Quark-like structures in the atomic f shell

Instead of thinking of the 15 electronic configurations f^N (0N 14) as distinct entities, we can construct all 16 384 states of the f shell by coupling together four independent quarks and including two parity labels. Each quark is eight-dimensional, has the angular momentum structure s + f, and belongs to the simplest spinor representation of the Lie group SO(7). Two coupled quarks provide the spin-up states of either even N or odd N configurations; the other two perform a similar role in the spin-down space. Transformations among the eight components of a quark bring the group SO(8) into play. Just how the representations of this group collapse into representations of the three possible SO(7) subgroups and their common subgroup G₂ is illustrated by projecting the four-dimensional weight space on to the two-dimensional space of the printed page. The quarks make themselves felt by providing selection rules and unexpected proportionalities between matrix elements. Among various examples so far investigated is that provided by the relation between some matrix elements of the spin-spin type for the quintet states of f⁴ (with even L) and some crystal-field matrix elements for the singlets of f².     

Microstructure and thermoelectric properties of NaxCo2O4/Ag composite synthesized by the polymerized complex method

The synthesis of a thermoelectric Na_xCo₂O₄/Ag composite was attempted by the polymerized complex (PC) process using AgNO₃ as an Ag source and subsequent sintering at 1153 K for 72 ks. The effects of the PC process and Ag addition to Na_xCo₂O₄ on microstructure and thermoelectric properties of the Na_xCo₂O₄/Ag composite were investigated. Ag was hardly substituted for Na and Co sites, and the sintered sample was composed of the Na_xCo₂O₄ and Ag phases. The electrical resistivity of the composite was smaller than that of the Na_xCo₂O₄ single phase and the Seebeck coefficient was slightly enhanced by Ag addition, resulting in the significantly large power factor. However, most of precipitated Ag particles in the Na_xCo₂O₄ matrix were coarse, 5–8 μm in size, and the thermal conductivity of the composite was high as compared to the Na_xCo₂O₄ single phase. From these results, the dimensionless figure of merit of the composite was almost the same as that of the Na_xCo₂O₄ single phase.     

Nanocrystalline LaNi4−xMn0.75Al0.25Cox electrode materials prepared by mechanical alloying (0≤x≤1.0)

Polycrystalline hydrogen storage alloys based on lanthanum (La) are commercially used as negative electrode materials for the nickel–metal hydride (Ni–MH_x) batteries. In this paper, mechanical alloying (MA) was used to synthesize nanocrystalline LaNi_4−xMn_0.75Al_0.25Co_x (x=0, 0.25, 0.5, 0.75 and 1.0) hydrogen storage materials. XRD analysis showed that, after 30 h milling, the starting mixture of the elements decomposed into an amorphous phase. Following the annealing in high purity argon at 700 °C for 0.5 h, XRD confirmed the formation of the CaCu₅-type structures with a crystallite sizes of about 25 nm. The nanocrystalline materials were used as negative electrodes for a Ni–MH_x battery. Cobalt substituting nickel in LaNi₄Mn_0.75Al_0.25 greatly improved the discharge capacity and cycle life of the LaNi₅ material. For example, in the nanocrystalline LaNi_3.75Mn_0.75Al_0.25Co_0.25 powder, discharge capacities up to 258 mA h g⁻¹ (at 40 mA g⁻¹ discharge current) were measured. Mechanical alloying is a suitable procedure to obtain LaNi₅-type alloy powders for electrochemical energy storage.     

Ba4In2−x(CO3)1+xO6−2.5x and Ba4In0.8Cu1.6(CO3)0.6O6.2—new indium oxycarbonates closely related to n=3 Ruddlesden–Popper phases

Investigations of phase relations in the Ba-rich part of the In₂O₃–BaO(CO₂)–CuO pseudo-ternary system at 900 °C have revealed the existence of new indium–copper oxycarbonate – Ba₄In_0.8Cu_1.6(CO₃)_0.6O_6.2. Rietveld refinement of the X-ray powder diffraction data combined with infrared studies gives evidence that this phase is a oxycarbonate crystallising in the tetragonal structure (space group I4/mmm) with unit cell parameters: a=4.0349(1) Å and c=29.8408(15) Å. In the binary part of the In₂O₃–BaO(CO₂) system we have identified the occurrence of Ba₄In_2−x(CO₃)_1+xO_6−2.5x oxycarbonate solid solution showing a crystal structure also described by I4/mmm space group, but with the unit cell parameters: a=4.1669(1) Å and c=29.3841(11) Å for x=1. The existence range of this phase, −0.153<x<0.4, includes chemical compositions of earlier found phases: Ba₅In_2+xO_8+0.5x with 0≤x≤0.45 (known as the -solid solution), as well as the binary Ba₄In₂O₇ phase. The crystal structures of both new oxycarbonates are isomorphic and related to n=3 member of the Ruddlesden–Popper family.     

Tailoring of misfit along interfaces between ZnxMn3−xO4 and Ag

This work is aimed at examining how the tetragonality of Zn_xMn_3−xO₄ spinel structures depends on the chemical composition when Zn_xMn_3−xO₄ is embedded in a metal matrix. The paper focuses on a wide range of Zn_xMn_3−xO₄ precipitates in a Ag matrix with x varying between 0 and 1.5. This variation of x has been obtained by internal oxidation of Ag–2at.%Mn–4at.%Zn in air followed by annealing in vacuo at different temperatures. It will be demonstrated that the Zn concentration x in Zn_xMn_3−xO₄ has a major influence on the interfacial misfit and orientation relation between Ag/Zn_xMn_3−xO₄. The degree of mismatch of 10.4% of 1 1 1 Ag–Mn₃O₄ and 2.4% of Ag–Zn_1.5Mn_1.5O₄ was visualized using the Bragg filtering technique on HRTEM micrographs of those interfaces. It was possible to identify misfit dislocations qualitatively with this technique at 1 1 1 Ag–Zn_xMn_3−xO₄ interfaces with different degree of mismatch.     

Luminescence based on energy transfer in xerogels doped with Ln2−xTbx(WO4)3

This paper presents preparation, optical absorption and photoluminescence properties of luminescent materials consisting of Ln_2−xTb_x(WO₄)₃ [where Ln = Gd(III) or La(III)] incorporated into silica xerogel. Photoluminescence behaviour of the salt in the rigid matrix was studied by the luminescence spectroscopy. The excitation spectra of the system Ln_2−xTb_x(WO₄)₃ show an intense broad band with a maximum placed at about 240 nm. This band is attributed to ligand–metal charge transfer (LMCT) inside the tungstate group. On the other hand, Tb³⁺ ion exhibits its characteristic emission in the material. Owing to energy transfer from the excited tungstate groups to the Tb³⁺ ions the emission intensity is improved. The energy transfer from WO₄²⁻ group to Tb(III) ion is particularly effective for such dopants as Gd_0.4Tb_1.6(WO₄)₃ or La_0.8Tb_1.2(WO₄)₃ incorporated into SiO₂ xerogel. Concentration of the emission quenchers such as water molecules and OH groups was reduced by thermal treatment. The high emission intensity and easy preparation of these systems make them potential candidates for application as luminescent materials.     

Synthesis and characterization of proton conducting Sr(Ce1−xZrx)0.95Yb0.05O3−δ by the citrate method

The citrate method was used to synthesize Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ (x = 0.1, 0.2, 0.3, 0.4) and to avoid the drawbacks of the conventional solid state reaction method. The products were characterized by thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe X-ray microanalyzer (EPMA). The results indicate that the citrate method is an advantageous route in producing Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ materials. Sr(Ce_0.9Zr_0.1)_0.95Yb_0.05O_3−δ powders are composed of nanoscaled crystallites with the average grain size in the range of 60–70 nm. Single phase is confirmed over the whole x range. In addition, chemical stability against CO₂ and electrical conduction behavior of the sintered Sr(Ce_1−xZr_x)_0.95Yb_0.05O_3−δ ceramics were investigated. The chemical stability of the ceramics against CO₂ is certified to increase with the increase in zirconium content. Impedance spectroscopy was used to study the electrical conduction behavior of Sr(Ce_0.9Zr_0.1)_0.95Yb_0.05O_3−δ ceramic.     

Study of extraction of the ndcl3-smcl3-hcl-h2o-di-(2-ethylhexyl)-phosphoric-acid-kerosine system

The extraction equilibrium relations of neodymium and samarium were studied in the system HCl (1.0 mol dm⁻³)-di-(2-ethylhexyl) phosphorie acid (HDEHP), for a wide range of neodymium and samarium concentration (0.02-1.20 mol dm⁻³), mole fraction (X_i=0.005-0.995) and acidity (C_H+=0.27-1.06 mol dm⁻³). A relatively simple, high precision model for the distribution ratio D_i was established by correlating the experimental data with a progressive regression program D_i=exp(a₁+a₂I+a₃C_H++a₄X_i)I^a5X_i^{a6 In C_H+ + a7 In(1-X_i})

The extraction behaviour of the neodymium-samarium binary system was studied using the above model. The study of acid equilibrium shows that the extraction proceeds according to the cation exchange reaction within this range of experimental parameters. The neodymium and samarium concentrations in the organic phase increase rapidly and reach a maximum, and then decrease as the aqueous concentration increases; the maximum concentration appears at about 0.3-0.4 mol dm⁻³ of aqueous neodymium and samarium concentration. The variation of separation factors, β_Sm/Nd, was also studied with the change in aqueous acidity, rare earth concentration and mole fraction, β_Sm/Nd vary between 6.5 and 10.2 and the average value is 8.77.     

Temporal evolution of the nanostructure of Al(Sc,Zr) alloys: Part I – Chemical compositions of Al3(Sc1−xZrx) precipitates

Atom-probe tomography (APT) and high-resolution transmission electron microscopy are used to study the chemical composition and nanostructural temporal evolution of Al₃(Sc_1−xZr_x) precipitates in an Al–0.09 Sc–0.047 Zr at.% alloy aged at 300 °C. Concentration profiles, via APT, reveal that Sc and Zr partition to Al₃(Sc_1−xZr_x) precipitates and Zr segregates concomitantly to the -Al/Al₃(Sc_1−xZr_x) interface. The Zr concentration in the precipitates increases with increasing aging time, reaching a maximum value of 1.5 at.% at 576 h. The relative Gibbsian interfacial excess of Zr, with respect to Al and Sc, reaches a maximum value of 1.24 ± 0.62 atoms nm⁻² after 2412 h. The temporal evolution of Al₃(Sc_1−xZr_x) precipitates is determined by measuring the time dependence of the depletion of the matrix supersaturation of Sc and Zr. The time dependency of the supersaturation of Zr does not follow the asymptotic t^−1/3 law while that of Sc does, indicating that a quasi-stationary state is not achieved for both Sc and Zr.     

Atomistic study on the site preference and thermodynamic properties for Cr23−xFexC6

The site preference of Fe in Cr_23−xFe_xC₆ is investigated based on the interatomic potentials obtained by the lattice inversion method. The calculated results show that Fe atoms preferentially substitute for Cr at 4a sites first and then 8c sites. The structural parameters of Cr_23−xFe_xC₆ with content x are calculated and the results are consistent with experimental results. The calculated cohesive energies indicate that the increase in x value is accompanied by the decrease in the stability of Cr_23−xFe_xC₆. The thermodynamic properties of Cr₂₃C₆, such as the phonon density of states and vibrational entropy, as well as the bulk modulus of Cr_23−xFe_xC₆ are evaluated. The calculated results are in good agreement with experimental results. This work provides a simple and promising method for studying the properties of carbides with complex structures.     

Study of the antiferromagnetic phase in Sm(Mn1−xCrx)2Ge2

Magnetic and thermal expansion measurements have been carried out on the polycrystalline Sm(Mn_1−xCr_x)₂Ge₂ samples to see how the antiferromagnetie (AFMII) region in SmMn₂Ge₂ is affected by Cr substitution. It is found that the antiferromagnetic region disappears for samples with less than 2 at.% of Cr. Sharp changes in the thermal expansivity (Δl/l) at FMI–AFMII and AFMII–FMII transitions are observed, indicating first order transitions. The decrease in relative thermal expansivity at the two transitions with the increase of Cr concentration is related to the decrease in the stability and the temperature-range of the AFMII phase observed in magnetization measurements. A spin reorientation transition (T_SR) has been observed for x=0, at 148 K. It is found that the T_SR increases with the increase of Cr concentration. A magnetic phase diagram as a function of Cr concentration in Sm(Mn_1−xCr_x)₂Ge₂ has been constructed.     

Effects of noble metal addition on microstructure and thermoelectric properties of NaxCo2O4

The synthesis of Na_xCo₂O₄/Ag and Na_xCo₂O₄/Au composites was tried by mechanical milling and subsequent sintering. Ag and Au particles were added to the Na_xCo₂O₄ powder prior to the mechanical milling. The microstructure and thermoelectric properties of the Na_xCo₂O₄/Au composite were compared to those of the Na_xCo₂O₄/Ag composite and the Na_xCo₂O₄ single phase, and the effects of the Ag and Au addition on the thermoelectric performance of Na_xCo₂O₄ were discussed. Au particles around 2 μm or smaller in size, which were significantly smaller than Ag particles around 10 μm in size, were dispersed in the Na_xCo₂O₄ matrix. The Seebeck coefficient and the electrical resistivity of Na_xCo₂O₄ were slightly enhanced and significantly reduced by these noble metals addition, resulting in the large power factor of these composites. On the other hand, the Na_xCo₂O₄/Au composite showed the electrical resistivity larger than that of the Na_xCo₂O₄/Ag composite. Ag and Au addition markedly increased the thermal conductivity, and the dimensionless figure of merit of Na_xCo₂O₄ could not be improved by these noble metals addition.     

Comparative analyses of Nd (4f) energy level structures in various crystalline hosts

We have performed an in-depth analysis of the energy-level structures in seven Nd³⁺ (4f³) crystal systems: [Nd(H₂O)₉]·3CF₃SO₃, Nd³⁺: Cs₂NaGdCl₈, four Nd³⁺-doped garnets (Nd³⁺: A₃B₅O₁₂), and Nd³⁺: CsCdBr₃. A model Hamiltonian employing 20 free-ion operators and the appropriate one-electron crystal-field interaction operators (plus selected two-particle correlation crystal-field (CFF) operators) was diagonalized within the full 364 SLJM, basis of the f³ electronic configuration. Ample spectroscopic experimental data allowed us to use least-squares fitting routines to produce a crystal-field energy-level structure for Nd³⁺ in each host. Particular attention is given in this report to some differences in the values of the free-ion parameters among the seven hosts and to the strong effects of three CCF operators on crystal-field level ordering in specific multiplets. The effects of CCF operators (originally proposed by Judd and later studies by Reid) are shown to be a small perturbation on the standard one-particle (B_q^k) formalism.     

Effect of Nd doping on the grain-matrix contribution to ac losses in sintered Bi1.6Pb0.4Sr2(Ca1−xNdx)2Cu3Oδ superconductors

We have carried out measurements of complex ac susceptibility χ=χ′+χ″ as a function of temperature and ac field amplitude on rectangular bar-shaped high-temperature superconductors (HTS) with nominal composition of Bi_1.6Pb_0.4Sr₂(Ca_1−xNd_x)₂Cu₃O_δ superconducting samples prepared by the solid-state reaction method. The effect of Nd-substitution on the Bi–(Pb)–Sr–Ca–Cu–O system has been investigated in terms of ac susceptibility study. We estimated the effective volume fraction of the grains and the field dependence of the inter-granular critical current density comparing the maximum of the extracted matrix susceptibility and the corresponding calculated data which was obtained employing the power law critical state model.     

Preparation and structural characterization of perovskite-type LaxLn1−x″CoO3 by the thermal decomposition of heteronuclear complexes, LaxLn1−x″| Co(CN)6| · nH2O (Ln″ = Sm and Ho)

Perovskite-type La_xLn_1−x″CoO₃ oxides are prepared by the thermal decomposition of La_xLn_1−x″ [Co(CN)₆] · nH₂O hetero-nuclear complexes. Except for LaCoO₃ (hexagonal), the structures observed for La_xSm_1−xCoO₃ are othorhombic. While the perovskite-type oxide HoCoO₃ is not formed by decomposition at 1000°C of the corresponding hexacyano complex, the partial replacing of Ho with La is effective in forming the pervoskite-type oxide having an orthorhombic structure containing Ho even at 800°C. A monotonous correlation (quasi-linear relationship) was found between the b- and c-lattice constants of the orthorhombic structures of the perovskite-type oxides and the effective radii of Ln ions, defined as r_eff = xr_1.a + (1 − x)r_1.0″. The distortion parameter for the orthorhombie cell (3″a/b−1) increaseswith decrease in r_eff and is expected to be 0.270 for perovskite-type HoCoO₃. The crystal structure of the La_xLn_1−x″, CoO₃ oxides is mainly controlled by the effective radii of Ln ions.     

Bulk and lattice thermal expansion in Ce1−xSrxO2−x (0.0≤x≤0.10)

In this communication, we report on the bulk and lattice thermal expansion studies on a number of compounds, within the homogeneity range of solid solutions, in a series with the general composition Ce_1−xSr_xO_2−x (0.0≤x≤0.10). The XRD pattern of each product was refined to determine the solid solubility of SrO into the lattice of CeO₂, and the homogeneity range. The composition with maximum solid solubility limit of SrO in CeO₂ lattice, under the slow cooled conditions, was delineated as Ce_0.91Sr_0.09O_1.91 (i.e. 9 mol.% of SrO). The bulk thermal expansion measurements from ambient to 1123 K, as investigated by a dilatometer, revealed that the _l (293 to 1123 K) values for the compositions within the homogeneity range increase from 11.58×10⁻⁶ to 12.13×10⁻⁶ K⁻¹ on increasing the Sr²⁺ content from 0 mol.% (i.e. CeO₂) to 9 mol.%, i.e. the upper solubility limit of SrO into the lattice of CeO₂. A similar trend was observed in the lattice thermal expansion coefficients _a (293 to 1473 K) as obtained by a high temperature-XRD.     

An anode for solid oxide fuel cells: NiO+(Ce0.9Ca0.1O1.9)0.25(YSZ)0.75 solid solution

A (CCO)_0.25(YSZ)_0.75 solid solution (YSZ is Y₂O₃-stabilized ZrO₂; CCO is an abbreviation of Ce_0.9Ca_0.1O_1.9) was successfully synthesized by a solid-state method. A composite anode material, NYC_x (60 wt% NiO+40 wt% (CCO)_x(YSZ)_1−x), for SOFCs (solid oxide fuel cells) was prepared. XRD results suggest that a solid solution reaction occurs between YSZ and CCO after sintering at 1400 °C for 10 h. From the impedance results, the conclusion can be drawn that the (CCO)_0.25(YSZ)_0.75 solid solution is a mixed conductor (ionic and electronic). Its total conductivity is greater than that of YSZ. The NYC_x (x>0) anodes exhibited better performance than the commonly used Ni+YSZ anode with regards to overpotential and anode interfacial impedance, with the NYC_0.25 anode exhibiting the best performance     

New clues for understanding the magnetic behavior of UPt3

We present results of a fairly complete study on which dopants M result in single-phase U_1−xM_xPt₃ samples. The resulting new samples. M = Y, Lu, Sc, Hf, Zr, have been characterized by low temperature magnetic susceptibility and specific heat for x 0.07. Based on a previously proposed model, the c/a ratios of the present alloys would be expected to result in magnetic behavior as observed in U_1−xTh_xPt₃ and U(Pt_1−xPd_x)₃. On the contrary, no magnetism is found; the Sc-doped samples actually show a suppression of spin fluctuations. A new proposal is made to explain which properties are critical for the magnetic behavior of UPt₃, as well as its doped derivatives.     

Stability of AB5-type hydrogen storage alloys

In this paper, we deduced an empirical equation for predicting the stabilities of the hydrides of AB₅-type compounds with the consideration of the effects of geometrical factor and electric factor. The theoretical curves derived from this equation are in good agreement with the test results of La_1−xCa_xNi₅, Ml_1−xCa_xNi₅ and La_1−xCe_xNi₅ compounds. With this model, we can easily interpret the phenomenon that the initial partial substitution of R by Ca in R_1−xCa_xNi₅ (R is rare earth metal) compounds cause an increase of hydrogen desorption pressure to a maximum value, whereas in the range of larger Ca content x the hydrogen desorption pressure decreases with increasing x.     

Constitution, structural chemistry and magnetism in the ternary system Ce–Ag–Si

Phase relations were established for the Ce–Ag–Si system at 850°C by means of X-ray diffraction, light optical microscopy and quantitative electron probe microanalysis. Phase equilibria are characterised by the existence of extended solid solutions starting from the binaries: Ce(Ag_xSi_1−x)_2−y (ThSi₂-type), Ce(Ag_1−xSi_x)_1−y (unknown structure type) and Ce(Ag_1−xSi_x)_2−y (unknown structure type). Three ternary phases were found to exist, CeAg₂Si₂ (ThCr₂Si₂-type), Ce(Ag_xSi_1−x)_2−y (AlB₂-type) and the new ternary compound CeAgSi₂ with unknown structure type. Magnetic behaviour was studied from magnetic susceptibility and magnetisation measurements down to 1.7 K and employing magnetic fields up to 5 T. Soft ferromagnetism is observed for CeAg_xSi_2−x (AlB₂-type) below 5 K. Alloys Ce(Ag_xSi_1−x)_2−y with 0.08<x_Ag<0.30 (ThSi₂-type) encounter ferromagnetic order below 7 K. For x_Ag=0.31 the ferromagnetic interaction changes to antiferromagnetism with T_N=5.7 K. For CeAgSi₂ ferrimagnetic or canted antiferromagnetic order is indicated below 7 K.