Structure and properties of rhombohedral CePd3Ga8: A variant of the cubic parent compound with BaHg11 structure type

Single crystals of a new intermetallic gallide, R-CePd₃Ga₈, have been synthesized from excess molten gallium. Single-crystal X-ray diffraction reveals that R-CePd₃Ga₈ crystallizes in the R-3m space group with a=b=c=8.4903(10) Å and α=β=γ=89.993(17). R-CePd₃Ga₈ is a variant of the cubic BaHg₁₁ structure type with three structural units: a Ce-centered polyhedron, a distorted cube of Pd₂Ga₆ and a Pd-centered cuboctahedron. The distortions of these units are compared to undistorted analogous units in intermetallic compounds with BaHg₁₁ structure type. Field and temperature-dependent magnetization measurements on R-CePd₃Ga₈ reveal a paramagnetic material with strong antiferromagnetic correlations and a magnetization consistent with Ce³⁺. Electrical resistance measurements indicate Kondo behavior between localized Ce³⁺ magnetic moments.     

CrxRe1−xO2 oxides with different rutile-like structures: changes in the electronic configuration and resulting physical properties

Mixed chromium-rhenium oxides, Cr_xRe_1−xO₂ with 0.31?x?0.66, have been synthesized for the first time by high-pressure high-temperature synthesis and in evacuated quartz tubes. The crystal structures of the compounds have been determined by single crystal and powder X-ray diffraction. Depending on synthesis conditions (pressure and temperature) these phases crystallize either in a tetragonal structure (P4₂/mnm) with statistical distribution of metal ions on one site (rutile-type), with cation ordering along c-axis (trirutile-type), or in a monoclinic rutile-like structure (C2/m) with ordering of Cr- and Re-cations and metallic Re-Re bonds. The “a” parameter of the tetragonal unit cell increases with increasing Re content whereas the “c” parameter decreases, indicating a strengthening of the Re-Re bond. The thermal stability of tetragonal Cr_xRe_1−xO₂ in Ar-atmosphere depends on the Re-content, decomposition is observed at 1241 K for x=0.34, but already at 966 K for x=0.5. The thermal expansion of Cr_xRe_1−xO₂ is anisotropic with a larger expansion coefficient in the “c” direction. Tetragonal Cr_xRe_1−xO₂ with 0.31?x<0.54 order antiferromagnetically at low temperatures with T_N depending on the Cr-content x.     

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.     

Crystal structure of the parent misfit-layered cobalt oxide [Sr2O2]qCoO2

The crystal structure of our newly discovered Sr-Co-O phase is investigated in detail through high-resolution electron microscopy (HREM) techniques. Electron diffraction (ED) measurement together with energy dispersive X-ray spectroscopy (EDS) analysis show that an ampoule-synthesized sample contains an unknown Sr-Co-O ternary phase with monoclinic symmetry and the cation ratio of Sr/Co=1. From HREM images a layered structure with a regular stacking of a CdI₂-type CoO₂ sheet and a rock-salt-type Sr₂O₂ double-layered block is observed, which confirms that the phase is the parent of the more complex “misfit-layered (ML)” cobalt oxides of [M_mA₂O_m+2]_qCoO₂ with the formula of [Sr₂O₂]_qCoO₂, i.e. m=0. It is revealed that the misfit parameter q is 0.5, i.e. the two sublattices of the CoO₂ sheet and the Sr₂O₂ block coexist to form a commensurate composite structure. We propose a structural model with monoclinic P2₁/m symmetry, which is supported by simulations of ED patterns and HREM images based on dynamical diffraction theory.     

O-H and O-D stretching vibrations in isotopically dilute HDO molecules in some solid hydrates

Vibrational frequencies v_OH and v_OD have been measured for isotopically-dilute HDO molecules in eleven solid hydrates at 90 K. The results have been used to prepare a plot of the ratio v_OH/v_OD versus v_OH. The ratios fall on a smooth curve and decrease with decreasing frequencies v_OH. The anharmonicity constants ω_ex_e have been estimated. They were found to increase with decreasing v_OH.     

A relaxation time study of the electron-beam induced polymerization of 1,6-hexanediol diacrylate

NMR measurements have been used to characterize the electron-beam (EB) induced polymerization of 1,6-hexanediol diacrylate. Hydrogen T₁, T_1θ, and T₂ values have been measured as a function of radiation dose. The T₂ signal consists of a Gaussian component and a longer component consisting of two exponentials. The sum of the Gaussian and the short exponential intensities is strongly correlated with the polymer fraction determined by gel extraction. T₂ values associated with the exponential component decrease rapidly with increasing dose. The T₁ relaxation consists of two exponentials for low dose and a single exponential for high dose. The T_1θ relaxation consists of from two to three exponential components, and the departure from a single exponential decreases with increasing dose. T₁ and T_1θ intensities are poorly correlated with gel extraction results. T₁ and T_1θ minima occur at intermediate radiation dose, and T_1θ depends on H₁ at high dose. Limited spin diffusion plays an important role in the T₁ and T_1θ relaxation. Hydroquinone has little effect on the EB-induced polymerization.     

Nickel deficiency in RENi2-xP2 (RE=La, Ce, Pr). Combined crystallographic and physical property studies

Large single crystals from RENi_2-xP₂ (RE=La, Ce, Pr) were synthesized from the pure elements using Sn as a metal flux, and their structures were established by X-ray crystallography. The title compounds were confirmed to crystallize in the body-centered tetragonal ThCr₂Si₂ structure type (space group I4/mmm (No. 139); Pearson's symbol tI10), but with a significant homogeneity range with respect to the transition metal. Systematic synthetic work, coupled with accurate structure refinements indicated strong correlation between the degree of Ni-deficiency and the reaction conditions. According to the temperature dependent dc magnetization measurements, LaNi_2-xP₂ (x=0.30(1)), as expected, is Pauli-like paramagnetic in the studied temperature regime, while the Ce-analog CeNi_2-xP₂ (x=0.28(1)) shows the characteristics of a mixed valent Ce³⁺/Ce⁴⁺ system with a possible Kondo temperature scale on the order of 1000 K. For three different PrNi_2-xP₂ (x?0.5) samples, the temperature and field dependence of the magnetization indicated typical local moment 4f-magnetism and a stable Pr³⁺ ground state, with subtle variations of T_C as a function of the concentration of Ni defects. Field-dependent heat capacity data for CeNi_2-xP₂ (x=0.28(1)) and PrNi_2-xP₂ (x=0.53(1)) are discussed as well.     

Thermodynamics of proton dissociations from aqueous l-proline: apparent molar volumes and apparent molar heat capacities of the protonated cationic,zwitterionic, and deprotonated anionic forms at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa

Apparent molar volumes V_φ and apparent molar heat capacities C_p,φ were determined for aqueous solutions of l-proline, l-proline with equimolal HCl, and l-proline with equimolal NaOH at the pressure p=0.35 MPa. Density measurements obtained with a vibrating-tube densimeter at temperatures (278.15⩽T/K⩽368.15) were used to calculate V_φ values, and heat capacity measurements obtained with a twin fixed-cell, differential-output, power-compensation, temperature-scanning calorimeter at temperatures (278.15⩽T/K⩽393.15) were used to calculate C_p,φ values. Speciation arising from equilibrium was accounted for using Young’s Rule, and semi-empirical equations describing (V_φ, m, T) and (C_p,φ, m, T) for each aqueous equilibrium species were fitted by regression to the experimental results. From these equations, the volume change Δ_rV_m and heat capacity change Δ_rC_p,m for the protonation and deprotonation reactions were calculated. Additionally, the Δ_rC_p,m expression was integrated symbolically to yield values of the reaction enthalpy change Δ_rH_m, reaction entropy change Δ_rS_m, and equilibrium molality reaction quotient Q for both reactions. The results provide a much-improved thermodynamic characterization of aqueous l-proline and of its protonation and deprotonation equilibria.     

The anisotropy of the intermolecular potential for H2—inert gas systems,determined from total collision cross section measurements with state selected molecules; potential models and their anisotropy parameters

Total collision cross section measurements with a state selected H₂ beam are discussed in terms of a V₂ (R) P₂ (cos?) angle dependent part of the intermolecular potential. It is shown that the V₂ (R) factor has a shallow minimum ?₂ of about 10% of the angle averaged well depth ?; its position R_e2 is displaced a few percent to larger distances, with respect to the position R_e of the angle averaged minimum.     

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.     

Disordered modifications of cobalt molybdate

The structures of recently discovered new high-temperature modifications of cobalt molybdate, a′-and a″-CoMoO₄, were determined. a′-and a″-CoMoO₄ appear after the phase a-CoMoO₄ is heated above the temperature range 700–1000°C. They seem to be the disordered modifications of a-CoMoO₄ with metal atoms distributed at random in an a-CoMoO₄ oxygen network.The F(hkl) values, calculated for variously disordered a-CoMoO₄ structure, were compared with the observed intensities of diffraction lines changing in the course of a → a′ and a → a″ transitions. It was concluded that a″-CoMoO₄₄ has a completely disordered structure with random distribution of both Co and Mo atoms in oxygen interatomic voids. The a′-CoMoO₄ is a partly disordered modification, with random distribution of some cations only.The temperature and the kind of order-disorder transition depend on the method of preparation of a-CoMoO₄ samples.The disordered modifications of cobalt molybdate may be supercooled—even to room temperature—before it transforms rapidly into low-temperature b-CoMoO₄ form.     

Vibrational spectra and vibrational states of simple gas-phase hydrogen-bonded dimers

The article aims to outline the growth of evidence and ideas about infrared band broadening for simple, gaseous, moderately strong, hydrogen bonded dimers B…HA, to draw attention to areas in need of further development and to collect together experimental information available at the present time about vibrational states associated with such dimers. The band associated with the modified HA stretching mode, ν_s, is observed for several dimers not only to be broad, but to have sub-band structure, which is satisfactorily interpreted as arising from combination bands of ν_s with the low frequency stretch-mode ν_σ, giving a progression ν_s ± nν_σ as a result of strong anharmonic coupling. For weaker dimers with lower values of ν_σ the sub-band structure is less evident, and may appear only as shoulders, while for still weaker dimers, the sub-bands may be merged into a featureless broad band. A major factor contributing to the breadth of the individual sub-bands in the band structure is the presence of hot-bands, especially a long series based on successive excited states of the low-frequency bending mode ν_β. A link is indicated with the interpretation of band-broadening for moderately strong hydrogen-bonded complexes in the liquid state. The anharmonic coupling of ν_s and ν_σ is again a central feature but there is a new factor, namely the coupling of ν_σ through a fluctuating potential with the surroundings, which has the result that the ν_s mode rapidly loses phase coherence resulting in a broad structureless band.     

A mass spectral study of some symmetric and unsymmetric sulfides,disulfanes, and trisulfanes

The low-resolution mass spectra of 13 trifluoromethyl and/or methyl sulfur-containing compounds (CF₃S_nCF₃, CF₃S_nCH₃, CH₃S_nCH₃, CF₃S_mCl, and CH₃S_mCl; n = 1, 2, 3; m = 1, 2) are reported and discussed. There is a considerable similarity between the fragmentation pattern of compounds with the same terminal groups. However, with a fluorinated and a non-fluorinated methyl group, the major fragmentation products produced are similar to those for the dimethyl compounds.     

Magnetic properties of La0.5−xLnxSr0.5MnO3 (Ln=Pr, Nd, Gd and Y)

Magnetic and electron transport properties of four series of manganates of the composition La_0.5−xLn_xSr_0.5MnO₃ (Ln=Pr, Nd, Gd and Y) have been investigated to examine how the ferromagnetic metallic nature of the parent La compound changes over to antiferromagnetic insulating behavior, with change in Ln and x due to the associated changes in the A-site cation radius as well as the size disorder. When Ln=Pr and Nd, there is a transition from the tetragonal I4/mcm structure to the orthorhombic Immm and Imma structures at x=0.2 and 0.35, respectively. There is a gradual evolution of the properties from those of La_0.5Sr_0.5MnO₃ to those of Pr_0.5Sr_0.5MnO₃ or Nd_0.5Sr_0.5MnO₃ with increase in x. Thus, when x>0.2 and >0.35, respectively, the Pr- and Nd-substituted manganates show ferromagnetic transitions followed by antiferromagnetic transitions at low temperatures, with the ferromagnetic T_C decreasing with increasing x. The Gd and Y series of compounds are all orthorhombic and show a decrease in T_C with the increase in x, the ferromagnetism disappearing at high x. At a value of x corresponding to the A-site cation radius of Pr_0.5Sr_0.5MnO₃, the Gd and Y series of compounds exhibit ferromagnetism in the 250-300 K region and undergo an antiferromagnetic transition on cooling. The T_C−T_N gap is sensitive to the disorder arising from the size mismatch.     

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.     

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.     

Electrocrystallization of PrO2 and TbO2−x from alkali hydroxide melts and characterization of the fluorite-related TbO2−x

Single crystals of PrO₂ and of the oxygen deficient fluorite related phase TbO_2−x have been obtained by anodic electrocrystallization from alkaline hydroxide melts containing PrCl₃ and TbCl₃, respectively. Magnetic measurements, X-ray diffraction data and TEM investigations confirm the identity of the products. PrO₂ crystallizes in a fluorite type of structure with a=5.3945(3) Å and shows paramagnetic behaviour with a magnetic moment as expected for Pr⁴⁺ (μ_eff=2.49μ_B, Θ=−109 K, T_N=10 K). According to precession photographs and an examination with an area sensitive X-ray detector, TbO_2−x exhibits a superstructure of fluorite with a pseudocubic subcell with a_f=5.2810(1) Å. This lattice constant is intermediate between those of TbO₂ and Tb₂O₃ (C-type), the same is true for its paramagnetic behaviour (μ_eff=8.58μ_B, Θ=−22 K, T_N=5 K). The supercell was found to be hexagonal with a=25.836(1) Å and c=36.672(2) Å, the symmetry of the intensity distribution being monoclinic. Electron diffraction revealed a cubic cell with a=10.6 Å, space group Ia-3, indicating reduction of the material in the electron beam.     

(Mn1−xPbx)Pb10+ySb12−yS26−yCl4+yO, a new oxy-chloro-sulfide with ∼2 nm-spaced (Mn,Pb)Cl4 single chains within a waffle-type crystal structure

The new oxy-chloro-sulfide (Mn_1−xPb_x)Pb_10+ySb_12−yS_26−yCl_4+yO (x ∈ [0.2-0.3]; y ∈ [0.3-1.6]) was synthesized by dry way at 500-600 °C. A single crystal ∼Mn_0.7Pb_11.0Sb_11.3S_25.3Cl_4.7O indicates a monoclinic symmetry, space group C2/m, with a = 37.480(8), b = 4.1178(8), c = 18.167(4) Å, β = 106.37(3)°, V = 2690.2(9) Å³, Z = 2. Its crystal structure was determined by X-ray single crystal diffraction, with a final R = 5.11%. Modular analysis of the crystal structure reveals a “waffle” architecture, where complex rods with lozenge section delimitate an internal channel filled by a single chain of (Mn_0.7Pb_0.3)Cl₆ octahedra connected by opposite edges. Minimal inter-chain distances are close to 18 Å. The rod wall, two-atom thick, presents, in alternation with S atoms, Pb or (Pb,Sb) cations with prismatic coordination in the internal atom layer, while the external atom layer is constituted exclusively by Sb cations with dissymmetric square pyramidal coordination. A (Pb,Sb)₂S₂ fragment connects two successive rods along (2 0 1) to form a waffle-type palissadic layer. The unique O position, half filled, presents the same environment than the isolated O positions in the oxy-sulfide Pb₁₄Sb₃₀S₅₄O₅, or oxy-chloro-sulfides Pb₁₈Sb₂₀S₄₆Cl₂O and (Cu,Ag)₂Pb₂₁Sb₂₃S₅₅ClO. This compound belongs to a pseudo-homologous series of chalcogenides with waffle structure, ordered according to the size of their lozenge shape channel. Such a complex senary compound of the oxy-chloro-sulfide type illustrates the structural competition between three cations, on one hand, and, on the other hand, three anions. This compound is of special interest regarding the 1D distribution of magnetic Mn²⁺ atoms at the ∼2 nm scale.     

Tetrachloroferrate (III) Salts of BDH-TTP [2,5-Bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene] and BDA-TTP [2,5-Bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene]: Crystal Structures and Physical Properties

Three FeCl₄ salts based on non-tetrathiafulvalene (TTF) donors, 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene (BDH-TTP) and 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP), have been prepared and characterized as κ-(BDH-TTP)₂FeCl₄, β-(BDA-TTP)₂FeCl₄, and (BDA-TTP)₃FeCl₄ · PhCl. The κ-(BDH-TTP)₂FeCl₄ salt, with a room-temperature conductivity (σ_rt) of 39 S cm⁻¹, is metallic down to 1.5 K, and its magnetic susceptibility obeys the Curie-Weiss law with a Curie constant (C) of 4.25 emu K mol⁻¹ and a Weiss constant (θ) of 0.041 K. β-(BDA-TTP)₂FeCl₄ exhibits metallic behavior (σ_rt=9.4 S cm⁻¹) with a sharp metal-to-insulator (MI) transition (T_MI=113 K) and antiferromagnetic ordering with the Néel temperature of near 8.5 K, whereas the solvated (BDA-TTP)₃FeCl₄ · PhCl salt is a semiconductor with a thermal activation energy of 0.11 eV (σ_rt=2.0× 10⁻² S cm⁻¹) and exhibits Curie-Weiss behavior (C=4.42 emu K mol⁻¹, θ=−0.35 K).     

Effect of cation size and size-disorder on the magnetic and electron transport properties of La0.7−xLnxSr0.3MnO3 (Ln=Gd and Dy)

Magnetic and electron transport properties of La_0.7−xLn_xSr_0.3MnO₃ with Ln=Gd and Dy have been investigated over a wide range of compositions to explore the effects of the radius of the A-site cation, 〈r_A〉, and the size-disorder as measured by σ². These manganates exhibit ferromagnetism and insulator–metal transitions up to a critical value of x (x_c), with the T_c (T_IM) values decreasing with 〈r_A〉. When x⩾x_c, however, the manganates cease to be ferromagnetic and instead they show an abrupt decrease in magnetization and increase in resistivity, suggesting the likely occurrence of electronic phase separation.