Structure and Magnetism of Pr1−xSrxFeO3−δ

Crystal structure, redox, and magnetic properties for the Pr_1−xSr_xFeO_3−δ solid-solution phase have been studied. Oxidized samples (prepared in air at 900°C) crystallize in the GdFeO₃-type structure for 0≤x≤0.80, and probably in the Sr₈Fe₈O₂₃-type (unpublished) structure for x=0.90. Reduced samples (containing virtually only Fe³⁺) crystallize as the perovskite aristotype for x=0.50 and 0.67 with randomly distributed vacancies. The Fe⁴⁺ content increases linearly in the oxidized samples up to x≈0.70, whereupon it stabilizes at around 55%. Antiferromagnetic ordering of the G type is observed for oxidized samples (0≤x≤0.90) which show decreasing Néel temperature and ordered magnetic moment with increasing x, while the Néel temperature is nearly constant at 700 K for reduced samples. Electronic transitions for iron from an average-valence state via charge-separated to disproportionated states are proposed from anomalies in magnetic susceptibility curves in the temperature ranges 500–600 K and 150–185 K.     

Preparation and characterization of lithium ion-conducting glass-ceramics in the Li1 + xCrxGe2 − x(PO4)3 system

Li₂O–Cr₂O₃–GeO₂–P₂O₅ based glasses were synthesized by a conventional melt-quenching method and successfully converted into glass-ceramics through heat treatment. Experimental results of DTA, XRD, ac impedance techniques and FESEM indicated that Li_1.4Cr_0.4Ge_1.6(PO₄)₃ glass-ceramics treated at 900 °C for 12 h in the Li_1 + xCr_xGe_2 − x(PO₄)₃ (x = 0–0.8) system exhibited the best glass stability against crystallization and the highest ambient conductivity value of 6.81 × 10⁻⁴ S/cm with an activation energy as low as 26.9 kJ/mol. In addition, the Li_1.4Cr_0.4Ge_1.6(PO₄)₃ glass-ceramics displayed good chemical stability against lithium metal at room temperature. The good thermal and chemical stability, excellent conducting property, easy preparation and low cost make it promising to be used as solid-state electrolytes for all-solid-state lithium batteries.     

Analysis of thermo-magnetic fluctuations above the glass-transition temperature in Bi1.6Pb0.5Sr2−xEuxCa1.1Cu2.1O8+δ (0.000 ≤ x ≤ 0.180) system

The resistivity of Bi_1.6Pb_0.5Sr_2−xEu_xCa_1.1Cu_2.1O_8+δ (0.000 ≤ x ≤ 0.180) superconductor has been measured as a function of temperature and magnetic field. The resistivity shows a glassy behavior even at higher temperatures and magnetic fields for the Eu-doped samples as compared with the Eu free sample. The values of glass-transition temperature [T_g], magnetic field dependent activation energy [U₀(B)] and the temperature and magnetic field dependent activation energy [U₀(B,T)] are found to be maximum for optimal doping levels (x = 0.135) which shows that the flux lines are effectively pinned in this sample. Also for temperatures below the superconducting transition temperature (T_C), a scaling of measured resistivity curves in magnetic field (B = 0.4 and 0.8 T) is obtained and this scaling is quite useful for better understanding of the behavior of the flux vortices in high temperature superconductors.     

Crystal chemistry, modulated structure, and electrical conductivity in the oxygen excess scheelite-based compounds La1−xThxNbO4+x/2 and LaNb1−xWxO4+x/2

For La_1−xTh_xNbO_4+x/2, three phases with broad homogeneity regions occur, for 0.075 ≤ x ≤ 0.37, 0.41 < x < 0.61, and 0.65 ≤ x ≤ 0.74. All are related to the scheelite structure type, with at least the first exhibiting an incommensurate structural modulation. An analogous structurally modulated phase was found for LaNb_1−xW_xO_4+x/2 for 0.11 ≤ x ≤ 0.22. Additional phases occur at La_0.2Th_0.8NbO_4.4 and LaNb_0.4W_0.6O_4.3. The electrical conductivity and the direction and wavelength of the structural modulation have been characterized for the La_1−xTh_xNbO_4+x/2 phase with 0.075 ≤ x ≤ 0.37.     

A study of the perovskite solid solution series LaxSr1−xRuO3 and LaxCa1−xRuO3 by ruthenium-99 Mössbauer spectroscopy

The magnetic, electronic, and structural properties of the solid solutions La_xSr_1−xRuO₃ and La_xCa_1−xRuO₃ have been studied by ⁹⁹Ru Mössbauer spectroscopy and other techniques. The La_xCa_1−xRuO₃ phases are reported for the first time and have been shown by powder X-ray diffraction measurements to be orthorhombically distorted perovskites. Electrical resistivity measurements on compacted powders show that all the phases are metallic with p 10⁻³, ohm-cm. Progressive substitution of Sr²⁺ by La³⁺ in ferromagnetic SrRuO₃ leads to a rapid collapse of the magnetic hyper-fine splitting at 4.2°K. For x = 0.25 some ruthenium ions still experience a magnetic field but for 0.4 x 0.75 only single, narrow resonance lines are observed, consistent both with the complete removal of the ferromagnetism and with the presence of an averaged ruthenium oxidation state in each phase, i.e., La_x³⁺Sr_1−x²⁺Ru^(4−x)+O₃ rather than La_x³⁺Sr_1−x²⁺Ru_x³⁺Ru_1−x⁴⁺O₃. LaRuO₃ and CaRuO₃ both give essentially single-line spectra at 4.2°K, indicating that the ruthenium ions in these oxides are not involved in long-range antiferromagnetic order but are paramagnetic. The solid solutions La_xCa_1−xRuO₃ (0 < x 0.6) give sharp symmetrical singlets with chemical isomer shifts (relative to the Ru metal) which move progressively from the value characteristic of Ru⁴⁺ (−0.303 mm sec⁻¹) toward the value for Ru³⁺ (−0.557 mm sec⁻¹), consistent with the presence of intermediate ruthenium oxidation states in these phases also.     

The defect solid solution Na7/8(Fe7/8+xTi9/8−2xSbx)O4 (0 ≤ x ≤ 1/3): Evidence of Na mobility in the tunnels of a quadruple rutile-chain structure

A new defect solid solution, the series Na_7/8(Fe^III_7/8+xTi^IV_9/8−2xSb^V_x)O₄, was synthesized. Its homogeneity range is rather wide: 0 <- x ≤ 0.33. The incorporation of Sb^V gives rise to a progressive increase of the parameters of the orthorhombic unit cell. X-ray powder structure calculations point to a partial occupancy of the large double tunnels in a quadruple rutile-chain structure. A significant ordering of cations over the octahedral framework is observed, owing to a Ti^IV---Sb^V segregation. Electrical measurements emphasize a cationic conductivity, mainly related to a 1D motion of Na^I cations. A transition from a low activation energy process—E_A ≤ 0.20 eV—to a high activation energy one—E_A ≈ 0.75 eV—systematically occurs at T ≈ 440°C, independent of the Sb^V concentration. A possible skew motion from a half tunnel to another one is proposed as a tentative explanation of the high-temperature conductivity mechanism.     

Synthesis of ternary and quaternary CuInxGa1−xSe2 (0 ≤ x ≤ 1) semiconductor nanocrystals

Semiconducting CuIn_xGa_1−xSe₂ nanocrystals (20–30 nm) have been synthesized over the whole composition range using a facile solution-based method. Depending on the synthesis conditions, the nanocrystals exhibit a cubic or spherical morphology with a narrow size distribution. The band gap increases with increasing Ga concentration and the values are close to those observed in the bulk.     

Pillaring of Layered Perovskites, K1−xLaxCa2−xNb3O10, with Nanosized Fe2O3 Particles

Nanosized Fe₂O₃ clusters are pillared in the interlayer spaces of layered perovskites, H_1−xLa_xCa_2−xNb₃O₁₀ (0≤x≤0.75) by a guest-exchange reaction using the trinuclear acetato-hydroxo iron cation, [Fe₃(OCOCH₃)₇ OH·2H₂O]⁺. The interlayer spaces of niobate layers are pre-expanded with n-butylammonium cations (n-C₄H₉NH⁺₃), which are subsequently replaced by bulky iron pillaring species to form Fe(III) complex intercalated layer niobates. Upon heating at 380°C, the interlayered acetato-hydroxo iron complexes are converted into Fe₂O₃ nanoclusters with a thickness of ca. 3.5 Å irrespective of the interlayer charge density (x). The band-gap energy of the Fe₂O₃ pillars (E_g2.25 eV) is slightly larger than that of bulk Fe₂O₃ (E_g2.20 eV) but is smaller than that expected for such a small-sized semiconductor, which can be assigned to the pancake-shaped Fe₂O₃ pillars of 3.5 Å in height with comparatively large lateral dimension. X-ray absorption spectroscopic measurements at the Fe K-edge are carried out in order to obtain structural information on the Fe₂O₃ pillars stabilized between the niobate layers. XANES analysis reveals that the interlayer FeO₆ octahedra have coordination environments similar to that of bulk α-Fe₂O₃, but noncentrosymmetric distortion of interlayered FeO₆ is enhanced due to the asymmetric electric potential exerted by the negatively charged niobate layers. Scanning electron microscopic observation and nitrogen adsorption–desorption isotherm measurement suggest that the pillared derivatives are nanoporous materials with the highest BET specific surface area of ca. 116 m²/g.     

Oxygen-deficient SrTiO3−x, X = 0.28, 0.17, and 0.08. Crystal growth, crystal structure, magnetic, and transport properties

The grossly nonstoichiometric perovskites SrTiO_3−x with x = 0.28, 0.17, and 0.08 were prepared from a reaction of Sr₂TiO₄, TiO, and TiO₂ at 1500°C. For x = 0.28 relatively large single crystals were obtained. Also for this sample the crystal symmetry was found to depend on the rate of cooling from the reaction temperature and the annealing conditions. Rapidly cooled samples are tetragonal a = 3.9177(3) Å, c = 3.8878(5) Å. Samples annealed in vacuum at temperatures of 1000 to 600°C are cubic a = 3.9075(3) Å with no change in cell volume. Single crystal data from a tetragonal sample indicate slight preferential occupancy of one oxygen position in P4/mmm. No evidence for any supercell due to defect ordering could be seen by TEM in either cubic or tetragonal samples. The x = 0.28 crystals show metallic resistivity, (300 K) = 6 × 10⁻⁴ ohm-cm and temperature-independent paramagnetism, χ_m = 118 × 10⁻⁶ cm³ mole⁻¹. Hall effect data from 300 to 4.2 K analyzed on a single carrier model give a temperature-independent n-type carrier density of 2.4 × 10²¹ cm⁻³. This is a factor of 3.9 less than that expected if the creation of each oxygen vacancy results in the production of two carriers in a single band. Hall data for x = 0.17 and 0.08 samples give similar results corresponding to densities of 2.1 and 1.4 × 10²¹ cm⁻³, respectively, in the same temperature range. These densities are 2.7 and 1.9 times less than the expected single-band value, respectively. Such results point to a two-band model with a large effective mass in one of the bands.     

Thermal analysis and thermal expansion studies on high density YBa2−xLaxCu3O7−δ, 0x0.2

The compositions in the YBa_2−xLa_xCu₃O_7−δ (0x0.2) system were prepared by the solid state reaction, employing a novel high-temperature oxygen sintering route. The modified sintering route yields dense slab like microstructures with large grains. The decomposition (incongruent melting) temperature of the YBa₂Cu₃O_7−δ (Y-123) phase was found to shift to higher temperatures with increasing oxygen partial pressure and lanthanum content. Structure remained orthorhombic up to x=0.2 with a decrease in the orthorhombic strain ((b−a)/b). Iodometric titration indicated a systematic increase in the oxygen content with increasing lanthanum content. Thermo-gravimetric studies in various oxygen partial pressures revealed that the oxygen diffusion in to the YBa₂Cu₃O_7−δ (δ>0.5) lattice is an exothermic event and takes place at temperatures not less than 573 K. High-temperature thermal-expansion measurements in air indicated that the nonlinearity in thermal expansion behaviour was reduced by the substitution of lanthanum.     

Magnetic structure of the UCuxSi2−x system

Neutron powder diffraction experiments were performed on selected compositions of the UCu_xSi_2−x system exhibiting an interesting magnetic phase diagram towards the composition: spin fluctuation behaviour for x<0.49, ferromagnetism for 0.49≤x<0.80, spin glass state for 0.80<x<0.92 and finally antiferromagnetism for 0.92<x≤0.96. At 1.5 K, the compounds UCu_0.49Si_1.51 (hexagonal AlB₂ modification) and UCu_0.65Si_1.35 show a collinear ferromagnetic structure where the uranium magnetic moments equal to 1.1(1) and 2.5(1)μ_B, respectively, are aligned in the basal plane of the [U₆] trigonal prisms. On the contrary, UCu_0.96Si_1.04 adopts a non-collinear antiferromagnetic structure similar to that observed for UCuSn. Moreover, the study confirms the absence of long range magnetic order for UCu_0.90Si_1.10.     

Effect of annealing temperature on the structural and optical properties of Zn1−xMgxO particles prepared by oxalate precursor

Zn_1−xMg_xO particles were prepared using zinc and magnesium oxalate precursor by co-precipitated method. The lattice constants of Zn_1−xMg_xO proved that the interstitial Mg formed at 500 °C and Mg replaced Zn in ZnO tetrahedral coordination at 800 °C. Compared with the ZnO, the absorbing band edge of the Zn_1−xMg_xO displayed blue shifts. The room temperature photoluminescence was similar to ZnO and variation of Mg content did not change the shape or peak position of the emission spectra markedly when it was annealed at 500 °C. However, its blue emission band disappeared, and a relatively strong green light emission at 498 nm appeared after annealed at 800 °C. The photoluminescence intensity ratios I_(green)/I_(UV) of Zn_1−xMg_xO varied with Mg content and the green light emission peak shifted from 498 nm to 472 nm when Mg content increased from 0 to 2.0 at.%.     

Properties of the perovskites, SrMn1−xFexO3−δ (x=1/3, 1/2, 2/3)

The SrMn_1−xFe_xO_3−δ (x=1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x=1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe³⁺ and Fe⁴⁺. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x=1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x=1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn⁴⁺, Fe^(4+δ)+ and Fe⁽³⁺⁾⁺. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-e_g orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe⁽³⁺⁾+/Fe^(4+δ)+ disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.     

Conventional hydrothermal process versus microwave-assisted hydrothermal synthesis of La1−xAgxMnO3+δ (x = 0, 0.2) perovskites used in methane combustion

A study on the synthesis of La_1−xAg_xMnO_3+δ (x = 0, 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factor under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 200 °C, P = 20 atm) followed by a thermal treatment of the precursor at 700 °C (10 h).Structural and physico-chemical properties of the catalysts were investigated using X-ray diffraction (XRD), BET sorption, temperature-programmed reduction or desorption, mass spectrometry (TPR-MS and TPD-MS), and X-ray photoelectron spectroscopy (XPS). While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation method. The effect of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in methane combustion in the presence and in the absence of hydrogen sulphide has been studied. MWhyd-La_0.8Ag_0.2MnO_3+δ catalysts were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than CHhyd catalysts.     

Critical covalence and superconductivity in Ln2−xCexCuO4

The variations of superconductive properties with x of the n-type Ln_2−xCe_xCuO₄ (Ln = La_0.5Nd_0.5, Nd, or Gd) systems have been investigated. As the size of Ln³⁺ decreases, (i) the solubility limit x of Ce decreases, (ii) the value of x at which a transition from antiferromagnetic semiconductor to superconductor occurs increases, and (iii) the width Δx of the superconductive region decreases. The decreasing solubility of Ce with decreasing size of Ln³⁺ is due to decreasing tensile strain in the CuO₂ sheets. The progressive shift of the semiconductor to superconductor transition to higher x values with decreasing size of Ln³⁺ is explained on the basis of increasing electrostatic Madelung energy E_M caused by decreasing Cu---O bond length. A larger E_M means a larger charge transfer gap Δ and a smaller covalent-mixing parameter λ and bandwidth W; so a decreasing size of Ln³⁺ necessitates a higher level of Ce-doping in order to achieve a critical covalence essential for superconductivity to occur.     

Synthese et propriétés de conduction ionique des phases Li8−2xCaxCeO6 (0 < x ≤ 0,5) et Li6CaCeO6

The solid solution Li_8−2xCa_xCeO₆ (0 < x ≤ 0,5) and the definite phase Li₆CaCeO₆ have been obtained at 800°C through a study of Li---Ca---Ce---O system. Electrical measurements on the doped phases Li^tetr.₆ [Li_2-2xCa_xCe□]^oct.O₆ show that the conductivity varies slightly with the creation of vacancies in the octahedral layers. This result unambiguously confirms the following diffusion mechanism: the conduction is assumed essentially by lithium ions located in the tetrahedral layers. The compound Li₆CaCeO₆ is isostructural with Li₆In₂O₆. The cell is trigonal, Å, c = 10,603 Å, c/a = 1,058₇, and Z = 6. This new quaternary phase, which belongs to the same structural family of oxides of the type Li₈MO₆, either pure or doped with calcium, may be represented by the formula Li^tetr.₆[Ca Ce□]^oct.O₆. Electrical and structural data are correlated for this compound.     

Influence of ceramic process and Eu content on the composite multiferroic properties of the Ba6−2xLn2xFe1+xNb9−xO30 TTB system

The present work reports the optimisation of the composition and elaboration process of Ba_6−2xLn_2xFe_1+xNb_9−xO₃₀ (0.6 < x < 1.0), a composite multiferroic below 440 K. Both firing and sintering conditions have been investigated in order to lower the two spurious phases content and to improve the ceramic quality. The Eu content has also been varied, with subsequent adjustment of cationic content to maintain charge compensation and site occupancies. The evolution of phase content, dielectric and magnetic properties has been evaluated with respect to these parameters. It was found that optimal processing parameters allow a significant improvement of dielectric properties but leave magnetic properties unaffected. Eu content modification impacts both the dielectric and magnetic properties of the TTB samples and changes their dielectric behaviour from ferroelectric to relaxor.     

Phase equilibria and crystal structure of the solid solution LaFe1−xNixO3−δ (0 ≤ x ≤ 1)

Phase equilibria in the LaFeO₃–“LaNiO₃” were studied at 1100 °C in air. The samples were synthesized by standard ceramic and/or solution route via nitrate or citrate precursors. According to the results of XRD it was found that the homogeneity ranges of LaFe_1−xNi_xO_3−δ solid solution lay within 0.0 ≤ x ≤ 0.4 (sp.gr. Pbnm) and 0.6 ≤ x ≤ 0.8 (sp.gr. ). The structural parameters (bond lengths, atom coordinates) for the single-phase samples were refined using Rietveld analysis. The unit cell parameters versus LaFe_1−xNi_xO_3−δ composition are presented.     

Structures and energies of CnS (1 ≤ n ≤ 20) sulphur carbide clusters

C_nS (1 ≤ n ≤ 20) clusters have been investigated by means of the density functional theory. As a general rule, when 1 ≤ n ≤ 17 the energetically most favorable isomers are found to be the linear arrangement of nuclei (C_∞v) with the sulphur atom at the very end of the carbon chain. The electronic ground state is alternately predicted to be ¹∑⁺ for odd n or ³∑⁻ for even n with a conspicuous odd–even effect in the stability of these clusters. The C₁₈S cluster is predicted to have a S-capped monocyclic structure (¹A₁), but with a low barrier to linearity. On the other hand, C₁₉S and C₂₀S are unambiguously linear in the ¹∑⁺ and ³∑⁻ electronic ground states, respectively.     

High performance anode-supported intermediate temperature solid oxide fuel cells (IT-SOFCs) with La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolyte films prepared by electrophoretic deposition

Remarkable power density was obtained for anode-supported solid oxide fuel cells (SOFCs) based on La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte films, fabricated following an original procedure that allowed avoiding undesired reactions between LSGM and electrode materials, especially Ni. Electrophoretic deposition (EPD) was used for the fabrication of 30 μm-thick electrolyte films. Anode supports were made of La_0.4Ce_0.6O_2−x (LDC). The LSGM powder was deposited by EPD on an LDC green tape-cast membrane added with carbon powder, both as pore former and substrate conductivity booster. A subsequent co-firing step at 1490 °C produced dense electrolyte films on porous LDC skeletons. Then, a La_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (LSFC) cathode was applied by slurry-coating and calcined at 1100 °C. Finally, the porous LDC layer was impregnated with molten Ni nitrate to obtain, after calcination at 900 °C, a composite NiO–LDC anode. Maximum power densities of 780, 450, 275, 175, and 100 mW/cm² at 700, 650, 600, 550, and 500 °C, respectively, were obtained using H₂ as fuel and air as oxidant, demonstrating the success of the processing strategy. As a comparison, electrolyte-supported SOFCs made of the same materials were tested, showing a maximum power density of 150 mW/cm² at 700 °C, more than 5 times smaller than the anode-supported counterpart.