Co _x O_12?δ (x = 0.07) ceramic sample. Our results clearly demonstrate the lead free, multiferroic nature of Sm/Co-substituted Bi₄Ti₃O₁₂, which may find useful application in designing cost-effective electromagnetic devices.     

Structure and electrical conductivity of Ln2+x Hf2−x O7−x/2 (Ln = Sm-Tb; x = 0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln_2+x Hf_2?x O_7?δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln₂Hf₂O₇ (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm_2.096Hf_1.904O_6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-P1-P) phase transformations in the temperature range 1200–1670°C. The former transformation leads to a rise in 840°C conductivity from 10^?4 to 3 × 10^?3 S/cm in the samples synthesized at 1600°C, and the latter leads to a drop in 840°C conductivity to 6 × 10^?4 S/cm in the samples synthesized at 1670°C. The reduction in the conductivity of Sm_2.096Hf_1.904O_6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu_2+x Hf_2?x O_7?δ (x = 0.096) and Ln₂Hf₂O₇ (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu_2.096Hf_1.904O_6.952, Gd₂Hf₂O₇, and Tb₂Hf₂O₇ synthesized at 1670°C: 7.5 × 10^?3, 5 × 10^?3, and 2.5 × 10^?2 S/cm, respectively.     

Novel porous perovskite composite CeO2@LaMnO3/3DOM SiO2 as an effective catalyst for activation of PMS toward oxidation of urotropine

Perovskites with stable crystal structure and excellent catalytic performance have attracted extensive attention in peroxomonosulfate (PMS) activation, however, severe agglomeration has always been the main obstacle limiting the catalytic activity of them, so novel perovskite catalysts are urgently needed. In this study, three-dimensional ordered macroporous silica (3DOM SiO₂) was prepared by colloidal crystal template method, then CeO₂@LaMnO₃/3DOM SiO₂ was prepared by sol-gel method combined with impregnation method and used to activate PMS for urotropine (URO) degradation. CeO₂@LaMnO₃/3DOM SiO₂ activated PMS system exhibited high URO removal efficiency and quick kinetic, as 99.98 % URO was degraded even within 30 min. The catalyst has a wide pH range and still has high catalytic activity in the presence of organic matter and inorganic ions. The three components in CeO₂@LaMnO₃/3DOM SiO₂ showed a synergetic effect. CeO₂ and LaMnO₃ were uniformly loaded on 3DOM SiO₂, which effectively avoided agglomeration. The specific surface area of CeO₂@LaMnO₃/3DOM SiO₂ was 11.88 times that of LaMnO₃ prepared by sol-gel method. There are two redox cycles of Ce³⁺/Ce⁴⁺ and Mn²⁺/Mn³⁺/Mn⁴⁺ in CeO₂ and LaMnO₃, respectively, which synergistically realize the activation of PMS. Both quenching experiments and electron paramagnetic resonance (EPR) analysis revealed that that SO₄^?, OH and ¹O₂ jointly achieved the degradation of URO. In summary, CeO₂@LaMnO₃/3DOM SiO₂ would be a promising candidate for practical wastewater treatment.     

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.     

Cerium ion redox system in CeO2–xFe2O3 solid solution at high temperatures (1,273–1,673 K) in the two-step water-splitting reaction for solar H2 generation

CeO₂–xFe₂O₃ (x = 0.026–0.214) solid solutions with different Ce:Fe mole ratios (Ce:Fe = 9.5:0.5–7.0:3.0) were prepared as reactive ceramics with the combustion method for solar hydrogen production. The prepared CeO₂–xFe₂O₃ solid solutions were characterized by X-ray diffractometry, ICP atomic emission spectrometry, and Mössbauer spectroscopy. Two-step water-splitting reaction with the CeO₂–xFe₂O₃ solid solution proceeded at 1,673 K for the O₂-releasing reaction and at 1,273 K for the H₂-generation reaction by irradiation of an infrared imaging lamp as a solar simulator. The amounts of H₂ gas evolved in the H₂-generation reaction with CeO₂–xFe₂O₃ solid solutions were 0.97–1.8 cm³/g, the evolved H₂/O₂ ratio was approximately equal to 2 of the stoichiometric value. The amounts of H₂ and O₂ gases were independent of the Ce:Fe mole ratio in the CeO₂–xFe₂O₃ solid solution. It was suggested that the O₂-releasing and H₂-generation reactions with the CeO₂–xFe₂O₃ solid solution were repeated with the reduction and oxidation of Ce⁴⁺–Ce³⁺ enhanced by the presence of Fe³⁺–Fe²⁺.     

Preparation of porous Fe2O3/SiO2 nanocomposite abrasives and their chemical mechanical polishing behaviors on hard disk substrates

Abrasives play an important role in chemical mechanical polishing (CMP) processes. Compact solid silica particles, which have been widely used as abrasive in CMP slurries, may cause surface defects because of their high hardness. Porous silica abrasive exhibits better surface planarization and fewer scratches than traditional solid silica abrasive during the polishing of hard disk substrates. However, the improvement in material removal rate (MRR) was not significant. Therefore, porous Fe₂O₃/SiO₂ nanocomposite abrasives were prepared and their CMP performances on hard disk substrates were investigated. Experiment results indicate that the MRR of slurry containing porous Fe₂O₃/SiO₂ nanocomposite abrasives is obviously higher than that of slurry containing pure porous silica abrasive under the same testing conditions. MRR increases with the increase of the molar content of iron in porous Fe₂O₃/SiO₂ nanocomposite abrasives. Moreover, surfaces polished by slurries containing the porous Fe₂O₃/SiO₂ nanocomposite abrasives exhibit lower surface roughness, fewer scratches as well as lower topographical variations than that by pure porous silica abrasive.     

A Cobalt-Free Multi-Phase Nanocomposite as Near-Ideal Cathode of Intermediate-Temperature Solid Oxide Fuel Cells Developed by Smart Self-Assembly

An ideal solid oxide fuel cell (SOFC) cathode should meet multiple requirements, i.e., high activity for oxygen reduction reaction (ORR), good conductivity, favorable stability, and sound thermo-mechanical/chemical compatibility with electrolyte, while it is very challenging to achieve all these requirements based on a single-phase material. Herein, a cost-effective multi-phase nanocomposite, facilely synthesized through smart self-assembly at high temperature, is developed as a near-ideal cathode of intermediate-temperature SOFCs, showing high ORR activity (an area-specific resistance of ≈0.028 Ω cm² and a power output of 1208 mW cm⁻² at 650 °C), affordable conductivity (21.5 S cm⁻¹ at 650 °C), favorable stability (560 h operation in single cell), excellent chemical compatibility with Sm_0.2Ce_0.8O_1.9 electrolyte, and reduced thermal expansion coefficient (≈16.8 × 10⁻⁶ K⁻¹). Such a nanocomposite (Sr_0.9Ce_0.1Fe_0.8Ni_0.2O_3–δ) is composed of a single perovskite main phase (77.2 wt%), a Ruddlesden–Popper (RP) second phase (13.3 wt%), and surface-decorated NiO (5.8 wt%) and CeO₂ (3.7 wt%) minor phases. The RP phase promotes the oxygen bulk diffusion while NiO and CeO₂ nanoparticles facilitate the oxygen surface process and O²⁻ migration from the surface to the main phase, respectively. The strong interaction between four phases in nanodomain creates a synergistic effect, leading to the superior ORR activity.     

Preparation of an Alkali-Element or Alkali-Earth-Element-Doped CeO2–Sm2O3 System and Its Operation Properties as the Electrolyte in Planar Solid Oxide Fuel Cells

Ceria–samaria (CeO₂–Sm₂O₃) is one of the most interesting fluorite oxides since its ionic conductivity is higher than that of yttria-stabilized zirconia in air. However, these CeO₂ -based oxides are partially reduced and develop electronic conductivity under fuel cell operating conditions. In their application to the SOFC system, their current densities and power densities are not at a satisfactory level. For the development of high-performance CeO₂ electrolytes, it is important that the fluorite lattice of CeO₂-based oxide be improved from the viewpoint of crystallography. In this study, it is assumed that the reduction of Ce⁴⁺ in the fluorite lattice was inhibited by expansion of the CeO₂ lattice. In order to investigate the contribution of the expanded CeO₂ lattice to reduction resistance, CeO₂–Sm₂O₃ solid solution, calcia-doped CeO₂–Sm₂O₃ solid solution, and a small amount of alkali element-doped CeO₂–Sm₂O₃ -based oxide were prepared for comparison. It was found that the calcia or a small amount of alkali element-doped CeO₂ solid solution enhanced the oxide ionic conductivity. The power density of the latter showed a high value at 800°C. It is concluded that the improved fuel cell performance can be attributed to the good reduction resistance in the fuel cell atmosphere.     

Effects of some rare earth oxides and/or SiO2 additions to a commercial ZrO2 paint for coating SiGe thermoelectric materials

The effects of some rare earth oxides (Y₂O₃, CeO₂, Sm₂O₃ or Er₂O₃) and/or SiO₂ additions to a commercial oxide paint (mainly ZrO₂) were studied as coatings for SiGe thermoelectric materials. No spalling occurred in the samples prepared from coatings containing small amounts of Er₂O₃, SiO₂ or both CeO₂ and SiO₂ while heating for 1322 h at 1080° C in vacuum, and the sublimation of silicon and/or germanium was reduced significantly. The improvements imparted by these oxide additions to the commercial oxide paint is thought to be due to decreased internal stress of the surface oxides. The addition of Y₂O₃ and Sm₂O₃ had a detrimental effect.     

Reduction reaction of lanthanum-added cerium dioxide with carbon monoxide

The reaction of lanthanum-added cerium dioxide with carbon monoxide was examined by using a CO pulse reaction method and high-temperature X-ray diffraction (XRD). The lanthanum addition enhanced the activity of cerium dioxide for oxidizing carbon monoxide under moderately reducing conditions. Isothermal XRD observation at 500 and 700 °C indicated that the reduction reaction of CeO₂ and La-added CeO₂ with CO progressed in CO-N₂ flowing gas. The kinetics of the reaction CeO₂ + x/2CO → Cex1−x/4+Ce _x ³⁺ O_2−x/2 + x/2CO₂ + x/2V₀ was analysed by Jander's model: [1−(1−x)^1/3]²=kt.     

Mechanism of structure formation in samarium and holmium titanates prepared from mechanically activated oxides

We have studied the formation mechanism and phase transitions of samarium and holmium titanates prepared from mechanically activated oxide mixtures with the overall compositions Sm₂(Ho₂)Ti₂O₇ and Sm₂TiO₅. Mechanical activation of oxide mixtures leads to the formation of amorphous solid phases which crystallize in a distorted pyrochlore-like structure and contain OH groups on the oxygen site and structural vacancies up to 1000°C. In the range 800–1000°C, Sm_2?x Ti_1?y O_5?δ (OH) _n (x < 0.02; y < 0.08; δ, n < 0.19) converts to a distorted orthorhombic phase as a result of the relaxation of internal stress and removal of OH groups. Above 1000°C, the phases studied have the compositions Sm₂(Ho₂)Ti₂O₇ and Sm₂TiO₅ and ordered pyrochlore-like and orthorhombic structures, respectively. The lattice parameters of the titanates have been measured in the range 800–1350°C. The internal stress produced by mechanical activation in the phases studied here fully relaxes by ～1300°C.     

Abundant Ce3+ Ions in Au‐CeOx Nanosheets to Enhance CO2 Electroreduction Performance

The electroreduction of CO₂ to CO provides a potential way to solve the environmental problems caused by excess fossil fuel utilization. Loading transition metals on metal oxides is an efficient strategy for CO₂ electroreduction as well as for reducing metal usage. However, it needs a great potential to overcome the energy barrier to increase CO selectivity. This paper describes how 8.7 wt% gold nanoparticles (NPs) loaded on CeO_x nanosheets (NSs) with high Ce³⁺ concentration effectively decrease the overpotential for CO₂ electroreduction. The 3.6 nm gold NPs on CeO_x NSs containing 47.3% Ce³⁺ achieve CO faradaic efficiency of 90.1% at ?0.5 V in 0.1 m KHCO₃ solution. Furthermore, the CO₂ electroreduction activity shows a strong relationship with the fractions of Ce³⁺ on Au‐CeO_x NSs, which has never been reported. In situ surface‐enhanced infrared absorption spectroscopy shows that Au‐CeO_x NSs with high Ce³⁺ concentration promote CO₂ activation and *COOH formation. Theoretical calculations also indicate that the improved performance is attributed to the enhanced *COOH formation on Au‐CeO_x NSs with high Ce³⁺ fraction.     

Synthesis and characterization of Gd3+ and Nd3+ co-doped ceria by using citric acid–nitrate combustion method

A series of Ce_0.8Gd_0.2?xNd_xO_2?δ (x = 0–0.20) compositions have been synthesized by citric acid–nitrate combustion method. XRD measurements indicate that all the obtained materials crystallized in cubic fluorite-type structure. Lattice parameters were calculated by Rietveld method and the parameter a values in Ce_0.8Gd_0.2?xNd_xO_2?δ system obey Vegard's law, a (Å) = 5.4224 + 0.1208x. The obtained powders have good sinterability and the relative density could reach above 95% after being sintered at 1400 °C. Impedance spectroscopy measurements indicated that the conductivity of Ce_0.8Gd_0.2?xNd_xO_2?δ first increased and then decreased with Nd dopant content x. The maximum conductivity, σ_700 °C = 6.26 × 10^?2 S/cm, was found in Ce_0.8Gd_0.12Nd_0.08O_1.9 when sintered at 1300 °C. The corresponding activation energies of conduction had a minimum value E_a = 0.676 eV. The results tested experimentally the validity of the effective atomic number concept of recent density functional theory, which had suggested that co-dopant with effective atomic number between 61 (Pm) and 62 (Sm) was the ideal dopant exhibiting high ionic conductivity and low activation energy.