Journal of The Less Common Metals最新文献

The Invar alloy La(Fe_xAl_x−1)₁₃ with NaZn₁₃-type cubic structure was prepared. Using single crystals of CaF₂ as solid electrolyte, the activity and the relative partial molar Gibbs free energy of lanthanum in La(Fe_xAl_1−x)₁₃ alloys were determined by the e.m.f. method over the temperature range 983–1085 K. The melting point and the phase transition temperature of two La(Fe_xAl_1−x)₁₃ alloys were measured by differential thermal analysis. The average molar heat capacity was measured using a high temperature microcalorimeter in the temperature range 416–610 K.

A new ternary uranium compound, UPdGa, crystallizing in the hexagonal Fe₂P-type structure has been prepared. The magnetic and electrical properties of this material have been studied in the temperature range 4.2–300 K. The results obtained indicate the appearance of two magnetic phase transitions at 30 and 62 K, probably originating from an antiferromagnetic ordering.

A new quaternary compound Cr_5−x−yFe_xNb_ySi₃ has been studied using single-crystal X-ray diffraction data and electron microprobe analysis. The crystal structure was determined to be the same as that of the Mn₅Si₃ structure type: space group $\text{P6}{}_3\text{mcm}$ and Z = 2, a = 7.002(1), $\text{c = 4.765(1)}\text{A}\text{̊}$, D_x = 6.106 Mg m⁻³, μ(MoKα) = 15.03 mm⁻¹, F(000) = 345, T = 293 K. R = 0.055 for 118 contributing unique reflections.

Mixed oxides MTe₂O₆ (M  Ce, Th) were prepared at a temperature of 973 K. Structural characterization of these materials has been carried out by X-ray and electron diffraction. The structures have been refined by Rietveld analysis of X-ray powder diffraction data recorded at room temperature. The structures have monoclinic symmetry, space group P2₁/n, and the lattice parameters are $\text{a = 7.0197(8)}\text{A}\text{̊}$, $\text{b = 11.0434(8)}\text{A}\text{̊}$, $\text{c = 7.3327(1)}\text{A}\text{̊}$ and β = 108.007(8)° for CeTe₂O₆ and $\text{a = 7.1954(1)}\text{A}\text{̊}$, $\text{b = 11.2183(6)}\text{A}\text{̊}$, $\text{c = 7.4638(8)}\text{A}\text{̊}$ and β = 108.063(3)° for ThTe₂O₆.

We have developed new composite materials for hydrogen storage containing ZrFe_1.4Cr_0.6 or TiMn_1.5 as a storage material and magnesium as a binder. The influences of composition, compacting pressure and heat treatment on hydrogen capacity, kinetics and cyclic durability of the composite pellets were examined by repeating absorption-desorption cycles. The pellets obtained by sintering the mixtures at 773 K for 20–40 h absorb hydrogen readily and very quickly under a hydrogen pressure of less than 1 MPa without any activation treatment, and exhibit no disintegration after 1000 hydriding—dehydriding cycles. Observations of the microstructure and distribution of metals and oxygen atoms in the composites, using scanning electron microscopy, electron probe microanalysis and electron spectroscopy for chemical analysis, indicate that the heat treatment at 773 K not only promotes the so-called magnesium reduction and makes the surface of the hydride clean, but it also helps to form a new thin composite phase on the boundary between the original hydride and unreacting magnesium metal which acts as a binder, keeping the pellet intact upon hydrogenation.

Oxidation of reduced chlorides (KM₂Cl₅; M  Nd, Sm) or chloride-hydrides (MClH_0.67; M  La, Ce, Pr) of the lanthanides with sulphur in the presence of NaCl in the latter case (tantalum capsule, 850 °C, 7 days) results in the formation of binary sulphides and ternary chlorides with potassium and sodium respectively, both with trivalent cations (M³⁺), as the main products. Oxidic impurities (e.g. MOCl) react to yield M₁₀S₁₄O-type oxysulphides (M  La, Ce, Pr, Nd, Sm) in small amounts as bead-shaped single crystals of high quality (M₁₀S₁₄O, tetragonal, $\text{I4}{}_1\text{acd}$ (no. 142), Z = 8; M  La: a = 1536.51(4) pm, c = 2037.85(9) pm, R = R_w = 0.028; M  Ce: a = 1521.24(4) pm, c = 2018.43(9) pm, R = 0.014, R_w = 0.013; M  Pr: a = 1511.63(4) pm, c = 2006.27(9) pm, R = 0.028, R_w = 0.029; M  Nd: a = 1503.80(4) pm, c = 1996.46(8) pm, R = 0.015, R_w = 0.014; M  Sm: a = 1485.96(4) pm, c = 1974.04(8) pm, R = 0.016, R_w = 0.014). Their crystal structure is built up from isolated O²⁻-centred tetrahedra [OM₄]¹⁰⁺ which are surrounded by a “sea” of lanthanide sulphide according to the formulation [OM₄]S₇M₆S₇. Three crystallographically independent M³⁺ cations occur with coordination numbers of eight (M1: seven plus one S²⁻; M2: six plus one S²⁻ and one O²⁻, both bicapped trigonal prisms; M3: eight S²⁻, trigonal dodecahedron). Four different S²⁻ (coordination numbers from four plus one to six) provide for the necessary coupling of the isolated [O(M2)₄] tetrahedra to their metal sulphide vicinity.

The effect of partial substitution of iron by cobalt on the Curie temperature, demagnetization curves and corrosion behaviour under various conditions of sintered Nd₁₆Fe_76−xCo_xB₈ (O ⩽ x ⩽ 30) permanent magnets was investigated. The microstructure and chemical composition of phase structures were also determined. A new intergranular Nd(Fe, Co)₂B₂ phase was found for x ⩾ 20.