X-ray,electrical conductivity,and infrared studies of oxide spinels containing cobalt,titanium and manganese

X-ray, electrical conductivity and infrared studies of the system Co_2–x Ti_1–x Mn_2x O₄ were carried out with a view to investigate the cation distribution. Compounds 0 x 0.6 showed cubic symmetry, compounds 0.8 x 1 are tetragonal. X-ray intensity calculation indicated the presence Of Co²⁺ and Mn³⁺ at both octahedral and tetrahedral sites. A plot of activation energy against composition shows a break where crystal symmetry changes. The electrical conductivity-temperature behaviour obeys the Raschhinrichsen law. The mobility of holes calculated from infrared and conductivity data is of the order Of 10^–9 cm² V^–1 sec^–1. X-ray intensity calculations and conductivity measurements suggest the ionic configuration of the system to be Co _1–x ²⁺ Mn _x ³⁺ [Co _2–2x ²⁺ Mn _x ³⁺ Ti _x ⁴⁺ ]O ₄ ^2– .     

Preparation and electrochemical characterization of Sr- and Mn-doped LaGaO3 as anode materials for LSGM-based SOFCs

Sr- and Mn-doped lanthanum gallate powders (LSGMn, La_0.9Sr_0.1Ga_{1 – x} Mn _x O_{3 –} , x = 0.20, 0.35, 0.43) were prepared by glycine-nitrate combustion synthesis. X-ray diffraction patterns indicate the perovskite structure was formed without any second phase after calcining the powders at 1000°C for 4 h. Compacts of powders synthesized under stoichiometric combustion were sintered to densities over 95% of theoretical values. The electrical conductivity of this material in both air and H₂ were characterized using AC impedance spectroscopy. It showed that the conductivity in H₂ atmosphere is lower than that in air due to p-type electrical conduction in this material, and the electrical conductivity increases remarkably with increasing manganese content. Good chemical stability of La_0.9Sr_0.1Ga_{1 – x} Mn _x O_{3 –} in H₂ atmosphere as well as the relatively high conductivity makes it an appropriate anode material for Sr- and Mg-doped lanthanum gallate (LSGM)-based IT-SOFCs. Preliminary fuel cell performance measurements were performed, showing promising electrochemical properties of such anode materials.     

Structural,transport and infrared studies of oxidic spinels Zn1−xNixFeCrO4

X-ray, electrical conductivity and IR studies for the system Zn_1–x Ni _x FeCrO₄ were carried out. All the compounds, 0 x 1 showed cubic symmetry. X-ray intensity calculations and IR studies indicate the presence of Zn²⁺ at tetrahedral sites, Ni²⁺ and Cr³⁺ at octahedral sites, and Fe³⁺ ions are equally distributed at both the sites. Activation energy and thermoelectric coefficient decreases with the increasing values ofX. All the compounds exhibit P-type semiconductivity which may be due to transfer of Fe³⁺ ions from B-site to A-site creating holes. The electrical resistivity temperature behaviour obeys the relation= ₀ exp (E/KT). The mobility of holes as calculated from IR and conductivity data is of the order of 10^–9 cm² V sec^–1. The probable ionic configuration for the system is suggested to be Zn _1–x ²⁺ Fe _x ³⁺ [Fe _1–x ³⁺ Ni _x ²⁺ Cr³⁺] O ₄ ^2– .     

Effects of Al2O3 addition on the sinterability and ionic conductivity of nasicon

The effects Al₂O₃ doping on the sintering behaviour and ionic conductivity of nasicon have been investigated using formulations of Na_3+2x Zr₂Si₂Al _x P_1–x O₁₂ where Al³⁺ ions were substituted to P⁵⁺ sites within the range 0x0.2. The density of Na_3+2x Zr₂Si₂Al _x P_1–x O₁₂ was increased with increasing amount of Al₂O₃ doping due to the enhanced liquid-phase sintering. The relative density of Na_3.4Zr₂Si₂Al_0.2P_0.8O₁₂ reached a maximum value of 96% when sintered at 1200°C for more than 7 h. The maximum conductivity of 0.24 ^–1 cm^–1 at 300°C was obtained for the composition with x=0.1 when sintered at 1200°C for 10 h.     

Carbon monoxide oxidation on (Mg6−x Al x )MnO8 (0⩽x⩽0.6)

Murdochite-type (Mg_6–x Al _x )MnO_m8 (0x0.6) was synthesized at low temperature (700 °C) with the sol-gel process. The specific surface area, crystallite size, catalytic activity for the CO oxidation, and adsorption isotherms of oxygen on (Mg_6–x Al _x )MnO₈ were measured. The increase of the specific surface area with increasing Al³⁺ ion content was explained by the decrease of the particle size. The catalytic activity increased with increasing Al³⁺ ion content, and this increase was caused by the valence deviation from the Mn⁴⁺ ion to the Mn^3.5+ ion.     

Effect of Co substitution for Mn on Y1−x Sr x MnO3 properties for SOFC cathode material

Y_0.6Sr_0.4Mn_1–y Co _y O₃ (0 y 0.4) perovskite oxides were prepared by the coprecipitation method. The effect of Co substitution for Mn on the crystal structure, electrical conductivity and thermal expansion properties were investigated. By X-ray powder diffraction, the crystal structure was found to change from hexagonal symmetry of Y_0.8Sr_0.2MnO₃ to orthorhombic of Y_0.6Sr_0.4Mn_1–y Co _y O₃. The differences in the structure of the unsubstituted Y_1–x Sr _x MnO₃ (0.2 x 0.4) are attributed to the average ionic radii of the cations and the amounts of Mn⁴⁺ present. The results of electrical conductivity analysis can be described by the small polaron hopping conductivity model. With Co substitution, the activation energy increases, possibly due to an increase of Jahn–Teller distortion, at an extent higher than the increase of the concentration of charge carriers; thus, the electrical conductivity decreases. In addition, the relative densities of the materials reached 94% with sintering at 1350°C for 12 h and had higher concentration of the available lattice sites, thus showing higher conductivity, than that with sintering at 1300°C for 6 h, which achieved 70% relative density. It is also found that the thermal expansion coefficient (TEC) increases as the Sr and Co content of Y_1–x Sr _x Mn_1–y Co _y O₃ increases and those with Co content of y = 0.2 exhibit TEC compatibility with YSZ.     

Zr-Hf interdiffusion in polycrystalline Y2O3-(Zr+Hf)O2

Lattice and grain-boundary interdiffusion coefficients were calculated from the concentration distributions determined for Zr-Hf interdiffusion in polycrystalline 16Y₂O₃·84(Zr_1–x Hf _x )O₂ withx=0.020 and 0.100. The lattice interdiffusion coefficients were described byD=0.031 exp [–391 (kJ mol^–1)/RT] cm² sec^–1 and the grain-boundary diffusion parameters byD=1.5×10^–6exp [–309(kJ mol^–1)/RT] cm³ sec^–1 in the temperature range 1584–2116° C. Comparison of the results with those for the systems CaO-(Zr+Hf)O₂ and MgO-(Zr+Hf)O₂ indicated that the Zr self-diffusion coefficient was insensitive to the dopants in the fluorite-cubic ZrO₂ solid solutions.     

Study by thermogravimetry,electrical conductivity and IR spectrometry of the phase transition for manganese cation deficient spinels

Thermogravimetry, electrical conductivity and IR spectrometry were used to study the crystal structure change of the metastable manganese cation deficient spinels that occur during the oxidation of manganese ions above 500° C. The kinetic of the transformation is found to be governed by a nucleation growth mechanism with an activation energy varying with the amount of Mn³⁺ ions. The nature of the inversion products depends on the manganese substitution extent. We observe for a substitution extent x<0.72 a phase with a corundum structure which is a solid solution of -(MnFe)₂O₃ and for x>0.72 a mixture of two phases -(MnFe)₂O₃ + Mn₂O₃ of orthorhombic structure.     

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.     

Effect of Crystal Structure on the Electrical Properties of CaTi1 – xFexO3 – δ

The electrical conductivity of CaTi_{1 – x} Fe _x O_{3 –} (x= 0–0.5) was measured as a function of temperature and oxygen partial pressure. At 1000°C, the highest conductivity was observed at x= 0.2. The crystal structure of the materials with x= 0, 0.2, 0.25, and 0.3 was studied by x-ray powder diffraction and refined by the full-profile analysis method. The results were used to elucidate the mechanisms of the high-temperature (1000°C) formation, ordering, and transport of oxygen vacancies in CaTiO₃upon substitution of Fe³⁺for Ti⁴⁺. The composition dependences of ionic conductivity calculated for CaTi_{1 – x} Fe _x O_{3 –} agree well with experiment.     

Dielectric properties of (Ba,Sr)O-(Sm,La)2O3-TiO2 ceramics at microwave frequencies

The dielectric properties of (Ba, Sr)O-(Sm, La)₂O₃-TiO₂ material at microwave frequencies were investigated. By varying the amount of strontium from 0–25 mol% in the 0.15(Ba_1–x Sr _x )O-0.15Sm₂O₃-0.7TiO₂ composition, it was possible to adjust the frequency temperature coefficient, _f, from –13 p.p.m. °C^–1 to + 30 p.p.m. °C^–1. When 7 mol% Sr was substituted for barium, _f=0 p.p.m. °C^–1 was obtained. TiO₂ with rutile phase (_f400 p.p.m. °C^–1) acted as a dominant element in _f variation of the 0.15(Ba_1–x Sr _x )O-0.15(Sm_1–y La _y )₂O₃-0.7TiO₂ (0x0.25, 0y0.6) system. Additionally, increasing the quantity of lanthanum substitution for samarium had a greater positive effect on _f than strontium substitution for barium. When 60 mol% La was substituted for samarium with 7mol% Sr substitution barium, _f of the system reached 95 p.p.m. °C^–1. The effect on microwave dielectric characteristics of the 0.15(Ba_0.93Sr_0.07)O-0.15Sm₂O₃-0.7TiO₂ (BSST) ceramics by varying the calcination and/or sintering conditions or doping additives, were studied. The added SnO₂ acted as a firing agent to lower the sintering temperature, and the dielectricQ(Q _d) value was improved by properly adding CdO. With 1 wt% CdO addition, the highestQ _d value of the BSST resonator, after calcination at 1100 °C/2 h and sintering at 1370 °C/4 h, reached 4180 at 4 GHz with a small _f of –4 p.p.m. °C^–1 and an _r of 80.7 was obtained.     

The preparation,characterization and electrical properties of copper manganite spinels,CuxMn3−xO4, 0 ⩽x ⩽ 1

Single-phase copper manganite spinels Cu _x Mn_3–x O₄ with 0x1 were prepared by a careful thermal processing of copper-manganese co-precipitated hydroxide precursors. Powder X-ray diffraction (XRD) analysis of these spinels revealed the presence of a tetragonally distorted single spinel phase, with tetragonal distortionc/a > 1 which decreases with copper content. Thermogravimetric analysis (TGA) curves, run in an oxygen atmosphere for all the compositions studied, are characterized by a stability step up to 250 to 300° C, followed by a domain of oxidation between 300 and 900° C and finally a domain of reduction that restores the initial stoichiometry of the samples. The oxidation is observed to occur in two successive steps. The phenomenon appearing at the low temperature is due to the oxidation of Cu⁺ ions, while that at higher temperature corresponds to the oxidation of Mn²⁺ ions in tetrahedral sites. Further, electrical resistivity measurements confirm the presence of Cu⁺ ions on the tetrahedral sublattice of spinel. Correlation of the results obtained by XRD, TGA and electrical resistivity measurements permits one to infer the cation distribution, given by Cu _x ⁺ Mn _1–x ²⁺ [Mn _2–x ³⁺ Mn _x ⁴⁺ ] O ₄ ^2– .     

Proton Conductivity in H2 – x (NH4) x V9Mo3O32.5 – δ · nH2O Xerogel Films

The electrical conductivity of _{2 – x} (NH₄) _x V₉Mo₃O_{32.5 –} · nH₂O xerogel films containing different amounts of ammonium ions and water was measured as a function of temperature. The results demonstrate that, in a wide temperature range, the proton conductivity of the films reaches a maximum atx= 1.5. The activation energy of conductivity in the films, equal to 0.1–0.2 eV at low temperatures, rises upon partial dehydration above 100°C. The possible mechanisms of proton formation and transport are discussed.     

Reactions of MnO2, Mn2O3, α-Bi2O3, and Bi12Ti1 – x Mn x O20with Supercritical Isopropanol

Heterogeneous reactions between supercritical isopropanol and metal oxides were studied for the first time. The results demonstrate that Bi₂O₃ is reduced by isopropanol to metallic bismuth, while MnO₂ and Mn₂O₃ are reduced to Mn₃O₄ (hausmannite). The possibility of oxygen extraction from nonstoichiometric oxides is demonstrated by the example of the reaction Bi₁₂Ti_{1 – x} Mn⁵⁺ _x O_{20 +} Bi₁₂Ti_1–x Mn²⁺ _x O_{20 –} .     

Influence of Soaking in the Molten (Li0.62K0.38)2CO3Eutectic on the Structure and Conductivity of LiMnxFe1 – xO2and Li1 – yMnyFeO2 + δ

The phase composition, structure, and 920-K electrical conductivity of LiMn _x Fe_{1 – x} O₂(0 < x 1) and Li_{1 – y} Mn _y FeO_{2 +} (0 < y 1) materials prepared by solid-state reactions in air at 1270 K were studied before and after soaking in molten (Li_0.62K_0.38)₂CO₃. The highest conductivity of 32 S/m was found in the spinel solid solution Li_0.2Mn_0.8FeO_{2 +} ( = 2/3). Soaking in the carbonate melt converted all Li_{1 – y} Mn _y FeO_{2 +} samples into rocksalt solid solutions and reduced their conductivity.     

Structural and electrical properties of cadmium-sulpho-selenide solid solutions

Complete solid solutions of CdS _x Se_1–x (0x1) were synthesized by vacuum fusion of stoichiometric proportions of CdS and CdSe. X-ray diffraction data revealed that they possess the hexagonal wurtzite structure. The unit cell lattice constants vary linearly with the composition parameter,x, following Vegard's law.Thin films of CdS _x Se_1–x (0x1) solid solutions could be deposited onto glass substrates by thermal evaporation of the bulk material in 10^–4 Pa vacuum. Structural investigation showed that the films are polycrystalline with predominant appearance of the (002) reflecting plane. On annealing at 250°C in 10^–2 Pa vacuum atmosphere, aggregation and rearrangement of the as-deposited tiny crystallite occurred, preserving the same crystal structure.The dark electrical resistivity of the films is independent on the film thickness, but it varies appreciably with the composition parameter,x, showing a minimum resistivity of 0.02 cm atx=0.55. The temperature dependence of the resistivity follows the semiconducting behaviour with an extrinsic and an intrinsic conduction below and above 70°C, respectively. The determined activation energies 0.2 eV and 0.8 eV, respectively, correspond to shallow and deep trap levels, respectively.     

Electrical Conductivity of H2V12 – y W y O31 + δ · nH2O and H x V x W1 – x O3 · nH2O

The electrical conductivity of H₂V_{12 – y} W _y O_{31 +} · nH₂O (0 y 3) and H _x V _x W_{1 – x} O₃ · nH₂O (x = 0, 0.08, 0.17, 0.25, 0.33) prepared by the sol–gel method was measured between 293 and 473 K at a relative humidity of 12%. Partial substitution of tungsten for vanadium or V⁵⁺ and H⁺ for tungsten was found to reduce the conductivity of the phases studied. The only exception is the x = 0.33 phase. Below 373 K, the activation energy of conduction is 0.22–0.29 eV in the amorphous phases (0 y 3; x= 0.25, 0.33) and 0.06–0.11 eV in the crystalline phases (x = 0, 0.08, 0.17). The possible mechanisms of electrical transport are discussed.     

Optical Spectroscopy of Perovskite-Type Manganites

Review is given on late optical studies on colossal magnetoresistance (CMR) manganites with perovskite or related structures. In La_1–x Sr _x MnO₃ which undergoes a ferromagnetic transition, optical conductivity spectrum () shows a change from a gap-like feature to metallic but highly diffussive one with increasing spin-polarization. Such a remarkable spectral weight transfer is also observed in a layered manganite, La_2–2x Sr_1+2x Mn₂O₇ (x = 0.4), with a mid-infrared peak structure due to inter-orbital transition. In Pr_1–x Ca _x MnO₃ (x = 0.4) which undergoes the charge ordering phase transition, () shows an anisotropic feature reflecting charge/orbital ordering pattern at 10 K. The charge-ordered state is transformed into a ferromagnetic metallic state by a magnetic field of 6.5 T at 30 K, which is manifested in a huge change of optical spectra over a wide photon-energy region (0.05 eV–3 eV).     

Soft chemical synthesis and characterization of lithium nickel oxide electrode materials

Soft chemistry was used to prepare ordered nanophase Li_0.6NiO₂ electrode materials by completing the oxidation of Ni²⁺ to Ni³⁺ and/or Ni⁴⁺ species with H₂O₂ oxidant during solution reactions at 50–60 °C and evaporation at 105–150 °C rather than during sintering. Both elemental analysis and electron spectroscopy for chemical analysis (ESCA) results indicate that oxidation was completed. The deconvoluted ESCA spectra of nickel ions exhibited a semi-quantitative ratio of Ni⁴⁺Ni³⁺=6040 which presented no significant change with increase of sintering time. After sintering for up to 11 h at 700°C, ordered Li_0.61Ni_0.96O_2.0 ceramics were formed (R3m, a ₀=0.2837 nm, c ₀=1.417 nm). Distribution of the crystallite size was in the range of 80–200 nm. As sintering times were increased, the crystallite shapes exhibited a more distinct morphology, and the ordering degree of the cations was enhanced, while the conductivity was sharply enhanced up to 2.0×10^{–1 –1} cm^–1 at 30 °C. Compared to conventional ceramic and solution methods, the ordered nanophase Li_0.61 Ni_0.96O_2.0 ceramics was obtained at 700 °C with shorter sintering times ( 11 h).     

Cation Distribution in the CuxMn3 – xO4 and CuMxMn2 – xO4 (M = Cr, Al) Spinels

The Rietveld profile analysis method applied to x-ray diffraction data was used to assess the cation distribution in the Cu _x Mn_{3 – x} O₄ (x = 0.5, 0.75, 1.00) solid solutions at 900°C in air and after quenching and also in quenched CuCr _x Mn_{2 – x} O₄ and CuAl _x Mn_{2 – x} O₄ (0 x 2.0) solid solutions. The results show that the tetragonal distortion (c/a 1) of the spinel structure depends on the relative occupancies of the Jahn–Teller ions Cu²⁺ and Mn³⁺ on the octahedral and tetrahedral sites and also on whether the system is in an equilibrium or metastable state. The structure of the solid solutions may also be affected by clusterization of octahedral Jahn–Teller ions. The resultant contents of Jahn–Teller ions leading to one or the other type of tetragonal distortion are evaluated.