Laser ablation synthesis of actinide selenide, oxide and oxide–selenide clusters: AnSen, AnxOm and AnxOmSen [An=U, Np, Pu]

Actinide (An=U, Np, Pu) selenide, oxide and oxide–selenide molecular and cluster cations were synthesized by pulsed laser ablation of targets consisting of dilute mixtures of AnO₂ in a selenium matrix; selected lanthanides (Ln=Ho, Tm, Lu) were included for comparison. The compositions and abundance distributions of the molecules/clusters reflected distinctive f-element chemistries. Ablation of pure selenium resulted in Se_n⁺ clusters with n=2–9; the abundance distribution is compared with the high-temperature Se_n equilibrium vapor composition. Several types of molecular and cluster ions were synthesized during the ablation process. Binary metal selenides, MSe_n⁺ (n>1), presumably formed primarily via electrostatic attraction between co-ablated Se_n and M⁺. The compositions of binary metal oxide clusters, M_xO_m⁺ (x≤5) reflected the distinctive oxidation chemistries of the constituent metals. For the Ln_xO_m⁺, Lu^III was the highest oxidation state, whereas for An_xO_m⁺, oxidation states up to U^VI were exhibited. The formation of binary selenides and oxide clusters is attributed to the high volatility of selenium, which results in a dense ablation plume in which coalescence of MO_y, MO_y⁺, Se_n and Se_n⁺ is promoted. Among the most abundant clusters were AnO_mSe_n⁺, the compositions of most of which suggested aggregation of AnO_m⁺ with Se_n. The compositions of some AnO_mSe_n⁺ indicated Se²⁻ ions in analogy with O²⁻; thus, AnSe⁺ and AnOSe⁺ were produced along with AnO⁺ and AnO₂⁺, whereas both AnO₂Se⁺ and AnO₃⁺ were absent. The compositions of other AnO_mSe_n⁺, such as PuO₃Se⁺, indicated O–Se bonding, such as occurs in selenites. The feasibility of synthesis of metal oxide cluster ions by co-ablation of oxides and poly(acrylic acid) was assessed for UO₂ and Lu₂O₃. Oxide clusters were produced in appreciable quantities, but interpretation of the results was complicated by formation of complex ions from reactions with polymer fragments. Ablation of OH radicals resulted in species comprising M-(OH) as well as M=O moieties, the masses of which were often too similar to allow for definitive differentiation.     

EXAFS and Te spectroscopy study of the Yb1−xEuxTe, Pb1−xEuxTe and Yb1−xGdxTe solid solutions

We report EXAFS (Extended X-Ray Absorption Fine Structure) measurements on three solid solutions of rare-earth monotelluride in order to measure the x-dependence of the cation-anion distance and Debye-Waller factor. We find Yb_1−xEu_xTe is an ideal solid solution with a bimodal distribution of distances, accommodated by the Te sublattice, at contrast with the two other solid solutions Pb_1−xEu_xTe and Yb_1−xGd_xTe. The anomalous behaviour of the EXAFS parameters in the last system is related to the shallow nature of the Gd donor.     

Magnetic phase diagrams of the TbRh2−xPdxPdxSi2 and TbRu2−xPdxSi2 systems

The a.c. susceptibility and high field magnetization on TbRh_2−xPd_xPd_xSi₂ and TbRu_2−xPd_xSi₂ compounds were investigated up to 140 kOe. The (T,x) magnetic phase diagrams were determined. For both systems, an increase in the Pd content causes a decrease in the Néel temperature and changes the magnetization curves.     

The role of valence electron concentration in the cohesive properties of YBxN1−x, YCxN1−x and YNxO1−x compounds

The mechanical properties are not yet understood at basic levels. Previous works shows that the greatest hardness for rock-salt structures (such as TiC_xN_1−x) is attained for a valence electron concentration (VEC) of 4.2 electrons per atom. The present work is aimed to explore this concept for yttrium-based compounds. By means of first principles calculations we did a systematical investigation where nitrogen in YN (VEC = 4) was supplanted by either of B, C or O to reduce or increase its VEC, forming YB_xN_1−x, YC_xN_1−x and YN_1−xO_x ternary compounds. We have calculated the cohesive energy (E_O), cell volume (V_O), bulk modulus (B_O) and density of states (DoS) as a function of VEC. The Fermi level (E_f,) is shifted toward the valence band by substituting B or C in YN, and toward the conduction band by means of O. It is concluded that the optimal position for E_f (maximum B_O) is linked to the saturation of electronic states with e_g-symmetry. At this point the excess of electrons provided by O starts filling antibonding states with t_2g-symmetry. That is, B_O increases monotonically as a function of VEC until VEC  4.1, after that point B_O decrease.     

Fatigue behavior of duplex treated TiCxN1−x- and Ti1−xAlxN-hard coating steel compounds

Recently, modified surface treatment methods for cutting tools were developed combining a heat treatment and a subsequent hard coating (duplex treatment). As a consequence, the wear resistance has been improved considerably. As fatigue is an important failure mechanism during application, the present paper focuses on the improvement of the fatigue behavior by an optimization of heat treatment and hard coating. The problem is approached in two steps: first, the fatigue behavior of high-speed tool steel S6-5-2 is investigated, surface-treated with different plasma heat treatments (plasma nitriding and -carbonitriding). The results on bending fatigue testing indicate the superiority of plasma nitriding compared with both plasma carbonitriding and untreated substrate. Hence, in a second step the plasma nitrided tool steel is coated with PACVD hard coatings of the system Ti-Al-C-N and the fatigue behavior of these hard coating steel compounds is investigated. Obviously, the type and stoichiometry of the hard coating influence the fatigue behavior of the compound. For TiC_xN_1−x films, the fatigue strength increases with increasing carbon content, whereas for Ti_1−xAl_xN-films, the fatigue strength decreases with increasing Al content. While for an optimized hard coating the fatigue behavior of the compound is superior to hardened non-nitrided steel, no significant improvement was found as compared with plasma nitrided steel.     

The dielectric function of Mgy NiHx thin films ()

Mg_y Ni (2≤y≤10) thin films covered with a Pd cap layer are hydrogenated in 10⁵  Pa H₂ between room temperature and 80 ° C and their dielectric function is determined from reflection and transmission measurements. The hydrogenated Mg_y NiH_x thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielectric functions with predictions from an effective medium theory show that a considerable doping of the Mg₂ NiH₄ host takes place at least for y≤3.5 while no signature of MgH₂ is observed in that composition range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixture of semiconducting Mg₂ NiH₄ (energy gap E_g=1.6  eV) and MgH₂ (E_g=5.6  eV) is expected.     

Electronic properties of CePdxRh1−xIn

As part of a systematic research programme on equiatomic ternary compounds of the type RTX (R is a rare earth, T a transition metal and X a metal out of the p block of the periodic table) crystallizing in the ZrNiAl structure, results for CePd_xRh_1−xIn (x = 0, 0.2, 0.4, 0.6, 0.8, 0.85, 0.9, 0.95, 1) are presented. The magnetic properties, specific heat and electrical resistivity as a function of magnetic field and temperature are reported. The development of the electronic properties from CeRhIn with unstable 4f moments towards CePdIn with localized 4f moments is discussed in terms of the influence on the valence state of Ce of the increase in the d-electron concentration due to gradual substitution of Rh in CeRhIn by Pd. The almost trivalent state is found to be most favourable for the formation of the heavy-fermion state.     

XPS, TEM and SAD investigations of nanosized CoxByHz particles obtained by two different borohydride methods

The nanosized Co, B, H, particles synthesized by the ‘tea’ and ‘antigravity’ methods using a borohydride reduction process have been subjected to structure and composition studies by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected area diffraction (SAD). The amounts of the elements Co, B, O₂, H₂ and C as mean volume values, and surface values for the as-prepared particles, as well as after Ar⁺ etching to a depth of about 15 nm and 30 nm from the initial particle surface, are determined. About 1.5 atoms of cobalt per atom of boron correspond to samples obtained by the ‘antigravity’ method. The binding energy (BE) of 1s electrons of boron atoms has only one value. These particles are angular and are in the typical nanocrystalline state. In the case of samples prepared by the ‘tea’ method, two atoms of cobalt per atom of boron are found. The presence of two kinds of BE (B¹ and B¹¹) of 1s electrons of boron atoms in the particles obtained by the ‘tea’ method is observed and almost equal amounts of these two states are established in the spectrum. The particles' shape and structure are typical of the amorphous state. The fact that there is one peak when the ‘antigravity’ method is applied, in contrast to the two peaks with the ‘tea’ method indicates the presence of a metal amorphous state in the latter case.     

Physical properties of Mo6−xRuxTe8 and Mo6Te8−xSx

A series of the Chevrel phases, Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x (x=0, 1, 2), has been prepared and the various physical properties, such as the elastic modulus, Debye temperature, and electrical resistivity, have been evaluated. The relationships between several properties of the compounds have also been studied. Young’s modulus and Debye temperature of Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x increase with increasing x value. The relationship between the Vickers hardness and Young’s modulus shows ceramic characteristics for Mo_6−xRu_xTe₈, while they show glass-like characteristics for Mo₆Te_8−xS_x. The electrical resistivities of Mo_6−xRu_xTe₈ and Mo₆Te_8−xS_x increase with increasing x value.     

Power factor of La1−xSrxFeO3 and LaFe1−yNiyO3

The electrical conductivity (σ), Seebeck coefficient (S), and power factor (σS²) of perovskite-type LaFeO₃, La_1−xSr_xFeO₃ [0.1 ≤ x ≤ 0.4] and LaFe_1−yNi_yO₃ [0.1 ≤ y ≤ 0.6] were investigated in the temperature range of 300–1100 K to explore their possibility as thermoelectric materials. The electrical conductivity of LaFeO₃ showed semiconducting behavior, and its Seebeck coefficient changed from positive to negative around 650 K with increasing temperature. The electrical conductivity of LaFeO₃ increased with the substitutions of Sr and Ni atoms, while its Seebeck coefficient decreased. The Seebeck coefficient of La_1−xSr_xFeO₃ was positive, whereas that of LaFe_1−yNi_yO₃ changed from positive to negative with increasing Ni content. The substitutions of Sr and Ni were effective in increasing the power factor of LaFeO₃; 0.0053 × 10⁻⁴ Wm⁻¹ K⁻² for LaFeO₃ (1050 K), 1.1 × 10⁻⁴ Wm⁻¹ K⁻² for La_1−xSr_xFeO₃ (x = 0.1 at 1100 K) and 0.63 × 10⁻⁴ Wm⁻¹ K⁻² for LaFe_1−yNi_yO₃ (y = 0.1 at 1100 K).     

Thermoelectric performance of Mg2−xCaxSi compounds

Thermoelectric materials Mg_2−xCa_xSi (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1) compounds have been prepared by vacuum melting followed by hot-pressing. Effects of the substitution of Ca for Mg on phase structures and the thermoelectric properties of the hot-pressed compounds were investigated. It was found that the alloying of Ca in Mg₂Si based compounds increases the electrical conductivity and decreases the Seebeck coefficient of the compounds, due to the electronegativity difference between Ca and Mg. The dimensionless figures of merit of Mg₂Si and Mg_1.99Ca_0.01Si reach, respectively, 0.41 and 0.34 at 660 K.     

Thermoelectric properties of Sn1−x−yTiySbxO2 ceramics

Thermoelectric properties of Sn_1−x−yTi_y Sb_xO₂ ceramics were investigated in detail. The addition of Sb into SnO₂ matrix increased the electric conductivity, σ. The increase in the σ value should be caused by the increase in the carrier concentration. The Seebeck coefficients of all the samples were negative, which means that these samples have n-type conduction. The samples of this study have porous structure. The maximum Z value of all the samples measured in this study was 2.4 × 10⁻⁵ K⁻¹.     

Preparation and thermal conductivities of filled tetrahedral LixIn1−xZnxSb and the related compounds

Li-filling in tetrahedral InSb and related compounds was attempted to investigate its effect on their thermal conductivities. Li-filled Li_0.2In_0.8Zn_0.2Sb, Li_0.4In_0.6Zn_0.4Sb, LiZnSb, and Li_0.16Ga_0.84Zn_0.16Sb sintered samples were prepared by powder metallurgy. The filled samples had much lower room temperature lattice thermal conductivities than those of the corresponding unfilled materials; the values of the Li_0.4In_0.6Zn_0.4Sb, LiZnSb, and Li_0.16Ga_0.84Zn_0.16Sb were 23, 45, and 72 mW cm⁻¹K⁻¹, respectively. Filled tetrahedral compounds such as Li_xIn_1−xZn_xSb might be good thermoelectric materials.     

Existence, structure and valence properties of the skutterudites CeyFe4−xCoxSb12

We report on sample preparation, annealing effects, electron microprobe analysis in the series Ce_yFe_4−xCo_xSb₁₂ which shows that a phase separation occurs for substituted samples (0<x<4) annealed at 650 and 550 °C. Single phase samples are obtained for either Ce_yFe₄Sb₁₂ or Ce_yCo₄Sb₁₂ samples annealed at 650 °C and for all compositions when annealed at 700 °C. The valence state of Ce in homogeneous samples has been studied using X-ray absorption spectroscopy (XAS). Ce ions are trivalent throughout the series and the XAS spectra does not show effect of the crystal field on the 5d-final state.     

Thermoelectric properties of novel skutterudites with didymium: DDy(Fe1−xCox)4Sb12 and DDy(Fe1−xNix)4Sb12

In order to improve the thermoelectric properties via efficient phonon scattering Didymium (DD), a mixture of Pr and Nd, was used as a new filler in ternary skutterudites (Fe_1−xCo_x)₄Sb₁₂ and (Fe_1−xNi_x)₄Sb₁₂. DD-filling levels have been determined from combined data of X-ray powder diffraction and electron microprobe analyses (EMPA). Thermoelectric properties have been characterized by measurements of electrical resistivity, thermopower and thermal conductivity in the temperature range from 4.3 to 800 K. The effect of nanostructuring in DD_0.4Fe₂Co₂Sb₁₂ was elucidated from a comparison of both micro-powder (ground in a WC-mortar, 10 μm) and nano-powder (ball-milled, 150 nm), both hot pressed under identical conditions. The figure of merit ZT depends on the Fe/Co and Ni/Co-contents, respectively, reaching ZT > 1. At low temperatures the nanostructured material exhibits a higher thermoelectric figure of merit. The Vickers hardness was measured for all samples being higher for the nanostructured material.     

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd2−xYxFe17−yGay intermetallic compounds

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd_2−xY_xFe_17−yGa_y compounds with x = 0, 0.5, 1.0, 1.5 and y = 0, 1, 2, 3 have been investigated using X-ray and neutron powder diffractions. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure with only small amounts of alpha iron. It is found that the addition of Ga atoms lessens the decreasing rates of the a-axis and unit cell volume V on the Y content but almost does not affect the decreasing rates of the c-axis. However, the substitution of Y has a positive effect on the increasing rates of the a-axis and unit cell volume V on the Ga content but has a very slight effect on the increasing rate of the c-axis. The c/a ratio of Nd_2−xY_xFe_17−yGa_y as a function of Ga content exhibits a different increase for different Y content owe to the combined effects of Y and Ga on the crystallographic structure. The substitution of Y is found to have little effect on the site occupancy of Ga in Nd_2−xY_xFe_17−yGa_y. The combined effects of Y and Ga on the bond lengths and ASBL of Nd_2−xY_xFe_17−yGa_y indicate that more bonds detrimental to ferromagnetic exchange can be modulated into the desirable ferromagnetic exchange distance range through suitable combined substitution, which provides a valuable way to improve the magnetic properties of rare earth-transition intermetallic compounds.     

Transport properties of the CexLa1−xPt alloy system

In order to investigate the Kondo ferromagnetism in the CePt compound, we have measured transport properties, resistivity and thermopower, of the Ce_xLa_1−xPt alloy system from 2 K to 320 K. The extraction of the magnetic contribution to the resistivity reveals the existence of the Kondo interaction in all the alloys, but only at higher temperatures. We estimated that the first excited crystal field level of the Ce ion is 100 K. The Kondo characteristics, can be observed at low temperatures only for the lowest concentration measured, where we estimated that the magnetic ordering, if exists, is at 0.34 K. The low temperature minimum of the thermopower is also discussed.     

Magnetic properties of CeMn2−xCoxGe2

The structure and magnetic properties of CeMn_2−xCo_xGe₂ (0.0≤x≤1.0) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Co for Mn leads to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Co for Mn shows a depression of ferromagnetic ordering.     

Optical properties of amorphous Ge28-xSe72Sbx thin films

Bulk glasses of formal composition Ge_28−xSe₇₂Sb_x with 0≤x≤28 were prepared by applying the quench technique. The optical transmission spectra—using a melt were measured in the range from 200 to 1200 nm for Ge_28−xSe₇₂Sb_x films which are prepared by thermal evaporation technique. A simple, straightforward procedure suggested by Swanepoel, which is based on the use of interference fringes, has been applied to calculate the film thickness. On other hand the driving absorption coefficient (α), consequently the band tail width E_e and the optical band gap have been estimated. The real (ε′) and imaginary parts (ε″) of the dielectric constant have been determined and the optical band gap can also be calculated as a function of imaginary part (ε″). The dispersion parameters such as E₀(single-oscillator energy), E_d (dispersive energy) and M₋₁, M₋₃ (moments) were discussed in terms of the single-oscillator Wemple–DiDomenico model.     

Electronic structure and magnetism of PrNixPt1−x compounds

We have studied influence of the Pt–Ni substitution on the crystal structure and magnetic behavior of the PrNi_xPt_1−x compounds. Polycrystalline samples with x = 1, 0.9, 0.75, 0 were prepared and characterized by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). The analysis of XRPD data confirmed that the orthorhombic CrB-type structure owned by the parent binary compounds remains conserved through the entire series. The samples were subsequently investigated by specific heat (C_p), magnetization (M) and ac susceptibility measurements in the temperature range 2–350 K and in magnetic fields up to 9 T. All compounds were found to order ferromagnetically. The T_C values monotonously increase with increasing Ni content. To inspect the crystal-field (CEF) effects and magnetocaloric properties specific-heat data were analyzed in detail and the magnetic contribution to the specific heat together with the magnetic entropy have been determined. The results of first principles electronic structure calculations of the PrNi and PrPt confirmed that besides the stable Pr magnetic moments due to localized 4f-electrons only a very small magnetic moments of at most 0.2μ_B is induced at the Ni (Pt) site due to the polarized 3d-electron states (5d-electron states) hybridizing with the Pr 5d-electron states, i.e. the Ni (Pt) moment plays only minor role in the total balance of the magnetic moments in these compounds.