C14 Type Research Articles

Superstructure in the intermetallic compound TmNi 2

The results of neutron and X-ray diffraction experiments at 300 and 4.2 K on the intermetallic compound TmNi 2 indicate that the crystal structure of the two modifications of TmNi 2 observed by us are closely related to the cubic Laves phase structure (C15). From the fit between the experimental and calculated data a consistent set of parameters describing the atomic positions in the F4̄3 m space group is obtained for one of these modifications, the corresponding lengths of the unit cell being doubled in all three dimensions with respect to the C15 structure. The other observed modification of TmNi 2 has a simple cubic structure, the unit cell being the same size as found for the C15 type.

Crystal Structure and Magnetic Properties of Y1-xZrxFe2.9

Crystal structure of Y 1- x Zr x Fe 2.9 system were ascertained by X-ray diffraction to be classified in four types, PuNi 3 type in x <0.3, orthorhombic type in 0.3< x <0.7, C14 like hexagonal type in 0.7< x <0.85 and C15 type in 0.85< x <0.95. The Mossbauer spectra of the compounds were explained to be two superimposed hyperfine patterns. It is concluded that a direction of hyperfine field lies in the c plane for the compounds in 0< x <0.3 and lies along the c axis for the compounds in 0.4< x <0.95.

NMR Study of1H in Hexagonal C14 Type Laves Phase Metal-Hydrogen System ZrCr2H4.2: Diffusion and Korringa Relaxation of1H

The spin-lattice relaxation times T 1 and T 1ρ of protons have been measured in a hexagonal C14 type Laves phase metal hydride ZrCr 2 H 4.2 between 4.2 and 333 K. A Korringa type of relaxation due to conduction electrons is obtained as T 1e T =5.01 sec·K, which is about one order of magnitude smaller than 55.6 sec·K in the C15 type Laves phase ZrV 2 H 4 . T 1 exhibits a minimum owing to the diffusion of hydrogen and an asymmetric variation against 1 / T about this minimum. The results of T 1ρ measurements show that the asyrnmetric variation of T 1 is due to a distribution in the activation energy E for the diffusion of hydrogen. The central value and the width of the distribution of E are determined to be 0.29 eV arnd 0.05 eV, respectively.

ChemInform Abstract: Crystallization of the Laves Phases LnNiAl (C14 type) at High Pressure.

AbstractLnNiAl (Ln: Sm‐Lu, Y) compounds having Laves phase structures have been prepared by annealing stoichiometric mixtures of the elements under high pressure (1450‐1500 °C, 7.7 GPa, no yields given).

Magnetic moments and 57Fe hyperfine field interactions in Y(Fe1−xRux)2 ternaries

Measurements of magnetization and 57Fe Mössbauer spectra have been made for Y(Fe1−xRux)2. The C15 type cubic structure is stabilized for x</0.06 and the C14 type hexagonal structure for x 0.7. The C15 compounds is ferromagnetic with Tc200 K and its saturation moment decreases monotonically with increasing x, while the 57Fe hyperfine field decreases only slightly with x. From these results, it is deduced that the Ru atoms have an induced moment of ≈1μB in the range x ⩽ 0.2. In the C14 type phase, no magnetic ordering develops even at 4.2 K.

NMR study of coexistence of ferro- and antiferromagnetism in (Zr1−xNbx)Fe2

The magnetic phase diagram has been investigated in the C14 type (Zr1−xNbx)Fe2 with x⩾0.7 from 93Nb NMR and magnetization measurements. In the compound with x = 0.825 a first order-like transition has been found to occur around 25 K from a canted state with the ferromagnetic moment in the basal plane to a ferromagnetic state with decreasing temperature.

Crystallization of the Laves phases RNiAl (C14 type) at high pressure

The compounds RNiAl (R ≡ Sm — Lu, Y) with Laves phase structures (C14) were obtained from mixtures of the powdered initial components by annealing at 1450–1500 °C and a pressure of 7.7 GPa.

On the breakdown of the iron sublattice long-range magnetic order in RFeAl hydrides (R ≡ rare earth)

In the cubic C15 type Laves phase compounds RFe 2 (R ≡ rare earth), hydrogen absorption commonly leads to a rise in the moment on the iron site. If, however, half the iron is substituted by the non-magnetic aluminium, a remarkable weakening of the magnetic order is found for the resulting hexagonal C14 RFeAl hydrides. Distinct peaks in the initial susceptibility vs. temperature curve indicate the occurrence of magnetic order of the short-range type. In certain cases, where ferromagnetism is preserved in the hydrides, 57Fe Mössbauer spectra indicate that even here a substantial number of iron atoms have lost their moments upon charging.

Hydriding behaviors and equilibrium properties of Zr-Mn binary alloys

In the Zr-Mn binary alloys, ZrMn2 has the highest amount of reversibly occluded hydrogen. When hydrided at room temperature under 5MPa of hydrogen, a composition of ZrMn2H3.46 is attained. The crystal structure of ZrMn2 hydride also belongs to the C14 type Laves phase, and the ZrMn2-H system is described as a miscibility gap type pseudo-binary system. The other Zr-Mn alloys show rather complicated hydriding behaviors mainly due to the two phase features. This leads some modifications to the well published Zr-Mn phase diagram. Sieverts' plots for dissociative solution of hydrogen in ZrMn2 show the probable existence of two types of solutions in the α solid solution region.

A study of the structure of Zr(CrFe) 2 laves phase precipitates in a ZrCrFe alloy

C15 type Zr(CrFe) 2 Laves phase precipitates have been found in Zr-1.15 wt% Cr-0.1 wt% Fe alloy. Twinned, multiple twinned and dislocation structures have been found in the precipitates. Comparison of calculated and measured precipitate size show the growth of the Zr(CrFe) 2 Laves phase is controlled by diffusion. The orientation relationships (1̄11̄) L//(112̄0) α, [110] L//[0001] α between the Zr(CrFe) 2 Laves phase precipitates and ga-Zr matrix in the ZrCrFe alloy give the same type of model for the transformation as previously suggested for Zircaloy-4.

A TEM study of the C15 type Zr(CrFe) 2 Laves phase in Zircaloy-4

The intermetallic precipitates in Zircaloy-4 have been identified as the C15 type Zr(CrFe) 2 Laves phase using high order Laue zones and series diffraction patterns from the transmission electron microscope. A model for the transformation from the α-Zr matrix to the Zr(CrFe) 2 Laves phase has been constructed using the data on orientation relationships obtained from TEM diffraction patterns. A defect structure of R-type stacking variety has been found in the Zr(CrFe) 2 particles.

δ-Dependence of Magnetism in the C15 Laves Phase System HfCo2+δ

Investigations of the magnetic properties of non-stoichiometric HfCo2+δ led to the discovery of ferromagnetism for δ ≫ 0.8. Homogeneous samples formed by plasma jet-melting in argon were annealed to obtain single phase crystals, and then quenched. X-ray diffraction confirmed the existence of a single-phase with C15 type structure for -0.2 ≪ δ ≪ 1.2, but a single phase could not be obtained at other compositions. The range of ferromagnetic behavior, susceptibility, and other magnetic properties are described.

X-Ray Study of Crystal Distortions in RMn2Compounds (R: Y, Pr, Nd, Sm and Gd)

Powder X-ray diffraction measurements on intermetallic compounds RMn 2 (R: Y, Pr, Nd, Sm and Gd) have been made in a temperature range from 69 to 300 K. Below T N , every sample shows a large volume expansion, being as large as 4.6% for YMn 2 . In addition, PrMn 2 and NdMn 2 having the hexagonal C14 type of crystal structure are distorted into monoclinic below T N . On the other hand, SmMn 2 having the same C14 type structure is distorted into orthorhombic. On the basis of a theory of magnetoelastic coupling the directions of magnetization of these compounds below T N were determined. The cubic C15 type YMn 2 shows a very small and probably rhombohedral distortion below T N , whereas the same C15 type GdMn 2 shows no symmetry change.

CONVERGENT BEAM ELECTRON DIFFRACTION STUDY OF C14 TYPE LAVES PHASE IN GH 302 ALLOY

The dendritical spaped precipitates in GH 302 alloy were identified as C14 type Laves phase by means of CBED, SED and X-ray mitro-analysis.

Charge transfer and155Gd isomer shift in various intermetallic compounds of Gd

The isomer shift observed by means of 155Gd Mossbauer spectroscopy in intermetallic compounds composed of Gd and one of the metals M=Al, In, Pb, Sn, Zn, Cu, Ni, Co, Fe or Mn was analysed in terms of a model proposed earlier for the 197Au isomer shift in Au based materials by Miedema and van der Woude (1980). In combining Gd with s, p metals (Al, In, Zn, Pb, Sn) the s electron character at the Gd sites is decreased while the d electron character is increased. Upon compounding Gd with Cu or 3d metals, both the s and d electron characters are decreased. In the cubic C15 type compounds RM2, the decrease of the d character is strong in particular when M is Ni, Co or Fe. For most of the compounds investigated the 155Gd hyperfine fields were also determined.

Investigations of Dy(Fe xAl 1−x) 2 laves phases by x-ray, magnetometric and Mössbauer effect methods

The crystal and magnetic properties of the Dy(Fe x Al 1- x ) 2 (0≤ x≤1.0) system are investigated with the X-ray, magnetometric and Mössbauer effect methods. The system constitutes two cubic C15 structure regions separated by hexagonal C14 structure region. The X-ray investigations indicate the structure disorder of the C15 type phases. All compounds of the investigated system are ferrimagnets. Some of them exhibit a pronounced magnetic anisotropy. Substitution of Al for Fe decreases the Curie temperature and the Fe hyperfine magnetic field. The distribution of hyperfine magnetic field at iron sites is correlated with their nearest neighbours configurations. The Mössbauer effect data indicate the existence of the appreciable amount of the covalent apart from the long-range metallic bond in the investigated phases.

Hyperfine field anisotropy of Co in C15 RCo2 compounds (R = rare earth)

Results of 59Co NMR investigation on the C15 type RCo2 compounds (R = rare earth) in the ferro-, ferri-, and paramagnetic states are reviewed. An enormous anisotropy in the 59Co hyperfine fields, which results in antiparallel hyperfine fields at two magnetically inequivalent Co sites in RCo2 compounds with the [111] or [011] easy axis, is attributed to unquenched orbital moment of Co atoms. Analysis of temperature dependence of 59Co Knight shifts reveals the important role of the spin-orbit coupling. Observation of well-resolved satellite lines in Tb1−xYxCo2 yields an estimate of transferred hyperfine fields due to the rare earth spins as +42 kOe in TbCo2 at 4.2 K, where the positive direction is chosen to be the direction of the Co moment. From consideration of the sign and the magnitude of lattice dipole fields, Lorentz fields, and the transferred fields, the fraction of the orbital moment on Co in the RCo2 series is found to vary from 3% in GdCo2 to 26% in PrCo2.

Magnetic behavior of cubic laves phase RMn 2 ( rmR = Gd, DyandHo) compounds

Magnetization studies have been carried out on the cubic Laves phase (C15) type) intermetallic compounds GdMn 2, DyMn 2 and HoMn 2 at various applied fields in the temperature range 4.2 to 300 K. All compounds show evidence of magnetic ordering at low temperatures. A peak in the magnetization versus temperature curve is observed in zero field cooled samples of GdMn 2 and DyMn 2 but not in HoMn 2. This is taken to imply ferrimagnetic ordering between the rare earth and Mn moments. The Mn moment is estimated to be approximately 2μ B in GdMn 2. It appears that Mn carried no localized moment in HoMn 2. An anomaly in magnetization of GdMn 2 is observed around 95 K. The Mn moment appears to decrease in RMn 2 compounds while crossing the rare earth series from Gd to Ho. No Mn moment is known in hexagonal Laves phases (C14 type) compounds ErMn 2 and TmMn 2. A virtual bound state picture of Mn-3d electrons is suggested to explain the structure change and the Mn moment collapse in the RMn 2 compounds.

Composition and hydrogen absorption of C14 type ZrMn compounds

The hexagonal C14 type compound ZrMn 2 has been found to have a homogeneity region extending from ZrMn 1.8 to ZrMn 3.4. We studied the hydrogen absorption characteristics of various of these compounds at 50°C and H 2 pressures up to 40 atm. Several of the compounds, when not annealed before charging, absorb the H 2 gas in two distinct steps. Only a single plateau pressure was observed if the samples were annealed prior to the H 2 take-up. The changes in magnetic properties accompanying the hydrogen absorption were studied in ZrMn 1.8 and ZrMn 3.2.

Fe57Mössbauer effect in ScFe2and ScFe2H2

The intermetallic compound Sc${\mathrm{Fe}}_{2}$ is able to absorb large quantities of hydrogen gas without a change in crystal structure. The ternary hydride has the composition Sc${\mathrm{Fe}}_{2}$${\mathrm{H}}_{2}$. Its lattice constants are $a=5.279$ \AA{} and $c=8.507$ \AA{} (C14 type). By means of magnetic measurements we found that the moment per Fe atom changes from $1.4{\ensuremath{\mu}}_{B}$ to $2.2{\ensuremath{\mu}}_{B}$. Using $^{57}\mathrm{Fe}$ M\"ossbauer-effect spectroscopy we observed that an increase of the same magnitude (about 60%) also occurs in the hyperfine splitting. On the other hand, only a rather small change in isomer shift was observed to accompany the hydrogen absorption. Arguments are presented that it is difficult to explain the moment change in terms of a simple $3d$-band picture and charge-transfer effects.