Showing papers on "Laves phase published in 1981"

Magnetic properties and resonance studies of Gd(CoxAl1−x)2 compounds

Abstract: Results of magnetic measurements, ferromagnetic resonance and electron paramagnetic resonance (EPR) studies of the Gd(CoxAl1−x)2 system are presented. These compounds are ferrimagnetic in the concentration range x > 0.55. The cobalt moment is markedly dependent on composition. The reciprocal susceptibility has a non-linear temperature dependence. The magnetic behaviour of these compounds is analysed using three models, namely the molecular field approximation, the model which assumes that cobalt has an exchange-enhanced paramagnetic susceptibility and the local environment model. All these describe the experimental data rather well. For x < 0.55 the compounds are ferromagnetic. In this case the reciprocal susceptibility follows a Curie-Weiss law. For ferrimagnetic compounds the resonance data at 78 K are described using the Wangsness relation. The EPR measurements for x < 0.55 reveal that the slope of the linewidth b and the g values are not temperature dependent. For ferrimagnetic compounds both b and the g values are functions of temperature. These data are discussed in terms of a model based on the molecular field approximation. The composition dependences of gCo and gGd values are analysed. Finally, the magnetic behaviour of cobalt atoms in pseudo-binary Laves phase compounds is discussed.

Hydrogen ordering in the cubic laves phase HfV2

Abstract: We investigated the crystal structure of HfV 2 D 4 using a neutron diffraction technique The high temperature phase is shown to be a disordered solid solution in which deuterium atoms occupy two types of tetrahedral interstices in the cubic lattice of HfV 2 ; moreover, there is a short-range “blocking-type” order in the mutual arrangement of the interstitial deuterium atoms Below 278 K an order-disorder phase transition occurs, resulting in the formation of a superstructure belonging to space group I4 1 a We suggest, within the framework of the statistical thermodynamic theory of order-disorder transitions developed for this case, an explanation for the super-structure formation and we reach some conclusions about the nature of regularities of hydrogen behaviour which are common to metals and intermetallic compounds

The pseudo-binary system Zr(V1−xCrx)2: Hydrogen absorption and stability considerations

Abstract: Pressure-composition absorption isotherms were measured for the hydrides of Zr(V 1− x Cr x ) 2 ( x = 0.4, 0.5, 0.6). These cubic Laves phase alloys follow a linear relationship of free energy with chromium substitution. A free energy against cell volume plot of the cubic chromium-containing materials compared with earlier data on cubic as well as hexagonal iron-substituted alloys shows that electronic factors are probably more important than structural factors in determining the free energies of the ternary Zr(V 1− x B x ) 2 systems.

55Mn NMR Study in Magnetically Ordered State of RMn2 (R=Rate Earth) Intermetallic Compounds

Abstract: NMR of 55 Mn has been observed in magnetically ordered state of cubic Laves phase compounds GdMn 2 , TbMn 2 , DyMn 2 and HoMn 2 and in hexagonal Lave phases ErMn 2 and TmMn 2 . Magnetization and neutron diffraction studies have shown non zero Mn moment in cubic compounds only. Accordingly, we find that hyperfine fields at Mn site are larger in cubic than in hexagonal compounds. The observed hyperfine field values are used to estimate Mn moments in these compounds.

High critical currents obtained by heat treating rapidly quenched Hf-Zr-V metallic glasses

Abstract: We have found that metallic glasses can be formed in the Hf-V, Zr-V, and Hf-Zr-V systems by rapid quenching. The importance of this finding is threefold: 1) Like many metallic glasses these glasses exhibit excellent mechanical properties. 2) Using well established rapid quenching techniques the material can be economically produced as ribbon, filament, or powder. 3) Because the amorphous material is homogeneous on an atomic scale a variety of microstructures not obtainable by standard metallurgical procedures are possible. These glasses represent a potentially useful starting material for the fabrication of high field, high current superconducting devices. Completely amorphous samples were not found to be superconducting above 2 K. Controlled growth of the C-15 Laves phase compound by heat treating the glasses results in materials having J c (4.2 K) 105A/cm2at H applied = 150 kOe.

A study of the crystallisation process in the metallic glass Y0.66Fe0.34

Abstract: The crystallisation of the metallic glass Y0.66Fe0.34 has been investigated using 57Fe Mossbauer effect spectroscopy, differential scanning calorimetry, X-ray diffraction, and high-temperature resistivity measurements. The emphasis of this work is on the microscopic changes which occur in the local environments of the Fe atoms during the crystallisation process. The crystallisation of this glass occurs in two distinct steps associated with the formation of HCP Y and the Laves phase compound YFe2. The activation energies for these two steps were found to be 2.2 eV and 3.6 eV respectively.

X-ray investigations of laves phase intermetallic compounds in the Gd1−xErxAl2 system for 0 ⩽ x ⩽ 0.6

Abstract: The influence of the substitution of gadolinium atoms for erbium atoms in polycrystalline Gd1−xErxAl2 Laves phase compounds was investigated. X-ray diffraction studies showed that all the samples crystallize with a cubic structure of the MgCu2 type (C15). The lattice constants of the compounds satisfy Vegard's law for the whole range of x. Partial disorder of the crystal structure was found, i.e. the aluminium atoms partly exchange with the gadolinium and erbium atoms. The crystallographic ordering parameter y (defined as the number of aluminium atoms in the 8a sites) is equal to zero (“ideal ordering”) only for GdAl2. It was observed that the parameter y increases with increasing x up to a content of about 30 mol.% ErAl2. For this compound and for those with higher contents of ErA12 the exchange of the rare earth (gadolinium and erbium) and aluminium between the 8a and 16d sites is a maximum and equals about 1 atom per elementary cell.

Correlation of57Fe hyperfine interactions with Fe-Fe distance in Laves phase compounds RFe2 (R=rare earth)

Abstract: The57Fe Mossbauer isomer shifts and magnetic hyperfine fields in Laves phase compounds RFe2 (R=Pr, Nd and Sm) are studied with particular reference to the effect of the Fe−Fe interatomic distance on the hyperfine interactions. It is shown that the charge density at the Fe nuclei scales linearly with fractional volume change up to 20%. The57Fe hyperfine field corrected for the influence of rare-earth moment shows a systematic variation with the distance, which can be understood in terms of the Bethe-Slater curve arguments. The similarity of the atomic volume dependence of the57Fe hyperfine interactions in Lves phase compounds to those in iron with close-packed structure is emphasized.

Nuclear magnetic resonance study of Cu2-xZnxMg in the cubic Laves phase

Abstract: Pulsed nuclear magnetic resonance (NMR) techniques at high magnetic fields have been used to measure the Knight shift and spin-lattice relaxation rate of 63Cu in the Laves phase system Cu2-xZnxMg for 0

Method of preparing a hydrogen-absorbing alloy

Abstract: Alloys consisting of from 64 to 77 atomic percent of manganese with the remainder zirconium are not suitable without further processing as a material for storing hydrogen at technically desired pressures This property can be advantageously influenced by a heat treatment until a homogeneous C14 type of Laves phase has been obtained

NMR Study of Zr(Fe1-xMnx)2

Abstract: NMR spin echo technique was used to study magnetic properties of the Laves phase compound Zr(Fe 1- x Mn x ) 2 . For the C-15 type ferromagnetic compound ( x ≤0.3), 55 Mn spectra showed well-resolved and regular satellites between 100 and 230 MHz at 15 K in zero external field. This suggests that Mn atoms have small magnetic moments which hardly change with x for x ≤0.3. For the C-14 type compounds ( x ≥0.4), the experiment suggests that antiferromagnetism appears with the crystal structure transformation and disappears gradually with increasing x . The spin structure of the antiferromagnetic compound is discussed. The fine structure attributable to electric quadrupole interaction was observed in the 55 Mn spectrum of ZrMn 2 . The value of e 2 q Q / h derived from the structure is 7.3 MHz.

A study of the rhombohedral distortion of the magnetostrictive samarium‐iron compounds with addition of molybdenum or vanadium

Abstract: The mechanical hardness and magnetostriction of annealed pseudo‐binary compounds, based on the binary Laves phase, SmFe2 compound with additions of small percentage of Cu, Ni, V or Mo, have been studied. It is found that the compounds containing Mo have optimum hardness and values of magnetostriction. The pseudo‐binary compounds SmFe2−xMox and SmFe2−xVx (x = 0.05, 0.1 and 0.2) have been studied by using X‐ray counter diffractometer. Owing to the broadening and the splitting of line profiles at higher Bragg angles, a precision determination of lattice parameters becomes difficult. From the data the angles of rhombohedral cells and the relative change in lattice spacings of cubic (400) reflections in these compounds have been calculated.

Crystallographic and magnetic investigations of the intermetallic system ErCo2-ErAl2

Abstract: The crystalline structure and magnetic properties of polycrystalline ErCo2-ErAl2 Laves phase compounds with the C14 and C15 crystal structures were investigated. It was found that for the C15 cubic structure it is possible to obtain compounds with a disordered structure. X-ray examination showed that for the ErCo2 compound the exchange of cobalt atom between 8a and 16d sites is almost 1 atom per unit cell. After thermal treatment the crystal structure of the ErCo2 compound shows a tendency towards ordering. For compounds with the hexagonal C14 structure the possibility of a layer distribution of atoms, alternately magnetic and nonmagnetic, was tested. This assumption is well satisfied by the ErCoAl compound. For this compound a certain anomaly was also observed, i.e. a small lattice constant c. Magnetic investigations indicated that the magnetic properties of the compounds investigated, particularly the Curie temperature, are markedly dependent on the degree of crystal structure ordering. For the remaining compounds with the C15 structure it was not possible to obtain fully ordered compounds but after thermal treatment they exhibit a tendency towards ordering.

Method for the coproduction of hydrocarbons and alcohols.

Abstract: Methods for the synthesis of hydrocarbons and alcohols by contacting carbon monoxide and hydrogen over novel catalyst compositions are disclosed. The catalyst compostions are intermetallic compounds having a first and second phase, said first phase being selected from: (1) a single metal selected from V and Nb, (2) a single phase binary alloy selected from TiCo, TiCr, TiCu and TiMn and the binary Laves phase compounds TiCr2, ZrNi2, ZrMo2, ZrFe2, ZrCo2, ZrMn2, TiCo2, ZrCu2 and ZrCr2, and (3) single phase ternary or quaternary alloys selected from (a) TiFexMyNz where M and N are any metal that can substitute for Fe such that the resulting alloy is a single phase alloy, but not Ni or Ru, and where X is a number less than 1.0, y and z are each numbers less than 1.0 or either, but not both, may be 0, and x and y and z is approximately 1.0; (b) TiAXMYNZ where A is Co, Cr, Mn or Cu, M and N are any metal that can substitute for Co, Cr, Mn or Cu such that the resulting alloy is a single phase alloy, and where x is a number less than 1.0, y and z are each numbers less than 1.0 or either, but not both, may be 0, and x and y and z is approximately 1.0; and (c) ternary or higher Laves phase compounds based on Ti and/or Zr, and said second phase is free titanium hydride or zirconium hydride.

Magnetic and resonance studies of the laves phase compounds GdT2 (T ≡ Rh, Ir, Pt, Ru, Mn)

Abstract: A detailed study was made of the Laves phase compounds GdT2 (T ≡ Rh, Ir, Pt, Ru, Mn). The study was carried out by observing the spin-echo nuclear magnetic resonance (NMR) of 165Ho, which was introduced as a dilute substitutional impurity, the electron spin resonance (ESR) of Gd3+ and magnetic measurements. The NMR of holmium reveals the presence of a strong crystalline electric field in these compounds, particularly in GdRh2 and GdMn2. Except for GdMn2, for which the extraction of ESR parameters was not possible, the investigation suggests that the gadolinium ESR is bottlenecked in all the compounds studied. Both the magnetic and the ESR studies of GdMn2 reveal magnetic and structural phase transitions at 283 K and 106 K respectively. The g shift is negative for gadolinium in GdRu2 for which a saturation moment of (6.31 ± 0.15)μB is found. In other alloys the g shift is positive and the saturation moment for gadolinium is found to be approximately 7μB which is considerably higher than values reported earlier and in excellent agreement with values reported more recently.