Hexagonal MgZn2 Research Articles

Synthesis, Physical, and Magnetocaloric Properties of MgZn2-Type Gd2Al3Rh.

Gd2Al3Rh was synthesized from the elements using arc-melting techniques, along with subsequent annealing. The title compound adopts the hexagonal MgZn2 type structure (space group P63/mmc, hP12, Wyckoff sequence hfa) with the Gd atoms found on the Mg positions (4f), the Rh (2a) and Al (6h) atoms occupying the two Zn sites of the prototype. In addition, mixing of Rh and Al on both crystallographic positions is observed. The magnetic susceptibility and magnetization experiments were conducted indicating ferromagnetic ordering below the Curie temperature of TC = 51.3(1) K and very high magnetization of 6.96(1) μB at 3 K and 70 kOe. In addition, heat capacity and electrical resistivity measurements were conducted. The magnetocaloric properties of ferromagnetic Gd2Al3Rh have been determined by means of magnetization measurements. For a field change of ΔH = 0 → 50 kOe the magnetic entropy change equals ΔSMmax = -4.8 J kg-1 K-1, leading to a relative cooling power of RCP = 283 J kg-1 for the same field change. The adiabatic temperature change was estimated to be ΔTadmax = 2.1 K using the heat capacity data.

Searching for Laves Phase Superstructures: Structural and 27Al NMR Spectroscopic Investigations in the Hf-V-Al System.

Laves phases exhibit a plethora of different structures and a multitude of physical properties. Investigations in the ternary system Hf-V-Al led to the discovery of numerous members of the solid solution Hf(V1-xAlx)2, which adopt the hexagonal MgZn2 type (C14) for medium to high amounts of Al (x = 0.2-1) and the cubic MgCu2 type (C15) for small Al amounts (x = 0.05-0.1). While all members exhibit Pauli-paramagnetic behavior due to the absence of localized magnetic moments, the V-rich cubic member Hf(V0.95Al0.05)2 additionally exhibits a superconducting state below TC = 7.6(1) K. All synthesized compounds were characterized by powder X-ray diffraction, and selected samples were furthermore investigated by 27Al solid-state magic-angle spinning (MAS) NMR. HfAl2 exhibits two Al resonances, one rather sharp and one significantly broadened signal, in line with the crystal structure and respective coordination environments. The members of the solid solution exhibit extremely broadened resonances due to the mixing of V and Al on the same crystallographic sites. For nominal Hf(V0.125Al0.875)2, however, two distinct sharp NMR signals were observed. This contrasts with the description of a solid solution. Therefore, single-crystal X-ray studies were conducted, showing that Hf(V0.125Al0.875)2 really is an ordered compound with the sum formula Hf4VAl7 (P3̅m1), which exhibits an, thus far, unknown superstructure of MgZn2.

Laves type intermetallic compounds as hydrogen storage materials: A review

Laves type AB 2 intermetallics belong to the most abundant group of intermetallic compounds containing over 1000 compounds. A large variety of the chemical nature of A (Mg, Ca, Ti, Zr, Rare Earth Metals) and B (V, Cr, Mn, Fe, Co, Ni, Al) metals together with the existence of the extended solid solutions formed by mixing various selected components on both A and B sites dramatically extends the list the known binary and ternary individual compounds. A vast majority of the Laves type intermetallics crystallises with C15 / FCC MgCu 2 and C14 / hexagonal MgZn 2 types of structures, both formed for a large range of ratios between the atomic radii of the A and B components outside the ideal ratio r A / r B = 1.225. Their hydrogenation performance is defined by the chemical composition and structure of the alloys and proceeds according to the following alternative / parallel mechanisms: (a) Formation of the insertion type interstitial hydrides containing up to 6–7 at. H/f.u.AB 2 ; (b) Amorphisation of the alloys on hydrogenation; (c) Disproportionation with the formation of a binary hydride of the A metal and depleted by A metal B-components based alloys/hydrides. Equilibrium pressures of hydrogen desorption from the AB 2 -type hydrides span a huge range of ten orders of magnitude and thus Laves type-based intermetallics satisfy the requirements for various applications including getters of hydrogen gas, volume- and mass-efficient hydrogen storage materials operating at ambient conditions, materials for the efficient thermally driven compression of hydrogen gas with an output pressure of several hundred bar and high capacity and high rate anode materials for the metal hydride batteries operating in a challenging temperature range - at subzero temperatures and also above 60 °C. The paper contains references to 245 publications and will guide the future work in the areas of fundamental research and also in advancing the applications of the hydrides of the Laves type intermetallics. • Laves type AB 2 intermetallics are the most abundant group of alloys. • Hydrogenation performance is defined by the chemical composition and structure. • Mechanisms of hydrogenation: interstitial hydrides / amorphization / disproportionation. • Structure-properties relationships are described. • Important applications are reviewed: H stores/MH compression/MH batteries/H getters.

Argon-neon binary diagram and ArNe2 Laves phase.

Mixtures of argon and neon have been experimentally studied under high pressure. One stoichiometric compound, with ArNe2 composition, is observed in this system. It is a Laves phase with a hexagonal MgZn2 structure, stable up to at least 65 GPa, the highest pressure reached in the experiments. Its equation of state follows closely the one of an ideal Ar+2Ne mixture. The binary phase diagram of the Ar-Ne system resembles the diagram predicted for hard sphere mixtures with a similar atomic radius ratio, suggesting that no electronic interactions appear in this system in this pressure range. ArNe2 can be a convenient quasihydrostatic pressure transmitting medium under moderate pressure.

Zn‐rich Zincides of the Ternary System Ca–Mg–Zn: Synthesis, Crystal structure, Chemical Bonding

In the course of a study on the role of magnesium in polar zincides of the heavier alkaline‐earth elements, three intermetallic phases of the ternary system Ca–Mg–Zn were synthesized from melts of the elements and their structures were determined by means of single‐crystal X‐ray data. Starting from the binary zincide CaZn11, the phase width of the BaCd11‐type structure reaches up to the fully ordered stoichiometric compound CaMgZn10 [tI48, space group I41/amd, a = 1082.66(6), c = 688.95(5) pm, Z = 4, R1 = 0.0239]. The new compound CaMgZn5 (oP28, space group Pnma, a = 867.48(3), b = 530.37(5), c = 1104.45(9) pm, Z = 4, R1 = 0.0385) crystallizes in the CeCu6‐type structure, exhibits no Mg/Zn phase width and has no binary border equivalent in the system Ca–Mg–Zn. Similar to the situation in CaMgZn10, one M position of the aristotype has a slightly larger coordination sphere (CN = 14) and is accordingly occupied by the larger Mg atoms. The third phase, Ca2+xMg6–x–yZn15+y (hP92, space group P63/mmc, a = 1476.00(5), c = 881.01(4) pm, Z = 4, R1 = 0.0399 for Ca2.67Mg5.18Zn15.15) forms the hexagonal Sm3Mg13Zn30‐type structure also known as μ‐MgZnRE or S phase. A small phase width (x = 0–0.67; y = 0–0.58) is due to the slightly variable Ca or Zn content of the two Mg positions. The structure is described as an intergrowth of the hexagonal MgZn2 Laves phase and the CaZn2 structure (KHg2‐type). All compounds exhibit strong Zn–Zn and polar Mg–Zn covalent bonds, which are visible in the calculated electron density maps. Their structures are thus herein described using the full space tilings of [Zn4] and [MgZn3] tetrahedra, which are fused to polyanions consisting of tetrahedra stars, icosahedra segments etc. and the large (CN = 18–22) Ca cation coordination polyhedra. Pseudo bandgaps apparent in the tDOS are compatible with the narrow v.e./M ranges observed for other isotypic members of the three structure types.

CuxCrFeMoTi (x = 0.21, 0.44, 1) high entropy alloys as novel materials for fusion applications

High entropy alloys of type CuxCrFeMoTi (x = 0.21, 0.44, 1) were produced by arc melting and characterized using scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy, powder X-ray diffraction and electron backscattered diffraction. The observations revealed that all samples are multiphasic evidencing Cu-rich dendritic structures. The diffractograms of the samples indicate the presence of hexagonal type MgZn2, cubic type MgCu2 Laves phases and peaks of BCC structure. Electron backscattered diffraction results indicated that the Cr, Ti and Fe-rich dominant phase corresponds to the hexagonal Laves phase of type MgZn2 while the Cu-rich dendritic phase is related to the BCC cubic structure. In order to study the thermal diffusivity of these materials and their application as thermal barriers, a sample with nominal Cu0.17CrFeMo0.17Ti composition was prepared. This material shows low thermal diffusivity values in the 425–875 K temperature range.

Structure and Stoichiometry of MgxZny in Hot-Dipped Zn–Mg–Al Coating Layer on Interstitial-Free Steel

Correlations of stoichiometry and phase structure of MgxZny in hot-dipped Zn–Mg–Al coating layer which were modified by additive element have been established on the bases of diffraction and phase transformation principles. X-ray diffraction (XRD) results showed that MgxZny in the Zn–Mg–Al coating layers consist of Mg2Zn11 and MgZn2. The additive elements had a significant effect on the phase fraction of Mg2Zn11 while the Mg/Al ratio had a negligible effect. Transmission electron microscope (TEM) assisted selected area electron diffraction (SAED) results of small areas MgxZny were indexed dominantly as MgZn2 which have different Mg/Zn stoichiometry between 0.10 and 0.18. It is assumed that the MgxZny have deviated stoichiometry of the phase structure with additive element. The deviated Mg2Zn11 phase structure was interpreted as base-centered orthorhombic by applying two theoretical validity: a structure factor rule explained why the base-centered orthorhombic Mg2Zn11 has less reciprocal lattice reflections in the SAED compared to hexagonal MgZn2, and a phase transformation model elucidated its reasonable lattice point sharing of the corresponding unit cell during hexagonal MgZn2 (a, b = 0.5252 nm, c = 0.8577 nm) transform to intermediate tetragonal and final base-centered orthorhombic Mg2Zn11 (a = 0.8575 nm, b = 0.8874 nm, c = 0.8771 nm) in the equilibrium state.

First principles prediction of a new high-pressure phase and transport properties of Mg2Si

We have investigated the structural properties of seven different structure types of Mg2Si which include the cubic CaF2, orthorhombic PbCl2, hexagonal Ni2In, tetragonal Al2Cu, Laves phase (cubic MgCu2), hexagonal MgZn2 and dihexagonal MgNi2 type of structures, using a full potential linearized augmented plane wave method as implemented in WIEN2k within the framework of density functional theory. The exchange–correlation potential is treated by the new form of generalized gradient approximation (GGA-PBEsol). In total energy calculations it is clearly seen that cubic CaF2-type structure is stable at ambient conditions, and it undergoes a first-order phase transition to orthorhombic PbCl2-type, then to the hexagonal Ni2In-type structure and finally to the cubic Laves phase MgCu2-type. A new structure type is predicted to be stable at high pressure. Moreover, we intend to combine the electronic structure calculations performed by mean of generalized gradient approximation and modified Becke–Johnson potential with Boltzmann transport theory as incorporated in BoltzTraP code to interpret and predict the thermoelectric performance of each stable phase as a function of the chemical potential at various temperatures. We find a high thermoelectric thermopower values in cubic CaF2-type structure that could promise an excellent candidate for potential thermoelectric applications.

Contribution on the phase equilibria in Zr–Nb–Fe system

Phase equilibria of the Zr–Nb–Fe system at 700 °C were determined by using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). Previously reported ternary compounds, cubic Ti2Ni type (ZrNb)2Fe and hexagonal MgZn2 type Zr(NbFe)2, were confirmed. In the zirconium rich corner, equilibria of αZr with βZr and (ZrNb)2Fe, βZr with (ZrNb)2Fe and Zr(NbFe)2, and β-Zr with Zr(Nb,Fe)2 and βNb, were determined. Two four-phase invariant reactions, (ZrNb)2Fe + βZr ↔ αZr + Zr(NbFe)2 and βZr ↔ αZr + Zr(NbFe)2 + βNb, were revealed. A partial reaction scheme was proposed for the zirconium rich alloys. Close to the Nb–Fe side, NbFe phase were revealed to co-exist with (Zr1−xNbx)2Fe rather than Zr(NbFe)2.

Magnetism and large magnetocaloric effect in HoFe2−xAlx

The structural, magnetic and magnetocaloric properties of several HoFe2−xAlx compounds were investigated. The compounds in the Fe-rich region (0.36⩽x⩽0.4) crystallize in the cubic MgCu2 (C15) structure, while for the ones in the intermediate region (0.75⩽x⩽1.125) the hexagonal MgZn2 (C14) structure was observed. Electronic structure calculations were performed, showing a good agreement between theory and experiment. The Curie temperatures were found to decrease with Al content. For the Fe-rich compounds, these are close to room temperature, while for the compounds in the intermediate region, transition temperatures are well below 300K. No magnetic hysteresis was found around the Curie temperature for applied magnetic fields of up to 4T. All of the investigated compounds undergo a second-order magnetic phase transition at the Curie temperature. A maximum magnetic entropy change value of 7.6J/kgK was obtained for the sample with x=1.125, all of the samples displaying rather large RCP values. The possibility of incorporating these materials in magnetic refrigeration devices is discussed.

Transmission Electron Microscopy Observations on the Precipitates in Al-Zn-Mg Based Sacrificial Anode Alloy

nanoscale precipitates in the Al-Zn-In-Mg-Ti-Mn alloy were investigated by regular and high resolution transmission electron microscopy (TEM and HRTEM) and by selective area electron diffraction (SAED). The cast alloy contains the hexagonal structure MgZn2 and the orthorhombic structure Al6Mn precipitates, but after aged the precipitates of the cubic structure Mg2Zn11 were found. The Mg2Zn11 precipitates can act as activation centre, make the alloy pitting. But the activation of Al6Mn precipitates is very little.

HRTEM Investigation of Precipitates in Al-Zn-In-Mg-Ti-Ce Anode Alloy

The precipitates of Al-5Zn-0.02In-1Mg-0.05Ti-0.5Ce (wt %) anode alloy were studied by scanning electron microscopy, X-ray microanalysis, high resolution transmission electron microscopy and selected area electron diffraction analyses in the present work. The results show that the alloy mainly contains hexagonal structure MgZn2 and tetragonal structure Al2CeZn2 precipitates. From high resolution transmission electron microscopy and selected area electron diffraction, aluminium, Al2CeZn2 and MgZn2 phases have [0 1 -1]Al|| [1 -10]Al2CeZn2|| [-1 1 0 1]MgZn2orientation relation, and Al2CeZn2 and MgZn2 phases have the [0 2 -1]Al2CeZn2|| [0 1 -10]MgZn2orientation relation.

Orientation relationships between icosahedral clusters in hexagonal MgZn2 and monoclinic Mg4Zn7 phases in Mg-Zn(-Y) alloys

Intermetallic precipitates formed in heat-treated and aged Mg-Zn and Mg-Zn-Y alloys have been investigated via electron microscopy. Coarse spheroidal precipitates formed on deformation twin boundaries contained domains belonging to either the MgZn2 hexagonal Laves phase or the monoclinic Mg4Zn7 phase. Both phases are structurally related to the quasi-crystalline phase formed in Mg-Zn-Y alloys, containing icosahedrally coordinated zinc atoms arranged as a series of broad rhombohedral units. This rhombohedral arrangement was also visible in intragranular precipitates where local regions with the structures of hexagonal MgZn2 and Mg4Zn7 were found. The orientation adopted by the MgZn2 and Mg4Zn7 phases in twin-boundary and intragranular precipitates was such that the icosahedral clusters were aligned similarly. These results highlight the close structural similarities between the precipitates of the Mg-Zn-Y alloy system.

The structure of precipitates in Mg–Zn–Y alloys

It is shown here by high-resolution electron microscopy that the structure of rod-like precipitates perpendicular to the basal planes of the magnesium matrix in Mg–Zn–Y alloys consist of complex domains at nanoscale. These domains can be recognized to be those of monoclinic Mg4Zn7 phase and hexagonal Laves phase MgZn2 with axial orientations . Due to disorder, often complete unit cells of the Mg4Zn7 structure cannot be recognized. Inside a domain of Mg4Zn7, the structure may locally transform to that of MgZn2. The structure of both the phases are composed of similar units of icosahedral coordinations. Maintaining the above axial orientation, the Mg4Zn7 can exhibit two orientation relationships with the matrix, or . The hexagonal MgZn2 forms in the monoclinic Mg4Zn7 in two different orientations, related to rhombic units (with angle ∼72°) in the Mg4Zn7 unit cell. With the common axial relationship given above, these two orientation relationships can be given as or . One of these two variants forms a known orientation relationship , with the matrix. The structure of the MgZn2 was found to be modified by the ordering of zinc layers perpendicular to the hexagonal axis.

Structural relationships among MgZn2 and Mg4Zn7 phases and transition structures in Mg-Zn-Y alloys

Isothermal ageing of plastically deformed Mg-Zn-Y alloys resulted in precipitation along twin boundaries. The bulky precipitates so formed had structures similar to those recently reported for the rod-like precipitates, but afforded a more detailed study by high-resolution TEM due to their larger size. The core of the precipitates often had the structure of the monoclinic Mg4Zn7 phase, and had the orientation ; with either the matrix or the twin. On this Mg4Zn7 phase, the hexagonal MgZn2 phase grew in two orientations, both with . One of these orientations formed a known orientation relationship ; with the matrix. The part of the precipitate with the MgZn2 structure was usually in direct contact with the twin boundary. Both the Mg4Zn7 and MgZn2 phases have layered structures that can be described with similar building blocks of icosahedrally coordinated atoms. The atomic positions of zinc atoms comprise the vertices of these icosahedra and form ‘thick’ rhombic tiles. The orientations of these rhombuses remain unchanged across the interfaces between the two phases. Near the interface with MgZn2, transition structures formed in the Mg4Zn7 phase, with the Zn:Mg atom ratio between those of the Mg4Zn7 and MgZn2 phases. In these transition structures, the unit cell of the Mg4Zn7 phase is extended along [100] or [001] by half a unit cell length by continuation of the rhombic tiling. Structures of these extended unit cells are proposed.

Phase composition and morphology development in WE-type alloys modified by high Zn content

Abstract Quasicrystalline icosahedral equilibrium phase was found in Mg – Y – Nd (WE43) alloys with the addition of 12 wt.% and 25 wt.% Zn constituting grain boundary eutectic phases. In response to the isochronal annealing, morphologically similar precipitates of phases with the same structure as those developing during heat treatment of binary Mg-8 wt.% Zn alloy at temperatures higher than 150 °C were observed, namely β (b-centered monoclinic crystal structure a = 2.596 nm, b = 1.428 nm, c = 0.524 nm, = 102.5°) and β (hexagonal MgZn2). The evolution of the resistivity and microhardness during isochronal annealing corresponds to the phase transformations and the morphology changes of the phases observed. Dislocations with the c Burgers vector component generated due to the presence of icosahedral phase are responsible for the improved formability as compared to the WE43 alloy.

Phase relations in the U–Mo–Al ternary system

The phase relations in the U–Mo–Al system of quenched samples annealed at 800 °C for 2 weeks and at 400 °C for 2 months have been established using X-ray powder diffraction, scanning electron microscopy and energy dispersive spectroscopic analysis performed at room temperature. Two ternary Al-rich phases, UMo 2− x Al 20+ x and U 6Mo 4+ x Al 43− x are found stable at 800 °C and 400 °C. They show significant homogeneity ranges resulting from Mo/Al substitution mechanism on various mixed crystallographic sites, as evidenced by single-crystal structure refinements. Substitution of up to 25 at.% of Al by Mo atoms is also observed for UAl 2 (cubic MgCu 2-type) giving a quite large extension (UAl 2− x Mo x , 0 < x < 0.5) into the ternary system. Larger substitution (0.6 < x < 0.7 at T = 800 °C) stabilizes another ternary Laves phase, UAl 2− x Mo x with the hexagonal MgZn 2-type. There is no detectable solubility of Mo in UAl 4, and it is of the order of 1 at.% in UAl 3. The interaction layers between the γU–Mo alloys and the Al matrix in nuclear fuel plates can be successively estimated as composed of the two- and three-phase fields equilibrium indicated on the assessment of the phase relations drawn for samples heat-treated at 400 °C.

High-pressure modifications of CaZn 2, SrZn 2, SrAl 2, and BaAl 2: Implications for Laves phase structural trends

High-pressure forms of intermetallic compounds with the composition CaZn 2, SrZn 2, SrAl 2, and BaAl 2 were synthesized from CeCu 2-type precursors (CaZn 2, SrZn 2, SrAl 2) and Ba 21Al 40 by multi-anvil techniques and investigated by X-ray powder diffraction (SrAl 2 and BaAl 2), X-ray single-crystal diffraction (CaZn 2), and electron microscopy (SrZn 2). Their structures correspond to that of Laves phases. Whereas the dialuminides crystallize in the cubic MgCu 2 (C15) structure, the dizincides adopt the hexagonal MgZn 2 (C14) structure. This trend is in agreement with the structural relationship displayed by sp bonded Laves phase systems at ambient conditions.

Atomic ordering in the Laves phases L1 V(Co1–xSix)2 (x = 0.43 and 0.56)

Abstract The synthesis, phase relations, and crystal structures of two truly ternary, partially ordered Laves phases occurring in the V—Co—Si system are reported. The phases termed L1 V(Co1–xSix)2 are found in two distinct phase fields next to TiMnSi-type VCoSi. This so-called E-phase emerges in the centre of the miscibility gap which separates the Co- and Si-rich Laves phase fields. Both fields accommodate several structurally distinct L-phases. The L1-phases V(Co1–xSix)2 (x = 0.43 and 0.56) crystallise in √3a × √3a superstructures of the hexagonal MgZn2 structure type. A rigorous symmetry-based analysis reveals that, at the given compositions, this type of superstructure affords sufficient structural degrees of freedom for ordering of Co and Si atoms on 4 different sites in a manner that enables tuning of hetero-atomic Co—Si interactions to an optimum.

Structure and stability of Laves phases. Part I. Critical assessment of factors controlling Laves phase stability

Laves phases form the largest group of intermetallic phases. Although they are well known since long, there are still unsolved problems concerning the stability of the respective crystal structures. The Laves phases crystallize with a cubic MgCu 2- or a hexagonal MgZn 2- or MgNi 2-type structure which differ only by the particular stacking of the same four-layered structural units. It is still not possible to predict which of the structure types is the stable one for a Laves phase compound AB 2. Phase transformations from a cubic low-temperature structure to a hexagonal high-temperature structure were observed as well as stress-induced transformations from the hexagonal structure to the cubic one. In addition, deviations from the stoichiometric composition were reported to result in a change of the stable polytype in various systems. In this first of two consecutive papers dealing with fundamental aspects of the stability of Laves phases, some factors which are known to affect the occurrence and structure type of Laves phases are discussed and it is shown that, at least up to now, the existing models and calculations are not well suited to give a general description of the stability of Laves phases.