Mechanical and electrical properties of Ni0.65Zn0.35CuxFe2−xO4 ferrite

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2?xO₄ (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. X-ray analysis showed that they were cubic spinel (single phase). Young's modulus, the dielectric loss and the change in capacitance under mechanical stress were measured for the samples. Young's modulus decreased with increasing Cu content. This is due to the fact that Cu²⁺ ions entered the lattice substitutionally for Fe³⁺ ions at the octahedral sites, creating lattice vacancies gave rise to lattice strain. The minimum value of the dielectric loss corresponding tox=0.3 may be due to the formation of lattice vacancies retarding the jump frequency to be far from the frequency of the applied a.c. field. The increase in capacitance of the samples with mechanical stress may be explained via the mechanism of dielectric polarization.     

Solid potassium-conducting electrolytes in the K2 − 2x Ga2 − x V x O4 system

New potassium-conducting solid electrolytes based on potassium monogallate in the K_2?2x Ga_2?x V _x O₄ system are synthesized and studied. It is found that an introduction of V⁵⁺ ions leads to a considerable increase in the KGaO₂ conductivity due to the formation of vacancies in the potassium sublattice. The conductivity for optimal compositions is approximately 10^?3 S cm^?1 at 400°C and above 10^?2 S cm^?1 at 700°C. The results are compared with early obtained data for potassium monogallate dopped with four-charged cations.     

Simultaneous measurements of oxygen pressure,composition, and electrical conductivity in praseodymium oxides: II. Pr10O18 and PrO2−x phases

Simultaneous measurements of oxygen pressure, composition, and electrical conductivity have been conducted in Pr₁₀O_18±x (epsilon) and PrO_2?x (alpha) phases, between Pr₉O_16±x (zeta) and Pr₁₀O_18±x phases, and between Pr₇O_12±x (iota) and PrO_2?x phases. In Pr₁₀O_18±x phase, the predominant defects are assigned to be neutral oxygen interstitials and neutral or doubly charged oxygen vacancies, and electrical conduction is thought to be governed mainly by the concentration of 7-coordinated praseodymium ions, which are the easiest sites for hopping electrons between Pr³⁺ and Pr⁴⁺ ions. In PrO_2?x phase, the electrical conductivity increases with oxygen pressure and the $\text{O}\text{Pr}$ ratio and the predominant defects are assigned to be neutral oxygen interstitials, indicating that oxygen vacancies are ordered in a short range and this phase is expressed by PrO_1.78+x rather than PrO_2?x in the region measured. The electrical conductivity-composition measurement, as well as the oxygen pressure-composition measurement, shows a reproducible hysteresis loop between Pr₉O_16±x and Pr₁₀O_18±x and it is discussed in terms of a domain model.     

X-ray photoelectron spectroscopy investigation of perovskites La1−x Sr x FeO3−y (0 ≤x < 1.0), prepared via a mechanochemical route

Perovskite-structure oxides La_1?x Sr _x FeO_3?y (x = 0, 0.2, 0.6, 1) were synthesized by the mechanochemical method. In order to refine the stoichiometric composition and the charge state of ions, these samples were studied by X-ray photoelectron spectroscopy (XPS). An investigation of perovskites with x = 0, 0.2, and 0.6 in air at room temperature showed that these samples do not contain oxygen vacancies (y = 0), i.e., they are fully oxidized. Hence, to produce electrical neutrality, these samples should contain iron(4+) cations in an amount proportional to the degree of substitution (x) of strontium(2+) for lanthanum(3+). However, no Fe⁴⁺ cations were found in the oxides. All perovskites contain only Fe³⁺ cations, oxygen ions O^2? along with oxygen ions with reduced electron density (O^?). These data provid evidence of the possible electron density redistribution from oxygen ions to iron cations. The fact that the oxides contain oxygen ions with reduced electron density suggests that weakly bound lattice oxygen in substituted perovskites is represented by O^? ions.     

The synthesis and properties of solid solutions based on Ba<Subscript>4</Subscript>Ca<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>11</Subscript>

(Ba_{1 ? x} Ca _x )₆Nb₂O₁₁ solid solutions were synthesized. The compositions were shown to be single-phase at 0.23 ≤ x ≤ 0.47 and have a double perovskite cubic structure with an incomplete oxygen sublattice. The interaction of solid solutions with water vapor and their electrical properties were studied. In dry atmosphere, these complex oxides were mixed oxygen-hole conductors. In humid atmosphere, they intercalated water and exhibited protonic conductivity. The influence of Ba/Ca isovalent substitution, the dynamics of the oxygen sublattice, and the concentration of intercalated water on the value and contribution of protonic and hole conductivity was analyzed.     

Electronic state and local surrounding of <Superscript>119</Superscript>Sn in calcium-substituted holmium orthochromites

The influence of Ca²⁺ doped into the holmium sublattice on the magnetically active surrounding of Sn⁴⁺ ions located in the chromium sublattice of Ho_1–x Ca _x Cr_0.997Sn_0.003O₃ (x = 0, 0.003, and 0.1) compounds was studied by ¹¹⁹Sn Mössbauer spectroscopy. At concentrations [Ca] = [Sn] = 0.3 mol %, an increase was observed in the spectral contribution of Sn⁴⁺ sites, having the full number of nearest-neighbor Cr³⁺cations (n = 6), where they perceived a magnetic field H(Sn)_{4.2 K} = 82 kOe, compared to the contribution of the relevant sites in the undoped chromite (x = 0). This observation was interpreted as resulting from a reduced probability of appearance of Cr³⁺ vacancies in the nearest surrounding of heterovalent Sn⁴⁺ ions. For x = 0.1, on the contrary, the ¹¹⁹Sn spectrum revealed a reduced contribution from the Sn⁴⁺ sites with n = 6. This evolution is shown not to be due neither to the appearance of Cr⁴⁺ nor Cr⁶⁺ ions in the nearest neighborhood of Sn⁴⁺ in the chromium sublattice to balance the charge deficiency of the Ca²⁺ ions doped into the holmium sublattice. This allowed us to suggest that the observed effect was due to the onset of Sn⁴⁺ segregations in the structure of Ho_0.9Ca_0.1Cr_0.997Sn_0.003O₃, which contained a far greater amount of Ca²⁺ ions whose charge deficiency was balanced mostly by Cr⁴⁺ formation. Studies of samples that were prepared under a hydrogen atmosphere revealed the reduction of Sn⁴⁺ to the oxidation state +2, with the concomitant stabilization of the formed Sn²⁺ ions on crystallite surfaces on sites having low coordination numbers.     

Crystal chemistry of the (Ta6Si4O26)6− and (Ta14Si4O47)8− frameworks

The range of chemical flexibilities of the hexagonal frameworks (Ta₆Si₄O₂₆)^6? and (Ta₁₄Si₄O₄₇)^8? have been partially explored. This has been done with high-temperature preparations as in general ionic mobilities in these frameworks are too low to permit low-temperature ion exchange. Ionic site potential calculations indicate that preferential site-occupancy factors as well as geometric constraints are responsible for the absence of ionic motion. New phases K_6?xNa_xTa₆Si₄O₂₆ (x ? 4), K_8?xNa_xTa₁₄Si₄O₄₇ (x ? 5), and impure Ba_3?xNa_2xTa₆Si₄O₂₆ have been prepared. Introduction of up to 2 moles of Li⁺ and 1 mole of Mg²⁺ ions per formula unit into sites of the framework not normally occupied has been demonstrated as well as the possibility of partially substituting Zr⁴⁺ for Ta⁵⁺ ions. Substitutions designed to introduce large tunnel vacancies in the presence of only monovalent K⁺ or Na⁺ ions (P for Si, W for Ta and F for O) generally proved unsuccessful. Competitive phases also frustrated attempts to substitute either the larger Rb⁺ or the smaller Li⁺ ions into the large-tunnel sites. A large area of solid solution was discovered in the BaONa₂OTa₂O₅ phase diagram; it has a (TaO₃)-framework with the structure of tetragonal potassium tungsten bronze.     

Oxidation behaviour of uranium and neptunium in stabilised zirconia

Yttria stabilised zirconia (YSZ) based (Zr,Y,U)O_2−x and (Zr,Y,Np)O_2−x solid solutions with 6 and 20 mol% actinide were prepared with Y/Zr ratios ranging from 0.2 to 2.0 to investigate uranium and neptunium oxidation behaviour depending on the oxygen vacancies in the defect fluorite lattice. Sintering at 1600 °C in Ar/H₂ yields a cubic, fluorite-type structure with U(IV) and Np(IV). Annealing (Zr,Y,U)O_2−x with Y/Zr=0.2 at 800 °C in air results in a tetragonal phase, whereas (Zr,Y,U)O_2−x with higher Y/Zr ratios and (Zr,Y,Np)O_2−x retain the cubic structure. XANES and O/M measurements indicate mixed U(V)-U(VI) and Np(IV)-Np(V) oxidation states after oxidation. Based on X-ray diffraction, O/M and EXAFS measurements, different oxidation mechanisms are identified for U- and Np-doped stabilised zirconia. In contrast to U, excess oxygen vacancies are needed to oxidise Np in (Zr,Y,Np)O_2−x as the oxidation process competes with Zr for oxygen vacancies. As a consequence, U(VI) and Np(V) can only be obtained in stabilised zirconia with Y/Zr=1 but not in YSZ with Y/Zr=0.2.     

Electrical properties of Ba2(In1 − x Al x )2O5 solid solutions

The electric conductivity of perovskite-like Ba₂(In_{1 ? x} Al _x )₂O₅ solid solutions (0 < x ≤ 0.20) characterized by structural disordering in the oxygen sublattice was studied as a function of temperature and partial pressure of oxygen in an atmosphere with a low content of water vapors ( $p_{H_2 O}$ = 3 × 10^?5 atm). When In³⁺ was partially replaced by Al³⁺, the oxygen ion conductivity increased because of the disordering of oxygen structural vacancies, leading to a significant increase in the total electric conductivity of the samples.     

Synthesis, structure, and impedance spectra of iron-containing bismuth titanates

Iron-containing bismuth titanates with a cubic pyrochlore Bi_1.6Fe _x Ti₂O_{7 ? δ} structure were synthesized by the ceramic method. The region in which iron-containing pyrochlores are formed was determined. The pycnometric specific gravity of the samples was measured. Based on XRD and pycnometric specific gravity data, it was concluded that iron atoms lie in the bismuth sublattice. The electric properties of the materials were studied by high-temperature impedance spectroscopy. A strong effect of polarization on the impedance spectra was found.     

Cathodic materials for lithium-ion batteries based on spinels Li x Mn2−y Me y O4: Synthesis, phase composition, and structure of Li x Mn2−y Cr y O4 at x = 1.0−1.2 and y = 0−0.5

Extralithiated chromium-doped finely divided lithium-manganese spinels are synthesized as a result of a two-step solid-phase process with use made of the fusion-saturation method. The spinels are intended for application as cathodic materials in lithium-ion batteries. The phase composition and structural characteristics of samples of cathodic materials of the type Li _x Mn_2?y Cr _y O₄ are studied. The samples with x = 1.0?1.2 and y = 0?0.5 are characterized by phase purity and cubic syngony with parameter a = 0.817?0.823 nm and a disperseness equal to 1–2 nm. The maximum content of chromium and lithium in Li _x Mn_2?y Cr _y O₄ that does not lead to violation of cubic syngony is determined. Lithium excess in the cathodic material that does not exceed 0.2 formula units may be used for compensating the irreversible capacity. Replacing some manganese atoms by chromium may facilitate retention of the structures’s integrity in the course of cycling.     

High-pressure defect phase Tm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript>

Perovskite-related oxide Tm _x Cu₃V₄O₁₂ (space group Im \(\bar 3\), Z = 2, a = 7.262?7.273 Å) with vacancies in the cationic sublattice has been prepared for the first time under barothermal conditions (p = 7.0?9.0 GPa, T = 900?1100°C). Electric resistivity (10–300 K) and magnetic susceptibility (0–300 K) were studied as a function of temperature. Tm _x Cu₃V₄O₁₂ is shown to have a metallic conductivity and paramagnetism.     

Effect of chlorine on solid solution formation in ruthenium titanium dioxide coatings

Rutile-type dioxides of titanium and ruthenium form partial solid solutions at temperatures above 1000°C, whereas the miscibility is complete in the temperature range of 350–600°C, when the oxides are coated on titanium or alumina substrates. The present work tends to show that the Ru_xTi_t?xO₂ coatings are metastable solutions wherein the chloride ions are likely to replace the oxide ions on the sublattice O^2? sites. The increase of a and c parameters in the overall composition range are compatible with a Cl/Ru ratio of 2.5 at.% in the rutile lattice. This ratio does not depend upon the titanium content and therefore it is thought that the Cl^? ions are compensated by reduced Ru³⁺ (or Ti³⁺) species. The electrical conductivity retains its metal-like characteristics in the composition range of 30–100% RuO₂, this is accounted for by the existence of RuO π1 bands along (110) directions. The reduced ions in equilibrium with Cl^? would provide additional paths for electron transport along the c-axis, explaining the high values of conductivity in the concentration range of 30–50% RuO₂.     

Magnetic properties in the system BaCo1−xMnxO3 and SrCo1−xMnxO3 (0 ≦ x ≦ 1)

The compounds in the systems of BaCo_1?xMn_xO₃ (0 ≦ x ≦ 1) and SrCo_1?xMn_xO₃ (0 ≦ x ≦ 1) were prepared at an oxygen pressure of 1400 bars. The former had a two-layer hexagonal structure and that of the latter was cubic perovskite type. From the variation of the unit-cell parameters and of the magnetic properties, it is found that the Co⁴⁺ ions change from the low-spin to the high-spin state. In the system of SrCo_1?xMn_xO₃, the change of magnetic property from ferromagnet to antiferromagnet is related to the spin state of Co⁴⁺ ions located at the octahedral sites.     

Effect of hydration on conductivity of Ba4La x Ca2 − x Nb2O11 + 0.5x (x = 0.5, 1, 1.5, 2) phases

Substitution of Ca by La in initial cubic double perovskite Ba₄(Ca₂Nb₂)O₁₁[VO]₁ allowed obtaining phases with a similar structure with a lower content of structural oxygen vacancies, Ba₄(La _x Ca_{2 ? x} Nb₂)O_{11 + 0.5x} [VO]_{1 ? 0.5x} (x = 0.5, 1, 1.5, 2). The impedance technique was used to measure the temperature dependences of conductivity in the atmosphere of dry and humid air. Transport numbers determined using the EMF method in an oxygen-air and water steam concentration cells point to the predominantly hole nature of conductivity in the high-temperature region (T > 600°C) and to predominance of proton conductivity in the low-temperature region. Activation energies of hole and proton conductivity were calculated. Thermogravimetric measurements were carried out under heating from 25 to 1000°C with simultaneous mass-spectrometric determination of evolved H₂O and CO₂. The properties of the studied Ba₄(La _x Ca_{2 ? x} Nb₂)O_{11 + 0.5x} (x = 0.5, 1, 1.5, 2) phases were compared with the earlier studied Ba_{4 ? x} La _x (Ca₂Nb₂)O_{11 + 0.5x} phases with similar lanthanum content.     

Sur de nouveaux ferrites oxyfluorés d'yttrium ou de gadolinium à structure grenat

The oxyfluoride garnets of formula Y₃Fe_5?xM_xO_12?xF_x and Gd₃Fe_5?xM_xO_12?xF_x (M = 3d transition element) result from partial substitution of O^2? by F^? in Y₃Fe₅O₁₂ and Gd₃Fe₅O₁₂ oxides. The cationic charge compensation is obtained by replacing the Fe³⁺ ions by divalent ions as Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ ions. The site occupied by some of these ions (Mn²⁺, Ni²⁺, Zn²⁺) is determined by magnetic or Mössbauer measurements.     

Synthesis,structure, and properties of randomly mixed and layer-ordered SrMn1−xGaxO3−δ perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques, we have extended the solubility limit of Ga³⁺ in the cubic perovskite phase to x≈0.33. Higher Ga concentrations lead to mixed phases until a single-phase ordered double-perovskite structure is obtained at x=0.5, i.e., Sr₂MnGaO_6−δ. In the cubic perovskite phase the maximum oxygen content is 3−x/2, which corresponds to 100% Mn⁴⁺. All maximally oxygenated solid solution compounds are found to order antiferromagnetically, with the transition temperature linearly decreasing as Ga content increases. Reducing the oxygen content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.7Ga_0.3O_2.5 below 30 K. The brownmillerite phase at low oxygen content, Sr₂MnGaO₅, is found to have Icmm crystallographic symmetry. At 12 K its magnetic structure is found to order in the Icm′m′ magnetic symmetry corresponding to a G-type antiferromagnetic structure of Mn³⁺ ions. At higher oxygen content, Sr₂MnGaO_5.5 is found to have Cmmm crystallographic symmetry with disordered oxygen vacancies. At 12 K two competing long-range magnetic structures are found for the Mn⁴⁺ sublattice having C_Im′m′m symmetry (G-type), and C_Pm′m′m symmetry (C-type), together with a G-type short-range magnetic correlations.     

Distorsion cristalline et distribution des cations dans les spinelles de la serie CoMnxFe2−xO4

By using neutron diffraction together with anomalous dispersion X-ray diffraction, it has been possible to ascertain the distribution of close atomic numbered cations in CoMn_xFe_2?xO₄ system spinels.At 950°C, these compounds have a cubic structure in the range 0 ? x ? 1.25 and exhibit a macroscopic tetragonal distortion as soon as 60% of the Mn³⁺ ions occupy octahedral sites.The great mobility of cobalt between both types of sites has been pointed out; it can be related to oxidation and reduction phenomena. In these compounds, Fe³⁺ iron remains neutral towards four or six coordinences.     

Catalytic properties of Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zr<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> prepared using a template in the oxidation of CO

The catalytic activity in CO oxidation of Ce _x Zr_1–x O₂ double oxides prepared using pine sawdust and cetyltrimethylammonium bromide (CTAB) as templates is compared. It is found by means of SEM and the low-temperature adsorption of N₂ that biomorphic oxides reproduce the macropore structure of the template. It is shown via XRD and Raman spectroscopy that all samples contained mixed ceria-zirconia oxide. The double oxides form a cubic phase with a lattice of the fluorite type at a ratio of Ce: Zr = 4, regardless of the nature of the template; when Ce: Zr = 1, the biomorphic mixed oxide forms a tetragonal phase. According to Raman spectroscopy and XRD it was shown that the distortion of the oxygen sublattice is higher in biomorphic samples. Energy dispersive analysis shows that Ca impurities were present in the biomorphic samples, introducing additional distortions in the lattice of double oxide and leading to the formation of anionic vacancies. It is found that when Ce: Zr = 4, the conversion of CO on biomorphic oxide in the range of 100–350°C is higher than that observed for Ce _x Zr_1–x O₂ (CTAB); reducing the Ce: Zr ratio in the biomorphic sample to 1 results in a marked decrease in CO conversion at 100–200°C. It is concluded that these differences are due to changes in the mobility of the lattice oxygen.     

How synthesis parameters influence the quality of lead magnesioniobate PbMg1/3Nb2/3O3

Lead magnesioniobate PbMg_1/3Nb_2/3O₃ (PMN) has been prepared using lead oxide and magnesium niobate. Factors that influence the perovskite/pyrochlore ratio in the PMN structure have been studied. In order for the maximal amount of the perovskite phase to be obtained, synthesis should be carried out at 850?C900°C; an excess of MgO (1?C5 wt %) does not exert a positive effect. An excess of PbO (5?C7 wt %) allows one to obtain a phase that contains 85% PMN, with the perovskite structure. The perovskite lattice is found to be stabilized as a result of cationic substitutions (Yb, Lu) in the Mg-Nb sublattice due to the formation of solid solution Pb(Mg_1/3Nb_2/3)_{1 ? x} Yb(Lu) _x O₃, where 0 ?? x ?? 0.9.