Structural and electrochemical properties of TixZr7−xNi10

Simple ternary alloys with formula Ti_xZr_7−xNi₁₀ (x between 0 and 2.5) were studied as a potential replacement for Laves phase alloys used in the negative electrodes of nickel metal hydride batteries. The samples were prepared by arc-melting and were not annealed. The samples retained a high degree of disorder, which contributed positively to activation and other electrochemical properties. Before hydrogenation, the alloys have a Zr₇Ni₁₀ orthorhombic structure mixed with some C15 and ZrO₂ secondary phases. The amount of C15 secondary phase is important to the bulk diffusion of hydrogen and the surface electrochemical kinetics. That is, the diffusion coefficient and the exchange current both increase in the presence of C15 secondary phase. The proportion of C15 secondary phase is controllable by stoichiometry design. For instance, a slightly higher Zr content reduces the C15 content. Further, as the titanium substitution level increases: (1) the lattice constants decrease; (2) the PCT plateau pressure increases; (3) activation becomes easier; and (4) the high rate dischargeability improves.     

Magnetic properties and spin-reorientation transition in DyFe10−xNixSi2 compounds

The magnetic properties of DyFe_10−xNi_xSi₂ compounds with x = 0, 1, 2, 3, 4, 6, 9 and 10 have been investigated by means of X-ray diffraction and magnetic measurements. Substitution of Ni for Fe leads to a decrease in the lattice constants a, c and the unit-cell volume V. The Curie temperature reaches a maximum of 590 K at x = 2, then decreases strongly for x ≥ 2. The spin reorientations are observed for the compounds with x = 0, 1, 2 and 3. The spin reorientation temperature decreases strongly from 255 to 60 K as the Ni content is increased from x = 0 to 3. Below the spin reorientation temperature, the compounds exhibit ferrimagnetic ordering. For the Ni-rich compounds with x = 9 and 10, the magnetization of the Dy sublattice decreases strongly since the magnetization of the Dy sublattice is strongly affected by the molecular field produced by the 3d sublattice.     

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.     

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.     

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.     

A neutron diffraction study of the hexagonal pseudo-ternary compounds ErMn6−xFexSn6 (x = 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 3.0, 4.0)

The hexagonal ErMn_6−xFe_xSn₆ solid solution (0.2 < x < 4) has been studied by magnetisation measurements and neutron diffraction. The ordering temperature of the T = (Mn,Fe) sublattice almost continuously increases from T = 386 K for x = 0.2 to T = 498 K for x = 4. The T sublattice orders in the successive magnetic structures helimagnetic H1, antiferromagnetic AF2, helimagnetic H2 and antiferromagnetic AF1 with increasing iron content. While structures AF2 and H1 were already observed in ternary Mn compounds and AF1 in ternary iron compounds, the structure H2 is of a new kind characterized by an AF slab around the Er(1a) site. At low temperature, a change of the easy direction of the Er moment from easy plane to easy axis is observed. The iron-rich compounds display a ferromagnetic order of the Er sublattice. A new kind of magnetic structure characterized by a sine-wave modulated arrangement with a propagating vector Q = (0, 0, q_z) is also observed. The evolution of the magnetic properties (enhancement of the AF character of the (Mn,Fe) sublattice and magnetocrystalline anisotropy of erbium) is discussed.     

Crystallographic site of Mn in the icosahedral cluster of LaCo13−xMnx compounds

Studies on the structure and the crystallographic site of Mn in LaCo_13−xMn_x compounds were carried out by using X-ray diffraction and X-ray absorption fine structure (XAFS) of the Mn K-edge. These compounds with x≤3.0 adopt a NaZn₁₃-type structure consisting of icosahedral clusters. The lattice constant increases with the Mn concentration. The calculated XAFS curves of the center and the corner sites in the icosahedral clusters for the Mn K-edge are obtained by using the program . The fitting result for the corner site agrees much better with the observed XAFS spectrum than that for the center site. Therefore, the Mn site is determined to be the corner Co site in the icosahedral clusters of all the compounds. In comparison with the crystallographic parameters of LaCo₁₃, the icosahedral clusters composed of Mn atoms expand and the crystallographic Mn site is slightly more close to the La atom.     

Crystal structure, magnetic behaviour and valence state of ytterbium in the Yb4Ni10+xGa21−x phase

The ternary phase Yb₄Ni_10+xGa_21−x has been synthesised from the elements by high frequency melting in argon atmosphere. The homogeneity region has been established from X-ray powder data and confirmed by EDX analysis for 0.3≤x≤1. The crystal structure of Yb₄Ni_10+xGa_21−x has been estimated from X-ray single crystal data: space group C2/m (no. 12), Z=2, a=20.6815(9) Å, b=4.0560(4) Å, c=15.3520(7) Å, β=124.800(3)°, R(F)=0.023 for 1701 symmetry independent reflections with F(hkl)>4σ(F). A special feature of the structure is the local disorder within the gallium/nickel network. Neglecting atomic disorder in the region of the Ga9 and Ga11 positions, the Yb₄Ni_10+xGa_21−x structure is an occupation variant of the Ho₄Ni₁₀Ga₂₁ type with nickel atoms partially replacing the Ga atoms in the 2d sites at the centers of distorted icosahedra. From magnetic susceptibility and from L_III-XAS spectra, the valence state of ytterbium is near 3+.     

Structural and thermodynamic properties of the pseudo-binary TiCr2−xVx compounds with 0.0≤x≤1.2

The TiCr_2−xV_x compounds with 0.0≤x≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x>0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature (P–C–T) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P–C–T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.     

Magnetic properties of Pr1−xGdxMn2Ge2 compounds

The structure and magnetic properties of the Pr_1−xGd_xMn₂Ge₂ (0.0≤x≤1.0) compounds have been investigated by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. The lattice constants and the unit cell volume obey Vegard’s law. Samples in this alloy system exhibit a crossover from ferromagnetic ordering for PrMn₂Ge₂ to antiferromagnetic ordering for GdMn₂Ge₂ as a function of Gd concentration x. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. The results are summarized in the x–T magnetic phase diagram.     

Glass-forming ability and magnetic properties of mechanically solid-state reacted Co100−xTix alloy powders

A high-energy ball milling technique using the mechanical alloying method has been employed for fabrication of glassy Co_100−xTi_x (25≤x≤67) alloy powders at room temperature. The fabricated glassy alloys in the Co-rich (33≥x) side exhibit good soft magnetic properties. The binary glassy alloys for which the glass transition temperatures (T_g) have rather high temperatures (above 800 K), show large supercooled liquid regions before crystallization (ΔT_x larger than 50 K). The reduced glass transition temperature (ratio between T_g and liquidus temperatures, T_l (T_g/T_l)) was found to be larger than 0.56. We have also performed post-annealing experiments on the mechanically deformed Co/Ti multilayered composite powders. The results show that annealing of the powders at 710 K leads to the formation of a glassy phase (thermally enhanced glass formation reaction), of which the heat of formation was measured directly. The similarity in the crystallization and magnetization behaviors between the two classes of as-annealed and as-mechanically alloyed glassy powders implies the formation of the same glass state.     

Magnetic phase transitions in Nd1−xYxMn2Ge2 (0 ≤ x ≤ 0.6)

The structure and magnetic properties of Nd_1−xY_xMn₂Ge₂ (0.0≤x≤0.6) 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 Y for Mn led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Y for Nd in NdMn₂Ge₂ shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering.     

Effect of Mn concentration on the spin glass transition temperature of Fe0.70−xMnxAl0.30 alloys

Magnetization measurements on zero field cooled (ZFC) and field cooled (FC) samples as a function of temperature where made on Fe_0.70−xMn_xAl_0.30 alloys with 0≤x≤0.10. Our data show the existence of two regimes of the spin glass temperature T_f when increasing the concentration x of the Mn atoms. We found that T_f decreases slowly with increasing x up to x=0.06 and then shows a rapid linear decrease above this concentration. The existence of two regimes of T_f behavior with x may be attributed to the disappearance of ferromagnetic clusters which exist for low values of x (x<0.06). Compared to the Mn-free alloy, the relative changes in T_f are bigger and exceed 40% at x=0.10. The second regime (x>0.06) is associated with more influence of x on T_f.     

Structural and magnetic properties of Sm2Fe17−xCrxC2 nanocrystalline carbides with 0≤x≤2

The crystallographic and the Curie temperature of the Sm₂Fe_17−xCr_xC₂ (x=0.5, 1, 1.5 and 2) carbides have been extensively studied. X-ray diffraction studies have shown that all these alloys are approximately single phases corresponding to the Th₂Zn₁₇ type rhombohedral structure with a small amount of -Fe. The amount of this residual -Fe phase decreases with increasing the Cr atomic content. It decreases from 1 wt% for x=0.5 to 0.4 wt.% for x=2. The lattice parameter c increases as a function of the Cr atomic content x from x=0 to x=1.5 and then decreases. This is due to the Cr atoms which prefer to substitute the Fe atoms in the 6c sites located along the c-axis. The lattice parameter a and the unit-cell volume decrease in all substitution ranges. The insertion of the C atoms leads essentially to an increase of the distances between the 9d and 18h sites and the 9d–18f sites. The Curie temperature reaches a maximum value of 583 K for x=1.5 and then decreases to 551 K for x=2. The enhancement of the T_c for lower Cr contents is due to a lowering of the hybridization of the iron atoms with their neighbors, the magnetovolume effect and the reduction of antiferromagnetic interactions. However, the decrease in T_c for higher Cr content is due to the reduction in the number of Fe–Fe pairs due to the magnetic dilution effect. For given interatomic distances, the exchange coupling of the Cr–Cr atoms is not of antiferromagnetic type and the exchange integral of the Cr–Cr pair is higher than that of the Fe–Fe pair.     

Magnetocrystalline anisotropy of Nd3(Fe1−xCox)27.7Ti1.3Ny compounds

The effect of Co substitution for Fe in Nd₃(Fe_1−xCo_x)_27.7Ti_1.3N_y (0 ≤ x ≤ 0.4) compounds on the magnetocrystalline anisotropy has been investigated. The anisotropy constants K's and the anisotropy field H_A have been deduced from the magnetization curves measured on magnetically aligned powder (4–7 μm) samples. The obtained results show that at RT the anisotropy is uniaxial and H_A (about 10 T) does not change substantially upon the substitution. At 5 K the results for K's give evidence for the presence of easy-cone-type anisotropy. The cone angle as well as the anisotropy field decrease upon the substitution from 21.6° to 11.8° and from 22.8 to 18.6 T, respectively.     

The YbNi2−xSn compound, a new structure type of ternary stannides

The crystal structure of the new ternary stannide YbNi_2−xSn [space group P6₃/mmc (N 194), z=4] was investigated using single crystal X-ray diffraction data (automatic single crystal diffractometers Philips PW1100 and Bruker SMART CCD, Mo K radiation). Three crystals with different composition YbNi_1.695Sn [a=4.424(4) Å, c=15.232(6) Å, V=258.2(4) ų, ρ=10.066 g cm⁻³, μ=57.32 mm⁻¹], YbNi_1.705Sn [a=4.424(5) Å, c=15.179(7) Å, V=257.3(4) ų, ρ=10.116 g cm⁻³, μ=57.59 mm⁻¹] and YbNi_1.745Sn [a=4.303(6) Å, c=16.001(9) Å, V=256.7(5) ų, ρ=10.199 g cm⁻³, μ=58.00 mm⁻¹] were refined to R=0.0494, wR²=0.1330, to R=0.0782, wR²=0.1916 and to R=0.0643, wR²=0.1460, respectively. The compounds belongs to a new structure type of intermetallic compounds and are formed from two hypothetical structures YbNi_1.5Sn (space group P6₃/mmc, z=4) and YbNi₂Sn (space group P6₃/mmc, z=4) containing segments of the MnCu₂Al, ZrBeSi and ZrPt₂Al structures.     

HfFe6Ge6-type YbMn6Ge6−xGax compounds (0.07≤x<0.72) with large magnetocrystalline anisotropy

The HfFe₆Ge₆-type YbMn₆Ge_6−xGa_x solid solution (0.07≤x≤0.72) has been studied by X-ray diffraction, microprobe analysis and powder magnetization measurements. All the compounds order antiferromagnetically between T_N=481 K for x=0.07 and T_N=349 K for x=0.72 and display more or less pronounced spontaneous magnetization at lower temperature. The corresponding Curie points increase from 40 K for x=0.07 to 319 K for x=0.72. The maximum magnetization values of the Ga-rich compounds (M≈5 μ_B/f.u. at 6 K) is compatible with a ferrimagnetic order of the Mn and Yb sublattices whereas the smaller values measured in the Ga-poor compounds suggest the stabilization of non-colinear magnetic structures. All the studied compounds are characterized by rather large coercive fields at low temperature (4.0≤H_c≤8.2 kOe).     

Evolution of magnetic hardness in the HfFe6Ge6-type RMn6Sn6−xGax and RMn6Sn6−xInx compounds (R=Tb, Dy; 0.2≤x≤1.4)

The HfFe₆Ge₆-type RMn₆Sn_6−xX_x′ solid solutions (R=Tb, Dy, X′=Ga, In; x≤1.4) have been studied by powder magnetization measurements. All the series are characterized by ferrimagnetic ordering and by a decrease in Curie temperatures with the substitution (ΔT_c/Δx≈−39 K for X′=Ga and ΔT_c/Δx≈−75 K for X′=In). The RMn₆Sn_6−xGa_x systems are characterized by a strong decrease in the spin reorientation temperature with substitution (ΔT_t/Δx≈−191 K and −78 K for R=Tb and Dy, respectively) while this transition almost does not change in systems containing indium. The coercive fields drastically decrease with the substitution in the TbMn₆Sn_6−xGa_x system while the substitution of In for Sn has a weaker effect. The coercive fields of the Dy compounds do not vary greatly with the substitution in both series. The behaviour of the TbMn₆Sn_6−xGa_x is compared with the evolutions observed in the TmMn₆Sn_6−xGa_x series. This comparison strongly suggests that the replacement of Sn by Ga changes the sign of the A₀² crystal field parameter.     

Electrical, magnetic and elastic properties of La1.2(Sr1−xCax)1.8Mn2O7 (0.0 ≤ x ≤ 0.4)

Polycrystalline bulk samples of double layered manganite system La_1.2(Sr_1−xCa_x)_1.8Mn₂O₇ (0.0 ≤ x ≤ 0.4) were prepared by sol–gel method. After characterizing the samples using XRD and SEM, their electrical, magnetic and elastic properties were investigated. The lattice parameters and cell volume show a monotonous decrease with increase of Ca content, whereas the grain size is found to increase with increasing Ca content. The value of T_IM is found to decrease with Ca content up to x = 0.3 and then a slight increase of T_IM is observed. The low temperature upturn of resistivity is attributed to the spin-glass-like behavior, which is also evidenced by the irreversibility observed between ZFC and FC magnetizations. The conduction mechanism above T_IM can be explained by Mott VRH model. The present magnetization and ultrasonic studies indicate that the system shows a secondary transition at T^*, which decreases with increasing Ca content. Further, the T^* seems to be intrinsic to the present double layered manganite system.     

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.