Effects of vacancy concentration on the magnetic and transport properties of (La1−xPbx)1−y□yMnO3

(La_1−xPb_x)_1−y□_yMnO₃ with x=0.05-0.5 and y=0, 0.05, 0.1 (where □ is a vacancy) was studied to evaluate the effects of A-site vacancies on the physical properties. In this system manganese perovskites form with tolerance factors close to 1 and low A-site cation size mismatch due to similarities in the effective ionic radii of La³⁺ and Pb²⁺. Increasing vacancy concentration indicates no significant effect on the lattice parameters or volume. However, the vacancies introduce a greater A-site cation size mismatch, which leads to a lowering of the ferromagnetic and metal-insulator transition temperatures, although the transitions are not broadened with increasing vacancy content. Due to the vacancies a distribution of Mn-O-Mn angles and Mn-O distances are created, and long range order in (La_1−xPb_x)_1−y□_yMnO₃ appears to be determined by Mn-O-Mn angles and Mn-O distances which most distort from 180° and are the longest, respectively, in the structure.     

Oxygen permeability, thermal expansion and mixed conductivity of GdxCe0.8−xPr0.2O2−δ, x=0, 0.15, 0.2

The non-linear thermal expansion behaviour observed in Ce_1−yPr_yO_2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system Gd_xCe_0.8−xPr_0.2O_2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr⁴⁺. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t_o=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce_0.8Pr_0.2O_2−δ, with specific oxygen permeability values approximately 2×10⁻⁹ mol s⁻¹ cm⁻¹ at 950 °C, but offering much better thermal expansion properties.     

Synthesis, structure, and magnetic properties of SrMn1−xGaxO3−δ (x=0-0.5) 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 derived from thermogravimetric measurements, we have extended the solubility limit of random substitution of Ga³⁺ for Mn in the cubic perovskite phase to x=0.5. In the cubic perovskite phase the maximum oxygen content is close to 3−x/2, which corresponds to 100% Mn⁴⁺. Maximally oxygenated solid solution compounds are found to order antiferromagnetically for x=0-0.4, with the transition temperature linearly decreasing as Ga content increases. Increasing the Ga content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.5Ga_0.5O_2.67(3) below 12 K. These properties are markedly different from the long-range antiferromagnetic order below 180 K observed for the layer-ordered compound Sr₂MnGaO_5.50 with nominally identical chemical composition.     

Comparison of the Chemical Stability of Li1−xCoO2 and Li1−xNi0.85Co0.15O2 Cathodes

The chemical stability of the layered Li_1−xCoO₂ and Li_1−xNi_0.85Co_O.15O₂ cathodes is compared by monitoring the oxygen content with lithium content (1−x) in chemically delithiated samples. The Li_1−xCoO₂ system tends to lose oxygen from the lattice at deep lithium extraction while the Li_1−xNi_0.85Co_0.15O₂ system does not lose oxygen at least for (1−x)>0.3. This difference seems to result in a lower reversible (practical) capacity (140 mA h/g) for LiCoO₂ compared to that for LiNi_0.85Co_0.15O₂ (180 Ma h/g). The loss of significant amount of oxygen leads to a sliding of oxide layers and the formation of a major P3 and a minor O1 phase for the end member CoO_2−δ with δ=0.33. In contrast, Ni_0.85Co_0.15O_2−δ with a small amount of δ=0.1 maintains the initial O3 layer structure.     

Mixed conductivity and Mössbauer spectra of (La0.5Sr0.5)1−xFe1−yAlyO3−δ (x=0-0.05, y=0-0.30)

Aluminum incorporation in the rhombohedrally distorted perovskite lattice of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ (x=0-0.05, y=0-0.30) decreases the unit cell volume and partial ionic and p-type electronic conductivities, while the oxygen nonstoichiometry and thermal expansion at 900-1200 K increase on doping. The creation of A-site cation vacancies has an opposite effect on the transport properties of Al-substituted ceramics. The maximum A-site deficiency tolerated by the (La,Sr)(Fe,Al)O_3−δ structure is however limited, close to 3-4%. The Mössbauer spectroscopy revealed progressive localization of electron holes and a mixed charge-compensation mechanism, which results in higher average oxidation state of iron when Al³⁺ concentration increases. The average thermal expansion coefficients of (La_0.5Sr_0.5)_1−xFe_1−yAl_yO_3−δ are (12.2-13.0)×10⁻⁶ K⁻¹ at 300-900 K and (20.1-30.0)×10⁻⁶ K⁻¹ at 900-1200 K in air. The steady-state oxygen permeability (OP) of dense Al-containing membranes is determined mainly by the bulk ionic conductivity. The ion transference numbers at 973-1223 K in air, calculated from the oxygen permeation and faradaic efficiency (FE) data, vary in the range 1×10⁻⁴-3×10⁻³, increasing with temperature.     

Luminescent and structural properties of the series Ba6−xEuxTi2+xTa8−xO30 and Ba4−yKyEu2Ti4−yTa6+yO30

The series Ba_6−xEu_xTi_2+xTa_8−xO₃₀ and Ba_4−yK_yEu₂Ti_4−yTa_6+yO₃₀ have been synthesized at 1400°C in air. They exhibit efficient excitation at about 400 nm and typical emission of Eu³⁺ at about 580-620 nm, form solid solutions within 0.0?x?2.0 and 0?y?4 respectively, and crystallized in P4/mbm at room temperature with Eu atoms occupied at centrosymmetric site (0, 0, 0). Their conductivity is very low (2.8×10⁻⁶ Ω⁻¹ cm⁻¹ at 740°C for Ba₆Ti₂Ta₈O₃₀).     

Chemical Synthesis and Properties of Spinel Li1−xCo2O4−δ

Spinel Li_1−xCo₂O_4−δ samples with 0.44≤(1−x)≤1 have been synthesized by chemically extracting lithium with the oxidizer NO₂BF₄ in acetonitrile medium from the LT-LiCoO₂ synthesized at 400°C. Rietveld analysis of the X-ray diffraction data reveals that the Li_1−xCo₂O_4−δ samples adopt the normal cubic spinel structure with a cation distribution of (Li_1−x)_8a[Co₂]_16dO_4−δ. Redox iodometric titration data indicate that the LT-LiCoO₂ tends to lose oxygen on extracting lithium and the spinel Li_1−xCo₂O_4−δ samples are oxygen-deficient. Both infrared spectroscopic and magnetic susceptibility data suggest that the LiCo₂O_4−δ spinel is metallic with itinerant electrons. The tendency to lose oxygen on extracting lithium from the LT-LiCoO₂ and the observed metallic behavior of the spinel LiCo₂O_4−δ are explained on the basis of a qualitative band diagram.     

Phase equilibrium and electrical conductivity of SrCo0.8Fe0.2O3−δ

The phase diagram of the SrCo_0.8Fe_0.2O_3−δ compound has been determined at high temperatures (823?T?1223 K) and in the oxygen partial pressure range (10⁻⁵?pO₂?1 atm) by thermogravimetric measurements of the equilibrium pO₂, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo_0.8Fe_0.2O_3−δ is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo_0.8Fe_0.2O_2.5 stabilizes within a small range around 3−δ=2.5 at temperatures below 1073 K. Equilibrium pO₂ measurements under isothermal conditions show chemical hysteresis at the perovskite to brownmillerite transition. The hysteresis loop decreases its amplitude in pO₂ with decreasing temperature. This behavior is discussed considering the evolution from coherent intergrowth interfaces with elastic strain energy to incoherent interfaces without elastic strain energy as T decreases. The thermodynamic quantities h_O2^oxide and s_O2^oxide for the perovskite phase decrease when increasing the oxygen defects concentration. The electrical conductivity (σ) of the cubic phase exhibits a thermally activated behavior at high temperature. The variation of σ with the oxygen content is non-linear and the activation energy varies from 0.4 to 0.28 eV as the oxygen content increases from 2.4 to 2.6. These results are interpreted in the frame of the small polaron model.     

Polaron morphologies in SrFe1−xTixO3−δ

The morphologies of the charge carriers in the perovskite system SrFe_1−xTi_xO_3−δ are explored by transport and magnetic measurements. Oxygen vacancies are present in all samples, but they do not trap out the Fe³⁺ ions they introduce. The x=0.05 composition was prepared with three different values of δ. They all show small-polaron conduction above 225 K; but where there is a ratio c=Fe⁴⁺/Fe<0.5, the polaron morphology appears to change progressively with decreasing temperature below 225 K to two-Fe polarons that become ferromagnetically coupled in an applied magnetic field at lower temperatures; With an applied field of 2500 Oe, divergence of the paramagnetic susceptibility for zero-field-cooled and field-cooled samples manifests a greater stabilization of ferromagnetic pairs on cooling in the applied field. With a c>0.5, the data are consistent with a disproportionation reaction 2Fe⁴⁺=Fe³⁺+Fe(V)O_6/2 that inhibits formation of two-Fe polarons and, on lowering the temperature, creates Fe³⁺-Fe(V)-Fe³⁺ superparamagnetic clusters.     

Synthesis, Crystal Structure and Physical Properties of LixMg0.857−xCu2.143O3−y

Solid solutions of Li-doped Mg_0.857Cu_2.143O₃ (Li_xMg_0.857−xCu_2.143O_3−y) were prepared at 950°C for 12 h in air by the solid-state method using Li₂CO₃, MgO and CuO powders. The solid solutions were obtained as the single α phase with the güggenite structure in 0≦x≦0.06 region. With the increasing of the Li content x, the lattice parameters a, b and unit cell volume V decreased, while c increased. On the basis of the charge neutrality, hole carrier estimated by the oxygen content increased with the Li substitution. The Seebeck coefficient at room temperature of x = 0.03 sample was +400 μV/K. The electrical resistivity ρ at room temperature drastically decreased with the increasing x. Temperature dependences of ρ for x = 0.01, 0.03 and 0.06 samples were semi-conductive behavior from room temperature to about 12 K. Interaction between Cu²⁺ and Cu²⁺ through O²⁻ seems to be somewhat large antiferromagnetic one. Sperconducting transition was not detected in the temperature range.     

Room temperature photoluminescence of amorphous BaxSr1−xTiO3 doped with chromium

ABO₃ amorphous materials, such as BaTiO₃ (BT), SrTiO₃ (ST), PbTiO₃ (PT), and Ba_xSr_1−xTiO₃ (BST) have recently attracted a good deal of attention due to their ferroelectric and electro-optical properties. Intense photoluminescence at room temperature was observed in amorphous titanate doped with chromium (Ba_xSr_1−xTi_1−yCr_yO₃) prepared by the polymeric precursor method. Results indicated that substantial luminescence at room temperature was achieved with the addition of small Cr contents to amorphous Ba_xSr_1−xTi_1−yCr_yO₃. Further addition of Cr or crystallization were deleterious to the intensity of the luminescent peak obtained for excitation using λ=488.0 nm.     

The effect of a cation radii on structural, magnetic and electrical properties of doped manganites La0.6−xPrxSr0.4MnO3

Structural, magnetic and transport properties of La_0.6−xPr_xSr_0.4MnO₃ with x=0.0, 0.03, 0.06, 0.18, 0.3, 0.42, 0.54 and 0.6 are studied. The system exhibits a rhombohedrally distorted perovskite structure for x?0.3. A rhombohedral-orthorhombic (Pnma) structure transition is detected in the doping range from x=0.42 to 0.6. The structure refinement by Rietveld analysis of the X-ray powder diffraction data shows that the average distance Mn-O increases in the rhombohedral phases and decreases in the orthorhombic phases. Results show that the Curie temperature decreases from 374 to 310 K when 〈r_A〉 varies from 1.254 to 1.231 Å. Electrical measurements show that all samples exhibit a metallic to semiconducting transition with increasing temperature. Meanwhile, the size of the resistivity ρ increases near T_C. This phenomenon is interpreted as a gradual bending of the Mn-O-Mn bond angle, with decreasing 〈r_A〉, which causes the narrowing of the electronic bandwidth and the effect of the A-site variance σ².     

Superconductivity and crystal structure of the solid solutions of Ba8−δSi46−xGex (0?x?23) with Type I clathrate structure

The solid solutions of barium containing Type I clathrate, Ba_8−δSi_46−xGe_x (0?x?23) were prepared under high-pressure and high-temperature conditions of 3 GPa at 800°C. All the solid solutions showed superconductivity, and the transition temperature (T_c) decreased from 8.0 to 2.0 K as the germanium content increased from x=0 to 23 in Ba_8−δSi_46−xGe_x. The single crystals with five different compositions were obtained and the structures, compositions, and site occupancies were determined from X-ray single-crystal analysis. A slight barium deficiency was observed at Ba1 (2a) sites for all the clathrates. The Ge atoms replaced the Si atoms at the Si3 (24k) site in the composition range of x<8, and then at the Si2 (16i) site. The crystals had a slight deficiency in the covalent (Si, Ge) network and the deficiency increased with the increase of the Ge content.     

Crystal structure and physical properties of quaternary clathrates Ba8ZnxGe46−x−ySiy, Ba8(Zn,Cu)xGe46−x and Ba8(Zn,Pd)xGe46−x

Three series of vacancy-free quaternary clathrates of type I, Ba₈Zn_xGe_46−x−ySi_y, Ba₈(Zn,Cu)_xGe_46−x, and Ba₈(Zn,Pd)_xGe_46−x, have been prepared by reactions of elemental ingots in vacuum sealed quartz at 800 °C. In all cases cubic primitive symmetry (space group Pm3?n, a∼1.1 nm) was confirmed for the clathrate phase by X-ray powder diffraction and X-ray single crystal analyses. The lattice parameters show a linear increase with increase in Ge for Ba₈Zn_xGe_46−x−ySi_y. M atoms (Zn, Pd, Cu) preferably occupy the 6d site in random mixtures. No defects were observed for the 6d site. Site preference of Ge and Si in Ba₈Zn_xGe_46−x−ySi_y has been elucidated from X-ray refinement: Ge atoms linearly substitute Si in the 24k site whilst a significant deviation from linearity is observed for occupation of the 16i site. A connectivity scheme for the phase equilibria in the “Ba₈Ge₄₆” corner at 800 °C has been derived and a three-dimensional isothermal section at 800 °C is presented for the Ba-Pd-Zn-Ge system. Studies of transport properties carried out for Ba₈{Cu,Pd,Zn}_xGe_46−x and Ba₈Zn_xSi_yGe_46−x−y evidenced predominantly electrons as charge carriers and the closeness of the systems to a metal-to-insulator transition, fine-tuned by substitution and mechanical processing of starting material Ba₈Ge₄₃. A promising figure of merit, ZT ∼0.45 at 750 K, has been derived for Ba₈Zn_7.4Ge_19.8Si_18.8, where pricey germanium is exchanged by reasonably cheap silicon.     

Thermodynamic Quantities and Defect Structure of La0.6Sr0.4Co1−yFeyO3−δ(y=0–0.6) from High-Temperature Coulometric Titration Experiments

The partial energies and entropies of O₂in perovskite-type oxides La_0.6Sr_0.4Co_1−yFe_yO_3−δ(y=0, 0.1, 0.25, 0.4, 0.6) were determined as a function of nonstoichiometryδby coulometric titration of oxygen in the temperature range 650–950°C. An absolute reference value ofδwas obtained by thermogravimetry in air. The nonstoichiometry at a given oxygen pressure and temperature decreases with iron contenty. At low nonstoichiometries the oxygen chemical potential decreases withδ. The observed behavior can be interpreted by assuming random distribution of oxygen vacancies, an electronic structure with both localized donor states on Fe, and a partially filled itinerant electron band, of which the density of states at the Fermi level scales with the Co content. The energy of the Fe states is close to the energy at the Fermi level in the conduction band. The observed trends of the thermodynamic quantities can be interpreted in terms of the itinerant electron model only when the iron content is small. At high values ofδthe chemical potential of O₂becomes constant, indicating partial decomposition of the perovskite phase. The maximum value ofδat which the compositions are single-phase increases with temperature.     

Synthesis and NMR spectroscopic investigation of salts containing the novel [Au(CF3)nX4−n] (n=4-1, X=F, CN, Cl) anions

First examples for the syntheses of trifluoromethyl transition metal complexes by conversion of a cyano into a trifluoromethyl ligand are described. The fluorination of [][Au(CN)₄] with ClF in CH₂Cl₂ leads to the formation of a mixture of gold complexes of the type [AuF_xCl_y(CF₃)_4−x−y]⁻ (x=0-4, y=0-2). Ligand exchange reactions of [AuF_xCl_y(CF₃)_4−x−y]⁻ (x=0-4, y=0-2) with (CH₃)₃SiY (Y=Cl, CN) are performed resulting in anions of the type [AuY_x(CF₃)_4−x]⁻ (x=0-4). All products are characterised by - and NMR spectroscopy.     

Crystal growth and properties of Ln2Ag1−xGa10−y (Ln=La, Ce), a disordered variant of the Ce2NiGa10-structure type

We report the flux growth and characterization of Ln₂Ag_1−xGa_10−y (Ln=La, Ce), a disordered variant of the Ce₂NiGa₁₀ structure type. Single crystals of La₂Ag_1−xGa_10−y (x∼0.3; y∼0.6) and Ce₂Ag_1−xGa_10−y (x∼0.3; y∼0.9) were grown by the self-flux method and characterized using single-crystal X-ray diffraction. Transport measurements of Ce₂Ag_1−xGa_10−y (x∼0.3; y∼0.9) reveal metallic behavior with a transition at 3 K. Magnetic measurements indicate antiferromagnetic ordering at 3 K of localized Ce³⁺ moments for Ce₂Ag_1−xGa_10−y. Magnetoresistance is positive with a maximum value of 16% at 9 T. La₂Ag_1−xGa_10−y exhibits metallic behavior with magnetic susceptibility showing temperature independent paramagnetism. We will compare Ce₂Ag_1−xGa_10−y (x∼0.3; y∼0.9) to Ce₂NiGa₁₀ to examine the effects of transition metal substitution and to the related Ce(Ag,Ga)₄ phase to examine the effects of crystal structure on the physical properties.     

New Ba5M5−xPtxClO13 (M=Fe, Co) oxychlorides with layered perovskite-related structure

Crystals of Ba₅Fe_5−xPt_xClO₁₃ and Ba₅Co_5−yPt_yClO₁₃ were prepared for x=1.31, 1.51, 1.57, 1.59 and y=0, 0.97 and 1.08 in a BaCl₂ flux and investigated by X-ray diffraction methods. These compounds adopt a 10H perovskite structure built from the stacking of BaO₃ and BaOCl layers in the sequence (BaO₃)₄(BaOCl) with space group P6₃/mmc. The cation sites within the trimeric unit of face-sharing octahedra are occupied by Co or Fe and Pt ions, while the tetrahedral sites formed between BaO₃ and BaOCl layers are only occupied by Fe or Co. Moreover, oxygen stoichiometry indicates an average oxidation state for Co and Fe higher than +III, indicating the stabilization of Co⁴⁺ and Fe⁴⁺.     

Homoatomic Clustering in the Intermediate Valence Compounds Yb1−yAl3−xSix and Yb1−yAl3−xGex

Two new phases, Yb_1−xAl_3−xSi_x and Yb_1−yAl_3−xGe_x, were found by systematic investigations of the according ternary systems. The crystal structures of Yb_1−yAl_2.8Si_0.2 and Yb_1−yAl_2.8Ge_0.2 (defect HT-PuAl₃ type) were studied by X-ray powder methods (CuKα₁ radiation, λ=1.54056 Å, hexagonal system, space group P6₃/mmc (No. 194), a=6.009(1) and 6.015(1) Å, c=14.199(2) and 14.241(5) Å, V=444.0(2) and 446.2(3) ų, 93 and 92 reflections, and 8200 and 8000 profile points for silicide and germanide, respectively). Full profile refinements with 11 and 13 structural parameters resulted in R_I=0.049 and 0.054, and R_p=0.088 and 0.104, respectively. The ternary structures are distorted closest packings in comparison with the binary YbAl₃ compound with AuCu₃-type structure. They are characterized by the formation of Al₃-, Si₃-, and Ge₃-homoatomic clusters and aluminum networks. Magnetization measurements show that both the silicide and germanide are valence fluctuation compounds with enhanced electronic density of states at the Fermi level similar to the binary YbAl₃. The characteristic maximum of the magnetic susceptibility increases from ≈120 K for YbAl₃ to ≈140 K for Yb_1−yAl_2.8Si_0.2or Yb_1−yAl_2.8Ge_0.2 and further to ≈150 K for Yb_1−yAl_2.75Si_0.25. The S-shape of the electrical resistivity curves is also characteristic of valence fluctuations.     

Co3−δO4 paracrystal: 3D assembly of nanosize defect clusters in spinel lattice

Paracrystalline array of defect clusters ca. five times the lattice spacing of the average Co_3−δO₄ spinel structure occurred more or less in a relaxed manner when the sintered Co_1−xO polycrystals were air-quenched below the Co_1−xO/Co_3−δO₄ transition temperature to activate oxy-precipitation of cube-like Co_3−δO₄ at dislocations. The same paracrystalline spacing was obtained for Co_3−δO₄ when formed via oxidizing/sintering the Co_1−xO powders at 800°C in air, suggesting a nearly constant δ value for Co_3−δO₄ in the T-P_O2 conditions encountered. The extra cobalt vacancies and Co³⁺ interstitials, as a result of δ value, may form additional 4:1-derived defect clusters for further paracrystalline distribution in the spinel lattice. The nanosize defect clusters self-assembled by columbic interactions and lattice relaxation in ionic crystal may have potential applications as step-wise sensor of oxygen partial pressure at high temperatures.