A systematic study of magnetic order in divalent europium perovskites

The magnetic properties of a series of cubic perovskite materials, Eu(Al_0.5Ta_0.5)O₃, Eu(Mg_0.5W_0.5)O₃, and Eu(Lu_0.5Ta_0.5)O₃ have been investigated using bulk magnetic and Mössbauer techniques. Eu(Al_0.5Ta_0.5)O₃ is a ferromagnet (T_c ～ 5°K, θ_c = + 8°K), Eu(Lu_0.5Ta_0.5)O₃ is an antiferromagnet (T_N = 4.0, θ_c = ?8°K), while Eu(Mg_0.5W_0.5)O₃ is probably antiferromagnetic (T_N = 2.8°K, θ_c = ? 1°K). These data are compared with the known properties of EuLiH₃ and EuTiO₃ and a sharp drop in θ_c as a function of increasing lattice constant is noted. A molecular field theory analysis of the data yields two possible sets of nn (J₁) and nnn (J₂) exchange constants for each compound which are compared with existing theories.     

Factors affecting the precision of a new method for determining the reduced and oxidized forms of a redox couple by a single potentiometric titration

In a potentiometric titration of a solution that contains both the reduced form Red₂ and the oxidized form Ox₂ of a redox couple, at concentrations of c⁰_Red2M and c⁰_Ox2M, respectively, using as reagent a solution that contains the oxidized and reduced forms Ox₁ and Red₁ at concentrations of c_Ox1M and c_Red1M, respectively, the reaction that occurs is nRed₂ + n₂Ox₁ = n₁Ox₂ + n₂Red₁; K_t = exp[n₁n₂(E⁰₁′–E⁰₂′)/0.02569] if all of the reactants and products are monomeric and if the titration is done at 25°C. By applying weighted non-linear regression analysis to the data obtained in such a titration, if c_Ox1 is known, it is possible to evaluate four parameters: c⁰_Red2, c⁰_Ox2 , and the formal potentials E⁰₂′ and E⁰₁′ for the couple titrated and the reagent couple, respectively. For the common situation in which n₁ and n₂ are equal, this paper describes the ways in which the precisions of all four parameters depend, over a wide range of conditions, on their values and on the precisions of measurement of the volume of reagent and the potential of the indicator electrode.     

Subsolidus phase relations and crystal structures of R-Ca-Cu-O (R=Nd, Sm, Gd, Tm) systems

The subsolidus phase relations of R₂O₃-CaO-CuO ternary systems (R=Nd, Sm, Gd, Tm) have been investigated by X-ray powder diffraction. All samples were synthesized at about 950° in air. There exists a ternary compound Ca_14−xR_xCu₂₄O₄₁ (x = 4 for R=Nd, Gd and x = 5 for R = Sm) and a ternary solid solution Ca_2+xR_2−xCu₅O₁₀ (R=Nd, Sm, Gd, Tm) with a wide composition range Δx of about 0.6. The compound Ca_14−xR_xCu₂₄O₄₁ possesses a layered orthorhombic structure and is isostructural to Sr_14−xCa_xCu₂₄O₄₁. The lattice parameters a and c of the compound are basically independent of the ionic radius of R, while the lattice parameter b and unit-cell volume V decrease substantially with the decrease of the ionic radii of R. The Ca_2+xR_2−xCu₅O₁₀ solid solution is isostructural to Ca_2+xY_2−xCu₅O₁₀, the structure of which is based on an orthorhombic “NaCuO₂-type” subcell containing infinite one-dimensional chains of edge-shared square planar cuprate groups crosslinked by the layered cations Ca and R that locate in the inter-chain tunnels.     

Predicting retention data in gas-liquid chromatography by multivariate analysis

Satisfactory reproduction of the retention data matrix in the case of a series of mono-substituted benzenes and a series of aliphatic substances proves the general character of the prediction equation

where x_Qi, φ_j are the retention data x of unknown solute Q_i on phase φ_j; a_Qi, b_Qi, c_Qi and d_Qi are regression coefficients which to some extent account for forces of orientation, charge transfer and association; and x_STi, φ_j are independent variables representing the retention data of the four compounds of the standard set of substances on a given phase φ_j. These four compounds are selected by diverse methods of multivariate analysis. The predicted values show very satisfactory agreement with the observed values.     

Etude par diffraction neutronique de la phase “Cs4−xYb12F40−x” (0 ≤ x ≤ 1): Hypothèse structurale

The structure of the phase Cs_4?xYb₁₂F_40?x(0 ≤ x ≤ 1) has been determined by a single-crystal neutron diffraction study. It has been solved in the space group P6₃mc and refined to the best R factor of 0.0535 for the formula Cs_3.4Yb₁₂F_39.4 (324 independent reflections). Three edge-sharing pentagonal bipyramids surrounding three ytterbium atoms form Yb₃F₁₆ groups and the structure is described as the superposition, according to the sequence A₁A₂B₁B₂A₁A₂…, of sheets of corner-sharing Yb₃F₁₆ groups with a possible transformation of bipyramids into octahedra in the A₂ and B₂ layers. These sheets are joined together by the axial fluorine atoms of the bipyramids or octahedra. Cesium atoms are located in the tunnels formed by their stacking. It is shown that the Cs_4?xYb₁₂F_40?x phase (0 ≤ x ≤ 1) is an intermediate step of the Cs_4?xYb₁₂F_40?x solid solution observed with 0 ≤ x ≤ 2 and corresponds to a superstructure of the high-temperature YbF₃ phase.     

Synthesis of nanopowders of pentoxides Ta2y Nb2(1−y)O5

The processes of production of high purity nanopowders of niobium and tantalum pentoxide Ta_2y Nb_2(1–y)O₅ with a low content of fluorine and Nb₂O₅ in low-temperature polymorph were studied. Ceramic samples were prepared from a charge of solid solutions LiTa _y Nb_1–y O₃ and Li _x Na_1–x Ta _y Nb_1–y O₃ synthesized using coprecipitated pentoxide Ta_2y Nb_2(1–y)O₅. Therewith for solid solutions LixNa_1–x Ta _y Nb_1–y O₃ significantly larger values of high dielectric constant and ionic conductivity were achieved compared to the solid solutions obtained by using a mechanical mixture of Ta₂O₅ and Nb₂O₅. This converts solid solutions LixNa_1–x Ta _y Nb_1–y O₃ from acoustoelectronic and piezoelectric type of materials into the capacitor and ion-conductive type of solid materials.     

Exchange interactions in R2Fe17−xGax (R=Y, Sm, Gd, Tb, Ho and Tm) compounds

We calculated the molecular field coefficients, n_FeFe and n_RFe (R=Sm, Gd, Tb, Ho and Tm), for R₂Fe_17−xGa_x and the values of n_FeFe and n_SmFe for R₂Fe_17−xT_x (T=Al and Si) using the experimental values of the Curie temperature. The values of n_FeFe increase in spite of the decrease of μ_Fe for 0?x?5. The values of n_SmFe have large values when the magnetic anisotropy is axial. For 6?x?8, the values of n_FeFe, n_HoFe and n_TmFe increase largely, which is related to the change of the easy magnetization direction. For Y₂Fe_17−xT_x (T=Ga and Al), the values of n_FeFe have a maximum value with increasing those of μ_Fe. With increasing V⁻¹, the values of n_FeFe have a maximum value around the same value of V⁻¹ for Y₂Fe_17−xT_x (T=Ga and Al). For Y₂Fe_17−xSi_x, the values of n_FeFe increase with increasing V⁻¹.     

Solid-state ionics: The electrical conductivity in the Ag1−xCuxI-substituted ammonium iodide double salts

The temperature dependence of the electrical conductivity in the Ag_1?xCu_xIQI (Q = NH₄, (CH₃)₄N, and C₅H₅NCH₃) systems were measured at temperatures ranging from ?20 to 130°C under nitrogen gas flow. Temperature T_d, at which a high ionic conductivity compound decomposes to a poor conductivity compound, was determined as a function of x. Temperature T_d was raised with the increase of x in the Ag_1?xCu_xINH₄I and Ag_1?xCu_xI(CH₃)₄NI systems, but did not depend on the x value in the Ag_1?xCu_xIC₅H₅NCH₃I system; that is, the disproportionation reaction in the Ag_1?xCu_xINH₄I and (CH₃)₄NI systems was different from that in the Ag_1?xCu_xIC₅H₅NCH₃I system.     

Determination of K shell X-ray intensity ratios for some heavy elements

The K shell X-ray intensity ratios K_α2/K_α1, K_β1,3/K_α1 and K_β2/K_α1 for 21 elements with 65⩽Z⩽92 have been measured using an incident photon energy of 123.6 keV. The X-rays have been measured with a Si(Li) semiconductor detector. K_β and K_α X-rays have been analyzed into the components K_β1,3 and K_β2 and K_α1 and K_α2, respectively, using a computer program. The experimental results were compared with the theoretical values of Scofield and available experimental results. All X-ray intensity ratios values have been plotted versus atomic number.     

Hardness evaluation of cured urea–formaldehyde resins with different formaldehyde/urea mole ratios using nanoindentation method

To understand the influence of formaldehyde/urea (F/U) mole ratio on the properties of urea–formaldehyde (UF) resins, this study investigated hardness of cured UF resins with different F/U mole ratios using a nanoindentation method. The traditional Brinell hardness (H_B) method was also used for comparison. The H_B of cured UF resin films with different F/U mole ratios was determined after exposing the films to different post-curing temperatures. The nanoindentation method was employed for these films to measure Meyer hardness (H_M) and reduced modulus (E_r) which have been used to calculate the elastic modulus (E_s) of cured UF resins. As the F/U mole ratio decreased, the H_B decreased continuously, indicating a less rigid network structure in low F/U mole ratio UF resins. The higher the post-curing temperature, the greater the value of H_B. The H_M value also showed a similar trend as a function of F/U mole ratio. However, the E_r and E_s did not show a consistent trend as exhibited by H_M and H_B. Both H_M and E_r showed much greater variation in the coefficient of variation (COV) at lower F/U mole ratios 1.0 and 1.2, indicating a more heterogeneous composition of these resins. Linear relationships between H_M and E_r indicate that heterogeneity of the surface composition of samples contributes greatly to variations in the measured values. This variability is discussed in terms of crystal structures present in the cured UF resins of low F/U mole ratios.     

Metamagnetism in DyBaCo2O5+x, x≈0.5

The temperature dependence of the paramagnetic susceptibility χ_m(T) taken in 2500 Oe, the resistivity ρ(T), and the thermoelectric power α(T) of DyBaCo₂O_5+x, which has Ba and Dy ordered into alternate (001) planes of an oxygen-deficient perovskite, have revealed a phase segregation in the compositional range 0.3?x<0.5. Orthorhombic DyBaCo₂O_5.51 has, in addition, oxygen vacancies ordered into alternate rows of the DyO_0.51 (001) planes; a cold-pressed polycrystalline sample exhibits a first-order insulator-metal transition at T_IM=320 K, a Curie temperature T_C=300 K, and a broadened metamagnetic transition temperature T_M≈265 K in 2500 Oe. A ferromagnetic M-H hysteresis curve fails to saturate at 5 T, and a minority ferromagnetic phase below T_M has a volume fraction that decreases with decreasing temperature, vanishing below 50 K. Oxygen vacancies in the DyBaCo₂O_5.5 phase suppress the metallic state; interstitial oxygen does not. A thermoelectric power α(T)>0 of DyBaCo₂O_5.51 changing continuously across T_IM is interpreted to manifest a metallic minority phase crossing a percolation threshold; α(T) also provides evidence for a progressive excitation of higher-spin Co(III) with increasing temperature from below 50 K to above T_IM. A previous model of the RBaCo₂O_5.5 phase is extended to account for the Ising spin configuration below T_C, the magnetic order in the presence of higher-spin octahedral-site Co(III), and the α(T) data.     

Phase and valence transitions in Ba2LnSnxSb1−xO6−δ (Ln=Pr and Tb)

Compounds in the double perovskites series Ba₂LnSn_xSb_1−xO_6−δ (Ln=Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba₂PrSn_xSb_1−xO_6−δ series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr³⁺ to Pr⁴⁺ with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba₂TbSbO₆ and Ba₂TbSnO_6−δ are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb³⁺ to Tb⁴⁺. Multiple linear regression analysis of the Pr and Tb L_III-edge XANES indicates that the rate of Ln³⁺ transforming to Ln⁴⁺ is such that there are no oxygen vacancies in Ba₂PrSn_xSb_1−xO_6−δ but in Ba₂TbSn_xSb_1−xO_6−δ there is a small amount of oxygen vacancies, with a maximum of δ≈0.05 present.     

Atomistic simulation on the site preference and mechanical properties of Th3Co4+xAl12−x and U3Co4+xAl12−x

An atomistic study is presented on the phase stability, site preference and lattice constants of the actinide intermetallic compounds Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x. Calculations are based on a series of interatomic pair potentials related to the actinides and transition metals, which are obtained by a strict lattice inversion method. The lattice constants of Th₃Co_4+xAl_12−x and U₃Co_4+xAl_12−x are calculated for different values of x. The site preference of Co atoms at Al sites is also evaluated and the order is given as 6h, 4f, 2b and 12k for Th₃Co_4+xAl_12−x, and 6h, 4f, 12k and 2b for U₃Co_4+xAl_12−x. In addition, some simple mechanical properties such as the elastic constants and bulk modulus are investigated for the actinide compounds with complex structures.     

Neutron powder diffraction and magnetic measurements on RbTil3, RbVI3, and CsVI3

CsVI₃ (a = 8.124(1) c = 6.774(1)Å,Z = 2, P6₃/mmc at 293 K) adopts the BaNiO₃ structure. Three-dimensional magnetic ordering takes place atT_c = 32(1)K. At 1.2 K the magnetic moment is 1.64(5) μ_B and it forms a 120° spin structure in the basal plane. RbVI₃ (a = 13.863(2) c = 6.807(1) Å,Z = 6, P6₃cmor P3¯c1 at 293 K) and RbTiI₃ (a = 14.024(3) Å,c = 6.796(2) Å,Z = 6, P6₃cm orP3¯c1 at 293 K) adopt a distorted BaNiO₃ structure, probably isostructural with KNiCl₃.T_c of RbVI₃ is 25(1) K. At 1.2 K, RbVI₃ has a spin structure similar to the one of CsVI₃ with a magnetic moment of 1.44(6) μ_B. RbTiI₃ shows no magnetic ordering at 4.2 K. It is shown that a deviation from the 120° structure is expected for compounds with a distorted BaNiO₃ structure such as RbVI₃. The cell dimensions of CsTiI₃ are reported.     

A new method for assigning the CO stretching bands in cis-disubstituted octahedral metal carbonyl complexes

The paper describes a method for assigning the CO stretching bands in cis-disubstituted metal carbonyls of the types L₂M(CO)₄, (LL)M(CO)₄, LL′M(CO)₄ and (LL′)M(CO)₄. The method was based on the parameter δ, defined as δ=(λ₁−λ₂)/(λ₁+λ₂−2λ₄) where λ₁, λ₂ and λ₄ are the λ parameters of the a₁⁽¹⁾, a₁⁽²⁾ and b₂ modes, respectively. For a large number of complexes with the above general formulas, the average value of δ was found to be 0.62, with a standard deviation of 0.02. It was shown that this range of δ can be used as a criterion of the correct band assignment for the complexes understudy.     

Electronic structures and stabilities of sodium clusters: jellium-sphere dimer calculation

Bonding properties of sodium-cluster dimers, (X ₄)₂ and (X ₈)₂, whereX _n is a jellium sphere corresponding to a cluster ofn atoms, were investigated by the linear-combination-of-jellium-orbitals method with local-spin-density-functional approximation. The stability ofn=8 clusters, observed in the experiment, is discussed in relation to the binding properties of dimers. We have found that (1) the (X ₄)₂ bonding has a covalent character, which makes theX ₈ formation favorable, and (2)X ₈ has an inert property because the force between jellium spheres in (X ₈)₂ is due to a weak dispersion force.     

Critical temperatures,pressures, and densities for the mixtures CO2–C3H8, CO2–nC4H10, C2H6–C3H8, and C3H8–nC4H10

A simple method is developed to estimate mixture critical temperatures (T_c), pressures (P_c), and densities (ρ_c) as a function of overall composition (X) from near critical region experimental coexistence data. This three-step method is applied to four mixtures, CO₂–C₃H₈, CO₂–nC₄H₁₀, C₂H₆–C₃H₈, and C₃H₈–nC₄H₁₀. Isothermal liquid–vapor coexistence data, which includes temperature, vapor pressure, coexisting densities (ρ^ℓ and ρ^v), and coexisting compositions for the more volatile component (x₁^v and x₁^ℓ) are used. In the first step, the difference of the saturated liquid and vapor densities (ρ^ℓ−ρ^v) is fitted to an empirical function in ((P_c−P)/P_c) to obtain P_c. Then P/P_c and ((ρ^ℓ+ρ^v)/2ρ_c) are simultaneously fitted to functions of a polynomial in (X₁−(x₁^v+x₁^ℓ)/2) yielding estimates of ρ_c and X₁. Finally, the discrete estimated critical data points are fitted with an equation to provide a continuous representation of the critical lines. The method is successfully tested for the mixtures, CO₂–C₃H₈ and CO₂–nC₄H₁₀, for which there is a reasonable amount of isothermal data. The procedure is then applied to the mixtures, C₂H₆–C₃H₈ and C₃H₈–nC₄H₁₀, for which there are sparse data. For all four mixtures, the critical temperature line, T_c vs. X₁, matches literature values within ±0.5%. The critical pressure line, P_c vs. X₁, and critical density line, ρ_c vs. X₁, match literature values, in general, within ±2%.     

Reactions of the Dirhenium(II) Complexes Re2 X 4(μ-dppm)2 (X=Cl, Br; dppm=Ph2PCH2PPh2) with Isocyanides. Part XV: A Fifth Structural Isomer of the [Re2Cl3(μ-dppm)2(CO)(CNXyl)2]+ Cation (Xyl = 2, 6-Dimethylphenyl)

The reaction of the unsymmetrical, coordinatively unsaturated dirhenium(II) complex [(XylNC)(OC)CIRe(μ-dppm)₂ReCl₂]O₃SCF₃ (dppm = Ph₂PCH₂PPh₂) with one equivalent of XylNC in CH₂Cl₂ affords a fifth structural isomer of the [Re₂Cl₃(μ-dppm)₂(CO)(CNXyl)₂] ⁺ cation; this is believed to have a CO-bridged structure of the type [(XylNC)ClRe(μ-Cl)(μ-CO)(μ-dppm)₂ReCl(CNXyl)]⁺. The latter complex reacts with a further equivalent of XylNC in the presence of Tl⁺ to form the [Re₂Cl₂(μ-dppm)₂(CO)(CNXyl)₃]²⁺ cation, which has been shown by IR spectroscopy, and by the X-ray crystallographic characterization of its neutral congener Re₂Cl₂(μ-dppm)₂(CO)(CNXyl)₃, to contain a very weak and unsymmetrical CO bridge.     

Intercalation behavior of n-alkylamines into an A-site defective layered perovskite H2W2O7

Intercalation behavior of n-alkylamines into a protonated form of an A-site defective layered perovskite H₂W₂O₇ has been investigated. Results from XRD indicate these materials are layered with the corresponding interlayer spacing governed by the n-alkylamine chain length, and a reversible intercalation and deintercalation property is observed among these intercalation compounds. The IR spectra of the intercalation compounds with n-alkylamines clearly show that n-alkyl chains possess an all-trans conformation, and H₂W₂O₇ accommodate n-alkylamines (C_nH_2n+1NH₂: n=3, 4, 7, 8, 12, 16) to form intercalation compounds via an acid-base mechanism. A linear relationship between the interlayer distance and the number of carbon atoms in n-alkyl chains is observed to show a bilayer arrangement of the n-alkyl chains with a tilt angle of ∼71.6°. Elemental analysis studies reveal that the amounts of intercalated n-alkylamines are about 2.0 mol per [W₂O₇]. Despite the surface geometry of H₂W₂O₇ is almost identical to those of layered perovskites H₂[A_n−1B_nO_3n+1], the amounts of intercalated n-alkylamines of them are different. A reasonable explanation is given through our research.