Crystal Structure and Electrical Conductivity of Palladium Sulfide BronzesMP3S4(M=La,Nd, and Eu)

Palladium sulfide bronzesMPd₃S₄(M=La, Nd, and Eu) were prepared in single phase. The bronzes are cubic with twoMatoms in (0,0,0; 1/2,1/2,1/2) and six palladium atoms in (1/4,0,1/2⥀) positions. The sulfur positions (x,x,x⥀) were determined with a guide of theRfactors. Thexvalues were 1/4 for La and Nd compounds (i.e., space groupPm3n), while a plot of theRfactors of EuPd₃S₄gave a very broad curve showing thexvalue rather displaced from 1/4. This result is considered to be associated with the mixed valency of europium (Eu²⁺and Eu³⁺) in this compound.MPd₃S₄(M=La, Nd, and Eu) exhibited metallic conductions with the electrical conductivities decreasing with increasing temperature in the experimental range from ∼15 K to room temperature. At 300 K, σ were 2.77, 2.42, and 2.28 S m⁻¹forMPd₃S₄(M=La, Nd, and Eu), respectively. From the Hall coefficient measurements, the carriers were found to be the electrons with their numbers 1.71, 1.68, and 0.82 per unit cell of the crystals ofM=La, Nd, and Eu compounds, respectively. These values suggest the formulas to beM³⁺(Pd²⁺₃e⁻)S²⁻₄for La and Nd compounds, and to be Eu²⁺_0.5Eu³⁺_0.5(Pd²⁺₃e⁻_0.5)S²⁻₄for Eu compound.     

Impact of metal substitutions for cobalt in YBaCo4O7

The “114” YBaCo₄O₇ cobaltite undergoes structural transition just beyond room temperature at T_S∼310 K. Correspondingly, its signature in the physical properties is detected by T-dependent measurements of electrical resistivity, magnetic susceptibility and thermoelectric power. It is found that low-level substitutions of divalent (M=Zn²⁺) or trivalent (M=Ga³⁺, Al³⁺) cations for cobalt according to the YBaCo_4−xM_xO₇ formula with x?0.4 have a strong impact upon this transition. On the one hand, Zn²⁺ substitutions preserve the transition but with T_S decreasing as x increases. On the other hand, for x=0.2 Ga³⁺ or Al³⁺, the transition is suppressed, i.e., for only 5% trivalent foreign cation substituted for cobalt. Though at first, this contrasted behaviour between divalent and trivalent substituting cations appears to be linked to the opposite evolution of hole carriers “Co³⁺” concentration with x, a possible destabilization of 3Co²⁺: 1Co³⁺ charge ordering induced by the M³⁺ cations is considered.     

Magnetic interactions in ternary cobalt oxides with cubic perovskite structure

Magnetic properties were measured on the cubic perovskite systems SrCoO_3?δ, (La_1?xSr_x)CoO₃ (0.5 ≦ x ≦ 1.0), and Sr(Co_1?xMn_x)O₃ (0 ≦ x ≦ 1.0). It is found that S²⁺ and La³⁺ ions strongly affect the spin state of the Co²⁺ ion and that the Mn⁴⁺ ion located at the octahedral site affects the spin state of Co⁴⁺ ion. The magnetic properties (T_c, T_θ, and σ) are explained by the magnetic interaction Co³⁺OCo³⁺, Co³⁺OCo⁴⁺, Co⁴⁺OCo⁴⁺, Mn⁴⁺OMn⁴⁺, and Mn⁴⁺OCo⁴⁺ in these systems.     

Reactivity in The Solid State Between Ag2S and Ag2CrO4

Reactivity, in the solid state between Ag₂S and Ag₂CrO₄, was investigated by DTA, XRD and IR methods. It was found that, according to a composition of an initial Ag₂S/Ag₂CrO₄ mixture, the products of a reaction of Ag₂S with Ag₂ CrO₄ can be: solid solution with Ag₂CrO₄ structure (Ag₂Cr_1–xS_xO₄) and AgCrO₂; or solid solution Ag₂Cr_1–xS_xO₄, Ag₂SO₄, AgCrO₂ and metallic silver; or Ag₂S, β-Ag₈S₄O₄, Ag, AgCrO₂, Ag₂SO₄ and Ag₂Cr_1–xS_xO₄ solid solution. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phase relationships and some magnetic properties of spinels in the systems M1−xNixCr2S4 where M = Mn,Fe, Co

Phase relationships between spinel and defect NiAs structures in the systems M_1?xNi_xCr₂S₄ (where M = Mn, Fe, Co) were investigated. It was found that the spinel structure is stable between x = 0 and x = 0.3 when M = Mn or Fe. When M = Co the spinel is formed in the region x = 0 to x ～ 0.4. The apparent stabilization of the defect NiAs phase by Ni²⁺ may be related to the strong sixfold site preference of Ni²⁺. Curie temperatures of all three ferrimagnetic systems increases with increasing Ni²⁺ substitution. This is probably due to higher NiS covalency.     

Synthesis and magnetic properties of the quasi-one-dimensional compound Ca0.83(Cu1−xCox)O2

A new solid solution of the quasi-one-dimensional composite crystal, , has been synthesized under of O₂ at 830°C. The non-doped compound Ca_0.83CuO₂ consists of two interpenetrating monoclinic subsystems of the [Ca] atoms and the edge-shared square planar [CuO₂] chains. Upon increasing x, both the subsystems undergo a phase change from monoclinic to orthorhombic (M-O). The M-O change occurs at x∼0.04 for the [(Cu,Co)O₂] subsystem, while such a change occurs at x∼0.17 for the [Ca] subsystem. Magnetic susceptibility measurements show an evolution from a short-range ordered state near x=0 to a long-range antiferromagnetic state for the samples with x?0.15. The effective magnetic moment μ_eff is found to increase with increasing x from for x=0.10 to for x=0.30, suggesting that the solid solution can be regarded as Ca_0.83[Cu_0.66²⁺Cu_0.34−x³⁺Co_x³⁺]O₂, in which a mixed state of Cu²⁺(S=1/2), Cu³⁺(S=0) and high-spin Co³⁺(S=2) ions is realized.     

Kinetics of hydroquinone chemisorption at polycrystalline platinum electrodes: The effect of surface roughness

The rate of hydroquinone (HQ) chemisorption from aqueous H₂SO₄ onto annealed (smooth) and platinized (rough) polycrystalline Pt has been studied. Previously, equilibrium adsorption measurements indicated that at smooth Pt surfaces, adsorption of HQ at c_HQ ⩽ 0.1 mM yielded flat (η⁶)-oriented species while adsorption at c_HQ ⩾ 1 mM resulted in edge (2,3-η²)-attached intermediates. Edge-attached species were not formed efficiently at roughened Pt surfaces. The present data show that the rate of η⁶-HQ chemisorption was significantly lower at roughened than at smooth surfaces. Analysis of the rate data gave the following enthalpies and entropies of activation at smooth (sm) and platinized (pt) surfaces: ΔH^≠_sm,0.1mM = 12.5 kJ/mol; ΔH^≠_sm,2mM = 8.3 kJ/mol; ΔS^≠_sm,0.1mM = −83 J/mol K; ΔS^≠_sm,2mM = −117 J/mol K; ΔH^≠_pt,0.4mM = 12.5 kJ/mol; ΔH^≠_pt,2mM = 12.5 kJ/mol; ΔS^≠_pt,0.4mM = −92 J/mol K; ΔS^≠_pt,2mM = −100 J/mol K. The similarity between ΔH^≠_{pt,0.4 mM}, ΔH^≠_{pt,2 mM} and ΔH^≠_{sm,0.1 mM}, and between ΔS^≠_{pt,0.4 mM}, ΔS^≠_{pt,2 mM} and ΔS^≠_{sm,0.1 mM} correlate with the earlier finding that adsorption of HQ onto roughened Pt surfaces occurred primarily in the flat orientation at all concentrations studied.     

Influence of M2+ ions substitution on the structure of lanthanum hexaaluminates with magnetoplumbite structure

A series of compounds with the initial composition LaM_xAl₁₁O_18+x has been studied with x ranging from 0 to 1, and M = Mn, Co, Cu. They exhibit a more or less distorted and defective magnetoplumbite arrangement. Their effective compositions have been determined by X-ray structure refinement. The smaller the content of M²⁺, the higher the disorder in the crystal lattice, the worse the crystal growth, and more intense the diffuse scattering in (001) planes. The role of M²⁺ is compared to that of M²⁺ stabilizing γ-Al₂O₃ to give MAl₂O₄ spinels.     

Mo¨ssbauer studies of thiospinels. IV. The system FeCr2S4-Fe3S4

Spinel compounds of the composition Fe_1+xCr_2?xS₄, with 0 ≦ x ≦ 0.5, have been prepared in polycrystalline form. The ionic distribution Fe²⁺[Cr³⁺_2?xFe³⁺_x]S^2?₄ is derived from both X-ray and ⁵⁷Fe Mo¨ssbauer data. Room temperature Mo¨ssbauer spectra show the typical behavior of tetrahedral-site Fe²⁺ surrounded by different octahedral-site neighbors. Octahedral-site Fe³⁺ absorbs as a doublet with Δ ≈ 0.5 mm/s. Samples of overall composition FeCr₂S₄ consist mainly of a spinel Fe²⁺[Cr³⁺_2?yFe³⁺_y]S^2?₄, y ≈ 0.02.     

Physical and optical properties of the quaternary sulfides SrCu2MS4 and EuCu2MS4 (M=Ge and Sn)

Four quaternary sulfides SrCu₂MS₄ and EuCu₂MS₄ (M=Ge and Sn) were prepared from a thoroughly ground mixture of EuS or SrS, Cu, or Sn, and S in stoichiometric proportions. Electrical conductivity measurements on pressed pellets showed that all the phases are semiconductors. The optical band gaps were assessed at 2.8 eV for SrCu₂GeS₄, 2.1 eV for SrCu₂SnS₄, 2.2 eV for EuCu₂SnS₄, and 1.6 eV for EuCu₂GeS₄. Both Sr-based compounds present a temperature-independent paramagnetism, of about +135×10⁻⁶ and +92×10⁻⁶ emu/mol, for SrCu₂SnS₄ and SrCu₂GeS₄, respectively. In the case of the europium compounds, they follow a Curie-Weiss dependence above 1.8 K (EuCu₂GeS₄) and above 4 K (for EuCu₂SnS₄), with values of the magnetic effective moment μ_eff and the Curie-Weiss temperature Θ, equal to 6.27 μ_B and −2.8 K for EuCu₂GeS₄, and 6.81 μ_B and +0.7 K, for EuCu₂SnS₄. The experimental magnetic moments confirm that the europium ion is in divalent state, similar to Sr in the related compounds.     

Studies on 8-quinolinol-5-sulphonic acid complexes of dioxouranium(VI) and elucidation of their structures and bondings through physico-chemical analyses

Interaction between the uranyl ion and 8-quinolinol-5-sulphonic acid has been studied and the soluble free acid with balancing secondary cations M₃(I)H[UO₂X₃] · xH₂O (where M(I) = Na⁺, K⁺; X = (C₉H₅NSO₄)²⁻; x = 0.5, 1) has been isolated in the solid state. Serveal salts of this free acid have been isolated in the solid state having the formulas M₄(I)[UO₂X₃]·xH₂O and M₂(II)[UO₂X₃]·xH₂O (where M(II) = Ca²⁺, Sr²⁺, Ba²⁺: x = 0.5, 1 or 2). All these complexes have been characterized through elemental analyses, IR, far IR, electronic and nmr spectra; TGA and DTA data of some of these complexes have also been reported. The free acid formulated as Na₃H[UO₂X₃]·0.5H₂O has been characterized by methods mentioned above and also by potentiometric titration with alkali and by determination of dissociation constant. The pK_a-value of the acid determined by Bjerrum's method is 6.2 at 27°C. The conversions of M₃(I)H(UO₂X₃]·xH₂O to the corresponding M₄(I)[UO₂X]·xH₂O and vice versa have been done in solution and the resulting products have been isolated in the solid state.     

Structure and photoluminescence properties of Ce-doped novel silicon-oxynitride Ba4−zMzSi8O20−3xN2x (M=Mg, Ca, Sr)

Structural and photoluminescence properties of undoped and Ce³⁺-doped novel silicon-oxynitride phosphors of Ba_4−zM_zSi₈O_20−3xN_2x (M=Mg, Sr, Ca) are reported. Single-phase solid solutions of Ba_4−zM_zSi₈O_20−3xN_2x oxynitride were synthesized by partial substitutions of 3O²⁻→2N³⁻ and Ba→M (M=Mg, Ca, Sr) in orthorhombic Ba₂Si₄O₁₀. The influences of the type of alkaline earth ions of M, the Ce³⁺ concentration on the photoluminescence properties and thermal quenching behaviors of Ba₃MSi₈O_20−3xN_2x (M=Mg, Ca, Sr, x=0.5) were investigated. Under excitation at about 330 nm, Ba₃MSi₈O_20−3xN_2x:Ce³⁺ (x=0.5) exhibits efficient blue emission centered at 400-450 nm in the range of 350-650 nm owing to the 5d→4f transition of Ce³⁺. The emission band of Ce³⁺ shifts to long wavelength by increasing the ionic size of M due to the modification of the crystal field, as well as the Ce³⁺ concentrations due to the Stokes shift and energy transfer or reabsorption of Ce³⁺ ions. Among the silicon-oxynitride phosphors of Ba₃MSi₈O_18.5N:Ce³⁺, M=Sr_0.6Ca_0.4 possesses the best thermal stability probably related to its high onset of the absorption edge of Ce³⁺.     

A kinetic investigation of reaction of [Os(N)(CH2SiMe3)4][NBun4] with methyl iodide

A high oxidation state alkylnitridoosmium complex, [Os(N)(CH₂SiMe₃)₄][NBuⁿ₄] acts as a nucleophile in reactions with alkyl halides. Alkylimido complexes, Os(NR)(CH₂SiMe₃)₄, are produced. The reaction between [Os(N)(CH₂SiMe₃)₄]⁻ [NBuⁿ₄] and MeI is second order with k₂= 9.5 x 10⁻⁵ sec^t̄1 M⁻¹ at 23°C in CD₂Cl₂ under pseudo first order conditions. The entropy of activation, ΔS^≠, was found to be −10.6 ± 0.5 cal M⁻¹ K⁻¹ and the enthalpy of activation, ΔH^≠, was found to be 19.6 ± 0.2 kcal M⁻¹. The reaction proceeds faster in polar, non-coordinating solvents than in either non-polar solvents or in solvents which can coordinate to the osmium center.     

Kinetics of the solvolysis of trans-dichlorotetra(4-t-butylpyridine)-cobalt(III) ions in mixtures of water with methanol and with ethanol

For the solvolysis of Co(4-t-Bupy)₄Cl₂^? ions in water + methanol and water + ethanol, log (rate constant) does not vary linearly with the reciprocal of the dielectric constant. The Gibbs free energy, the enthalpy, and the entropy of activation are insensitive to changes in the solvent composition in these mixtures, although a slight broad maximum in ΔH* and ΔS* probably exists at mole fractions of about 0.2 in water + ethanol. This contrasts with the extrema in ΔH* and ΔS* found with more hydrophobic alcohols used as cosolvents. However, the application of a Gibbs energy cycle to the solvolysis in water and in the mixtures shows that there is a differential effect of changes in solvent structure on the emergent solvated Co^III cation in the transition state and on Co(4-t-Bupy)₄Cl₂⁺ in the initial state. The stability of the former increases relative to that of the latter as the cosolvent content of the mixture rises. © 1995 John Wiley & Sons, Inc.     

Synthesis and photoluminescence properties of the high-brightness Eu-doped M2Gd4(MoO4)7 (M=Li, Na) red phosphors

A series of red-emitting phosphors Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) have been successfully synthesized at 850 °C by solid state reaction. The excitation spectra of the two phosphors reveal two strong excitation bands at 396 nm and 466 nm, respectively, which match well with the two popular emissions from near-UV and blue light-emitting diode chips. The intensity of the emission from ⁵D₀ to ⁷F₂ of M₂(Gd_1−xEu_x)₄(MoO₄)₇ phosphors with the optimal compositions of x=0.85 for Li or x=0.70 for Na is about five times higher than that of Y₂O₃:Eu³⁺. The quantum efficiencies of the entitled phosphors excited under 396 nm and 466 nm are also investigated and compared with commercial phosphors Sr₂Si₅N₈:Eu²⁺ and Y₃A₅O₁₂:Ce³⁺. The experimental results indicate that the Eu³⁺-doped M₂Gd₄(MoO₄)₇ (M=Li, Na) phosphors are promising red-emitting phosphors pumped by near-UV and blue light.     

Synthèse et propriétés magnétiques de solutions solides entre l'oxyde de chrome CrO2 et l'oxyhydroxyde de cobalt CoOOH

Solid solutions of general formula (1 ? x)CrO₂, xCoOOH have been prepared by hydrothermal synthesis under very high pressure conditions (80 kb). Cr⁶⁺O₃ and CoCr⁶⁺O₄ were used as starting materials. Homogeneous ferromagnetic phases were obtained when 0 ? x ? 0.5. X ray powder patterns clearly demonstrate the isotype with InOOH, an orthorhombic distorted rutile type structure. The results of the magnetic measures performed on samples with different compositions indicate that part of the Cr⁴⁺ cation have been reduced to Cr³⁺ and that the general formula of the solid solutions should be written Cr⁴⁺_1?(x+y)Cr³⁺_yCo²⁺_xO₂H_x+y.     

Molecular-orbital treatment of complex cations of magnesium and beryllium with acetonitrile

Semiempirical MO calculations of the self-consistent charge and configuration (SCCC ) method are reported for the acetonitrile–metal solvated species (CH₃CN)_xMⁿ⁺, where M = Be²⁺ and Mg²⁺. Comparison of the delocalization energies for various chemical structures x = 1, 2, 3, 4, 6 leads to an expectation of a tetrahedral structure for the Be²⁺ species and an octahedral structure for the Mg²⁺ species. The electronic nature of the donor–acceptor interaction is also discussed.     

Enthalpy of solvation, ΔHsolv0(CrO42−) (g), of gaseous chromate ion as estimated from lattice energy calculations

The enthalpy of solvation of the gaseous chromate ion, ΔH_solv⁰(CrO₄^2?) (g) is estimated on the basis of recent lattice energy studies made in this laboratory, and a charge distribution assigned to the ion. On the basis of the assigned charge of ?0.57 proton units to the oxygen atoms of the CrO₄^2? unit, the total lattice potential energies are found to be: U_pot(Na₂CrO₄) = 1836 kJ mol^?1; U_pot(K₂CrO₄) = 1717 kJ mol^?1; U_pot(Rb₂CrO₄) = 1645 kJ mol^?1 and U_pot(Cs₂CrO₄) = 1598 kJ mol^?1. The corresponding value for ΔH_solv⁰(CrO₄^2?) (g) = ?1077 kJ mol^?1.     

Eu doping effects on structural and magnetic properties of (Sr2−xEux)FeMoO6 compounds

Double perovskite compounds (Sr_2−xEu_x)FeMoO₆ (0≤x≤0.3) were prepared by solid-state reaction at high temperature. Crystal structure, magnetic and transport properties of the compounds were investigated. The crystal structure of the compounds changes from an I4/m lattice to an Fm3¯m lattice around x=0.1. The unit-cell volume decreases with the doping level in both the I4/m lattice and the Fm3¯m lattice. The resistivity of the compounds shows a metal-semiconductor transition, and the transition temperature T_M-S increases with the doping level. However, Curie temperature (T_C) of the compounds exhibits a weak dependence on the doping level. The saturation magnetization (M_S) at 100 K varies almost linearly with the anti-site defect concentration and agrees well with the simple FIM model. In contrast to the Ce-, Pr-, Nd- and Sm-doped Sr₂FeMoO₆, the difference of M_S of (Sr_2−xEu_x)FeMoO₆ between 5 and 100 K indicates that the moment of Eu³⁺ is antiparellel to that of Fe³⁺ at low temperature.     

Preparation, structure and photoluminescence properties of Eu and Ce-doped SrYSi4N7

Undoped and Eu²⁺ or Ce³⁺-doped SrYSi₄N₇ were synthesized by solid-state reaction method at 1400-1660 °C under nitrogen/hydrogen atmosphere. The crystal structure was refined from the X-ray powder diffraction data by the Rietveld method. SrYSi₄N₇ and EuYSi₄N₇, being isotypic with the family of compounds MYbSi₄N₇ (M=Sr, Eu, Ba) and BaYSi₄N_7, crystallize with the hexagonal symmetry: space group P6₃mc (No. 186), Z=2, a=6.0160 (1) Å, c=9.7894 (1) Å, V=306.83(3) Å³; and a=6.0123 (1) Å, c=9.7869 (1) Å, V=306.37(1) Å³, respectively. Photoluminescence properties have been studied for Sr_1−xEu_xYSi₄N₇ (x=0-1) and SrY_1−xCe_xSi₄N₇ (x=0-0.03) at room temperature. Eu²⁺-doped SrYSi₄N₇ shows a broad yellow emission band peaking around 548-570 nm, while Ce³⁺-doped SrYSi₄N₇ exhibits a blue emission band with a maximum at about 450 nm. SrYSi₄N₇:Eu²⁺ can be very well excited by 390 nm radiation, which makes this material attractive as conversion phosphor for LED lighting applications.