Hexagonal MgZn2 Research Articles

On new rare-earth compounds LnIrGa and LnRhGa

Compounds of nominal composition LnIrGa with Ln = La ··· Nd, Gd ··· Ho, Tm and some LnRhGa were found to crystallize in the orthorhombic TiNiSi-type structure (ordered Co 2Si type) whereas off-stoichiometric ‘ScIrGa’ was obtained in the hexagonal MgZn 2-type structure. Ferromagnetic and antiferromagnetic ordering was observed with ordering temperatures of 4 to 66 K. The valence of Ce in CeIrGa is between three and four at room temperature, and close to four in CeRhGa.

Iron-bearing precipitates in Zircaloys: a Mössbauer spectroscopy study

Mössbauer spectroscopy of 57Fe was used to study the chemical form and distribution of iron in Zircaloy-2 and Zircaloy-4. The spectra of Zircaloy-4 consist of two overlapping quadrupole-split doublets, which can be ascribed to precipitates of the pseudobinary Laves-phase intermetallic compound Zr(Cr,Fe) 2, which has a hexagonal MgZn 2 (C14) type structure and cubic MgCu 2 (C15) type structure. In Zircaloy-2, a large fraction of iron was also found in the form of Zr 2(Fe,Ni) precipitates, having a tetragonal Al 2Cu (C16) type structure. Characteristic room-temperature values of Mössbauer spectral parameters were determined for the three types of intermetallic precipitates. At liquid helium temperature, both alloys exhibit a poorly resolved magnetic hyperfine splitting, attributed to the low-temperature ferromagnetism of cubic Zr(Cr,Fe) 2 Laves-phase particles. This effect is more pronounced in Zircaloy-4 than in Zircaloy-2. The possibility of determining volume fractions of different phases from their relative spectral areas is discussed.

On new rare-earth iridium aluminides LnIrAl

The compounds LnIrAl with Ln = Ce … Nd, Sm, Gd … Tm, Lu, Y, were found to crystallize in the orthorhombic TiNiSi-type structure (ordered Co 2Si type), wheres ScIrAl was obtained in the hexagonal MgZn 2-type structure. Ferromagnetic and antiferromagnetic ordering was observed with ordering temperatures of 7 to 67 K. Ce in CeIrAl is close to tetravalent.

On the rare-earth palladium aluminides LnPdAl

A hexagonal ZrNiAl- or Fe2P-type high-temperature modification was identified for the compounds LnPdAl with LnSm, Gd ... Tm, Lu, Y. YbPdAl and its Pt analogue were obtained only in the orthorhombic TiNiSi structure, LaPdAl only in the orthorhombic LaNiAl structure. CePdAl transformed to an unknown low-temperature modification, while PrPdAl and NdPdAl retained the ZrNiAl structure on annealing at 700 °C. ScPdAl crystallizes in the hexagonal MgZn2 structure. Antiferromagnetic ordering was observed in hexagonal TbPdAl, whereas DyPdAl and HoPdAl appear to undergo a transition to ferromagnetic order.

The influence of Si and Ge on the hydrogen sorption properties of the intermetallic compound ZrCr 2

X-Ray powder diffraction studies of the Zr(Cr 1- x T x ) 2 (TSi, Ge) systems revealed that chromium can be replaced by silicon or germanium up to Zr(Cr 0.75Si 0.25) 2 or Zr(Cr 0.875Ge 0.125) 2 without changing the hexagonal MgZn 2 structure type of ZrCr 2. It was observed that all ternary alloys react readily with hydrogen to form hydrides containing 1.7–2.5 hydrogen atoms per formula unit under a hydrogen pressure of 0.1 MPa at room temperature. This is significantly lower than the value of 3.4 absorbed hydrogen atoms per formula unit found for the prototype binary compound ZrCr 2 under the same conditions of pressure and temperature. Pressure-composition hydrogen desorption isotherms were determined over the temperature range 296–473 K and it was found that the replacement of chromium in ZrCr 2 by silicon or germanium increases the hydrogen dissociation equilibrium pressure of ZrCr 2. The hydrogen sorption properties of the systems described in this paper are compared with those of several related systems based on ZrCr 2 and discussed briefly.

Structural, magnetic and magnetostrictive studies of Tb 0.27Dy 0.73(Fe 1 − xAl x) 2

Measurements of magnetic properties, X-ray diffraction and magnetostriction were made on Tb 0.27Dy 0.73(Fe 1 − x Al x ) 2 ( x = 0.1, 0.2, …, 0.7) compounds. It was found that the system has the cubic MgCu 2 structure over almost the whole (Fe,Al) concentration range investigated, except for a narrow intermediate range ( x = 0.4–0.6) where the hexagonal MgZn 2 structure appears. With increasing Al content x, the lattice constant a increases linearly with x. The first replacement of Fe results in a marked decrease in the Curie temperature, which is followed by a slight decrease in T C with x. A linear decrease in magnetostriction of | λ | − λ ⊥| at room temperature with x was also observed from 1530 × 10 −6 for x=0 to 36×10 −6 for x=0.3. The saturation magnetization σ s exhibits a complex concentration dependence in the Tb 0.27Dy 0.73(Fe) 1 − x Al x) 2 system: in the range x < 0.5, σ s increases linearly with x and, for x = 0.5–0.6, σ s decreases and then increases again. An enhancement of the magnetic ‘hardness’ in this system was also observed at low temperature.

ChemInform Abstract: Structure-Bonding Relationships in the Laves Phases.

The Laves phase structures (MgCu2 MgZn2 and MgNi2 types) are adopted by a large number of binary intermetallic compounds of composition AB2. These phases belong to the class of tetrahedrally closepacked alloys. Two of the structural features of the Laves phases which have aroused interest are, firstly the dependence of the structure adopted (especially the choice between the cubic MgCu2 and hexagonal MgZn2 types) on the number of valence electrons of the constituent metal atoms and, secondly the structural distortions observed in a number of the hexagonal MgZn2 type phases. By performing extended Huckel band calculations on model AB2 compounds with both the cubic and hexagonal structures and on the B sub-nets alone, structure electron-count and deformation-electron count correlations are derived. These correlations depend on the highly peaked nature of the density of states of the Laves phases, primarily due to the B network.

Structure‐Bonding Relationships in the Laves Phases

AbstractThe Laves phase structures (MgCu2 MgZn2 and MgNi2 types) are adopted by a large number of binary intermetallic compounds of composition AB2. These phases belong to the class of tetrahedrally closepacked alloys. Two of the structural features of the Laves phases which have aroused interest are, firstly the dependence of the structure adopted (especially the choice between the cubic MgCu2 and hexagonal MgZn2 types) on the number of valence electrons of the constituent metal atoms and, secondly the structural distortions observed in a number of the hexagonal MgZn2 type phases. By performing extended Hückel band calculations on model AB2 compounds with both the cubic and hexagonal structures and on the B sub‐nets alone, structure electron‐count and deformation‐electron count correlations are derived. These correlations depend on the highly peaked nature of the density of states of the Laves phases, primarily due to the B network.

On the structural and hydrogen desorption properties of the Zr(Cr 1− xNi x) 2 alloys

X-ray powder diffraction studies of the Zr(Cr 1− x Ni x ) 2 system revealed that chromium can be replaced by nickel up to the composition Zr(Cr 0.5Ni 0.5) 2 without changing the basic hexagonal MgZn 2-type structure of ZrCr 2. It was also found that all these alloys react readily with hydrogen to form hydrides having about 3.4 hydrogen atoms per formula unit under a hydrogen pressure of 0.81 MPa at room temperature. Pressure-composition hydrogen desorption isotherms were determined over the temperature range 296–473 K. In general, the dissociation equilibrium pressure increases with increasing nickel content; for example, at 353 K it rises from 5 kPa for the ZrCr 2-H 2 system to 50 kPa for the Zr(Cr 0.5Ni 0.5) 2-H 2 system. The unit cell volumes of the hydrides are about 21% larger than those of the corresponding alloys.

X-ray diffraction and magnetic susceptibility studies of Lu(Fe xAl 1- x) 2 compounds (0.55 ⩽ χ ⩽ 0.65)

Powdered LuFe 1.1Al 0.9 and LuFe 1.2Al 0.8 samples analysed by X-ray diffraction have the hexagonal MgZn 2 structure, whereas the structure of LuFe 1.3Al 0.7 sample is of the cubic MgCu 2 type. A tendency for the Fe atoms to occupy the 6h sites in hexagonal structure samples was observed. The temperature dependence of magnetic susceptibility measurements χ( T) exhibit a cusp-like peak (at T M) for all examined samples. Moreover, below T M the χ( T) runs depend on the cooling process (without and with a magnetic field applied prior to the measurement). This behaviour of the magnetic susceptibility suggests that all the investigated compounds are cluster glasses.

On friauf-laves phases in the Ta 1 − xAl xT 2 and T 1 − xSi xT 2 (T  Cr, Mn, Fe, Co, Ni) systems

A study of the substitution of tantalum in TaT 2 (T ≡ Cr, Mn, Fe, Co, Ni) by aluminium or silicon was carried out using X-ray and metallographic techniques. It was observed that alloys with a Friauf-Laves structure exist up to the compositions Ta 0.5Al 0.5Cr 2, Ta 0.4Si 0.6Cr 2, Ta 0.5Al 0.5Fe 2, Ta 0.4-Si 0.6Fe 2, Ta 0.6Al 0.4Co 2 and Ta 0.5Si 0.5Co 2, or are separated from their binary prototypes and span the composition regions Ta 0.8Al 0.2Mn 2 to Ta 0.7Al 0.3Mn 2 and Ta 0.8Si 0.2Mn 2 to Ta 0.6Si 0.4Mn 2. In the systems Ta 1 − x Al x Ni 2 and Ta 1 − x -Si x Ni 2 (TaNi 2 does not exist) the single-phase alloys were observed only within the composition range Ta 0.8Si 0.2Ni 2 to Ta 0.7Si 0.3Ni 2. Most of the diffraction patterns were indexed on the basis of the hexagonal MgZn 2-type structure (space group P 6 3/ mmc). The cubic MgCu 2-type structure (space group Fd3m) and the hexagonal MgNi 2 structure (space group P6 3/ mmc) were observed in the systems with cobalt. The rather unusual behaviour of the lattice parameters and composition curves necessitated a detailed analysis of interatomic distances. From these results it was concluded that the substructure of the atoms of the B component, especially within the Kagomé net, plays a major role in the stabilization of these phases.

Structural and magnetic properties of Lu(Fe xAl 1− x) 2 ( x ≤ 0.5) compounds

The results of X-ray diffraction studies show that in the aluminium-rich system Lu(Fe x Al 1 − x ) 2 the cubic MgCu 2 type structure is stable up to x = 0.2. The X-ray line broadening observed for the compounds with x = 0.1, 0.2 is caused solely by the smeared-out values of the lattice parameters. The structure of the LuFeAl compound is of the hexagonal MgZn 2 type. The magnetic properties for the investigated cubic aluminium-rich Lu(Fe x Al 1 − x ) 2and Y(Fe x Al 1 − x ) 2 compounds were found to be similar. A maximum at T ≈ 8 K was observed in the susceptibility vs. temperature curve for the LuFeAl compound.

Y4Ir9Si5, a new substitution variant of the Ce2Ni7 type and isotypes of other members of the R2+nT3+3nM1+2n structure series

Y4Irg+xSi5_ x, x=0.12 (2), Mr= 2226 for Y4IrgSi 5, hP36, P63/mmc, a = 5.4894 (2), c = 22.6054 (9) A, V= 589-93 (6) A 3, Z= 2, Dx= 12.53Mgm -3 (Y4Ir9Sis), MoKa, :t = 0. 71073 A, :t =127.4mm -~, F(000)=1838, T=293K, wR= 0.061 for 308 independent reflections. The Y4IrgSi 5 type is a site-occupation variant of the Ce2Ni 7 type. It is a new member (n- 2) of the hexagonal branch of the structure series R2+nT3+3nll/11+2n , grouping types where ternary Laves-type slabs are intergrown with ternary CeCo3B2-type slabs. ErRh2Si, ErRh2Ge, YIr2Si and YRh2Ge were found to crystallize with the YRh2Si type (site-occupation variant of the CeNi 3 type), Er4RhgGe 5 and Y4RhgGe5 with the Y4RhgSi 5 type (site-occupation variant of the Gd2Co 7 type). The former structure type belongs to the hexagonal and the latter to the rhombohedral branch of the same structure series. Introduction. The well known Laves types and the CaCu 5 type are widely spread among binary com- pounds. In several R-T systems (R = rare-earth ele- ment, T= transition metal), structures where one Laves-type slab (R2T4) is intergrown with n CaCu:type slabs (RT 5) have been found at intermediate com- positions. The R2+nT4+sn structure series grouping these structures can be subdivided into two branches depend- ing on the stacking of the Laves-type slabs. In the rhombohedral branch the Laves-type slabs are stacked as in the cubic Laves-type MgCu 2 and in the hexagonal branch the same slabs are stacked as in the hexagonal MgZn 2 type (Parth6 & Lemaire, 1975). Simple ordered substitution variants with composition R2T3M (M = main-group element) are known for both Laves types: Mg2Cu3Si (Witte, 1939) with MgZn2-type stacking and Mg2Ni3Si (Nortus, Eriksson, Gtthe & Werner, 1985)or Y2RhaGe (Cenzual, Chabot & Partht, 1987) with MgCu2-type stacking. CeCoaB 2 (Kuz'ma, Kripyakevich & Bilonizhko, 1969) is a ternary ordered substitution variant of CaCu 5 with composition R T3M 2. Three ternary structure series can then be imagined where one or both kinds of slabs are ternary. The members of these series are substitution variants of the members of the binary R2÷nT4÷5~ structure series. In reality, several borides have been found to crystallize

On Friauf-Laves phases in the Zr 1− xAl xT 2, Zr 1− xSi xT 2 and Zr 1− xTi xT 2 (T ≡ Mn, Fe, Co) systems

A large number of samples of the general composition Zr 1− x Al x T 2, Zr 1− x Si x T 2 and Zr 1− x Ti x T 2 (T ≡ Mn, Fe, Co) were prepared and investigated using X-ray diffractometry and metallography. It was found that in binary Friauf-Laves phases, ZrT 2, zirconium can be replaced by aluminium or silicon up to the compositions Zr 0.7A1 0.3Mn 2, Zr 0.7Si 0.3Mn 2, Zr 0.6Al 0.4Fe 2, Zr 0.5Si 0.5Fe 2, Zr 0.4Al 0.6Co 2 and Zr 0.6Si 0.4Co 2. In all these systems the crystal structure of the prototype compounds (the cubic MgCu 2 or the hexagonal MgZn 2 type) is preserved. In the Zr 1− x Ti xT 2 systems, continuous solid solutions were formed throughout the investigated region and their crystal structures depend on the composition and/or heat treatment. An ordering phenomenon was observed only in the Zr 1− x AL x Co 2 system, where a superstructure of the MgSnCu 4 type was formed at the composition Zr 0.5A1 0.5Co 2. The stability of these phases is briefly discussed in terms of the relative atomic sizes.

Metal-metalloid exchange in the Zr 1− xM xMo 2 (M ≡; Ge, Si, Al) system

An extensive study of the possibility of substituting zirconium in ZrMo 2 by germanium, silicon or aluminium was carried out using X-ray diffractometry and metallography. In the Zr 1− x Ge x Mo 2 system the crystal structure of the prototype compound ZrMo 2 (cubic Friauf-Laves phase; space group, Fd3 m) was retained up to the composition Zr 0.8Ge 0.2Mo 2. Substitution in the Zr 1− x Si x Mo 2 and Zr 1− x Al x Mo 2 systems was possible up to the compositions Zr 0.75Si 0.25Mo 2 and Zr 0.3A1{0.7}Mo 2, respectively, but the cubic crystal structure transformed into a hexagonal MgZn 2-type structure (space group, P6 3 mmc ). This structure was stabilized if the substituent contents were in excess of 5 at.% Si or 6.7 at.% Al. A high temperature b.c.c. ternary solid solution was also observed in all the systems investigated. The unit cell parameters, cell volumes and interatomic distances were determined for the observed phases of all three systems. Some semiquantitative correlations of these phases with those found in the related Hf 1− x M x Mo 2 (M ≡; Ti, Ge, Si, Al), Zr 1− x Ti x Mo 2, Zr 1− x M x W 2 and Hf 1− x M x W 2 (M ≡; Ti, Si, Al) systems were also made.

Crystal structure, electron-to-atom ratio and hydrogen storage capacity of zirconium based intermetallic laves phases

Laves phases of the type AB 2, where A is zirconium, have potential for use as hydrogen storage materials. These Laves phases have one of two different crystal structures, namely the C15 (cubic MgCu 2-type) or the C14 (hexagonal MgZn 2-type) Strukterbericht types. Data are presented on the lattice parameters for the two phases and relationships are developed between the lattice parameters of the two phases both for the binary (ZrB 2) and the pseudobinary (Zr(B′ xB″ 1−x)2) Laves phases. Electron-to-atom ratios are calculated for both the binary and pseudobinary Laves phases and these are related to hydrogen storage capacity (hydrogen-to-metal ratio, H/M). The hydrogen-to-metal ratio is shown to decrease with increasing electron-to-atom ratio.

Site occupancies and stoichiometries in hydrides of intermetallic compounds: Geometric considerations

Hole sizes and intersite distances were calculated at various hydrogen concentrations for the interstices in hydrides of TiCr 1.8, ZrTi 2, ZrV 2, ZrCr 2, ZrFe 2, HfV 2 and TaV 2 with the cubic MgCu 2 C15 structure and of ZrMn 2 and ErMn 2 with the hexagonal MgZn 2 C14 structure. From a survey of other metallic hydrides, empirical criteria were chosen for the minimum hole size (0.40 Å) and the minimum H-H distance (2.1 Å) for hydrogen atom occupancy in stable hydrides. With these criteria, a model was developed for predicting which sites and how many of them could be occupied by hydrogen atoms in each of the intermetallic compounds listed. The resulting stoichiometries were determined by experimentally filling the interstices of model lattices constructed of Friauf polyhedra without violating either of the adopted geometric criteria. For each of the compounds rationales are presented to explain the experimental observations reported by other researchers.

Ferromagnetism in Fe2Sc with the Hexagonal MgZn2-Type Structure

Ferromagnetism in Fe₂Sc with the Hexagonal MgZn₂-Type Structure

The effect of high pressure on the formation of LRu 2 and LOs 2 (LLanthanide) compounds

The compounds SmRu 2, NdRu 2 and LaOs 2 have the cubic MgCu2-type structure when prepared at atmospheric pressure. By application of pressures up to 88 kbar, these compounds were prepared with the hexagonal MgZn 2-type structure. The hexagonal lattice parameters ( a, c) are (5.298, 8.939), (5.323, 9.004), and (5.42, 9.00), respectively. The formation of these hexagonal polymorphs is attributed to the effect of pressure on the relative atomic size of the lanthanide vs. that of Ru or Os. The amount of 4 f bonding in these compounds has no influence on the type of polymorph formed.

Electronic Specific Heats of Alloys of the MgCu2−xZnx System

Specific-heat measurements were made on 17 alloys of the pseudobinary system MgCu2−xZnx between 1.5 and 4.2°K. In addition to the two terminal phases MgCu2 and MgZn2, the compositions studied included nine alloys having the cubic MgCu2 structure, four having the hexagonal MgZn2 structure, and two samples consisting of a mixture of two phases. The density of states at the Fermi level N(EF) was calculated from the measured values of γ. The variation of γ with composition is compared with results from this laboratory previously reported on the MgCu2−xAlx and Mg2−xSix systems. Variation of γ with composition in the cubic range exhibited, qualitatively, the same trends that were shown in the MgCu2−xAlx system, namely, a maximum in γ at 1.44 electrons/atom, a rapid decrease in γ as the stability limit for the cubic phase at ≈ 1.73 electrons/atom is approached, and a shoulder on the falling curve at 1.59 electrons/atom. Values of the Debye θ, calculated from the slopes of the curves of C / T vs T2 are computed for the three systems.