Guido Kreiner

The Samson phase, β-Mg2Al3, revisited

Co-Authors: Michael Feuerbacher, Carsten Thomas, Julien P. A. Makongo, Stefan Hoffmann, Wilder Carrillo-Cabrera, Raul Cardoso, Yuri Grin, Guido Kreiner, Jean-Marc Joubert, Thomas Schenk, Joseph Gastaldi, Henri Nguyen-Thi, Nathalie Mangelinck-Noël, Bernard Billia, Patricia Donnadieu, Aleksandra Czyrska-Filemonowicz, Anna Zielinska-Lipiec, Beata Dubiel, Thomas Weber, Philippe Schaub, Günter Krauss, Volker Gramlich, Jeppe Christensen, Sven Lidin, Daniel Fredrickson, Marek Mihalkovic, Wieslawa Sikora, Janusz Malinowski, Stefan Brühne, Thomas Proffen, Wolf Assmus, Marc de Boissieu, Francoise Bley, Jean-Luis Chemin, Jürgen Schreuer Abstract. The Al−Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg 38.5 Al 61.5, this cubic phase, space group Fd-3m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214 °C a first-order phase transition to rhombohedral β′-Mg2Al3(hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd-3m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400 °C. It essentially agrees with Samsons model, even in most of the many partially occupied and split positions. The structure of β′-Mg2Al3is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.

Exchange bias up to room temperature in antiferromagnetic hexagonal Mn3Ge

Mn3.04Ge0.96 has a hexagonal crystal structure, which can be stabilized by high-temperature annealing, and shows antiferromagnetic order with a small ferromagnetic component of less than 0.1μB and a coercivity of 0.45 T. In the ordered phase, magnetization curves M(H) exhibit an exchange bias of 62 mT at T = 2 K after field cooling, which is observable up to room temperature. The exchange anisotropy is suggested to originate from the exchange interaction between the host of triangular-antiferromagnetic Mn3Ge units and embedded ferrimagnetic-like clusters. Such clusters develop when excess Mn atoms occupy empty Ge sites in the original triangular-antiferromagnetic structure of Mn3Ge.

A case study of complex metallic alloy phases: structure and disorder phenomena of Mg-Pd compounds

In order to achieve a deeper insight into the relation between the homogeneity range, crystal structure and disorder phenomena of complex metallic alloy phases (CMAs) we have redetermined the Mg–Pd phase diagram in the range 60–100 at.% Mg. The existence of the intermediate phases Mg6Pd (β), Mg57Pd13 (γ), Mg56.4Pd13.5 (δ), Mg306Pd77 (ε), Mg78.5Pd21.5 (ζ), Mg3Pd (η), Mg5Pd2 (θ) and Mg2Pd (ι) has been reconfirmed. The first five of these are CMAs. The β phase melts congruently whereas the CMAs γ, δ, ε and ζ form in a cascade of peritectoid and peritectic reactions in a narrow window of approximately 30°C and 3 at.%. It is assumed that the stability of the Mackay icosahedron plays an important role in the phase formation of these CMAs. However, the β phase reveals an intricate pattern of disorder at the atomic positions of the Mackay icosahedron. Therefore, the concept of the stability of the Mackay cluster should be used only as a rule of thumb.

First-principles study of the structural stability of cubic, tetragonal and hexagonal phases in Mn3Z (Z=Ga, Sn and Ge) Heusler compounds

We investigate the structural stability and magnetic properties of cubic, tetragonal and hexagonal phases of Mn3Z (Z=Ga, Sn and Ge) Heusler compounds using first-principles density-functional theory. We propose that the cubic phase plays an important role as an intermediate state in the phase transition from the hexagonal to the tetragonal phases. Consequently, Mn3Ga and Mn3Ge behave differently from Mn3Sn, because the relative energies of the cubic and hexagonal phases are different. This result agrees with experimental observations from these three compounds. The weak ferromagnetism of the hexagonal phase and the perpendicular magnetocrystalline anisotropy of the tetragonal phase obtained in our calculations are also consistent with experiment.We investigate the structural stability and magnetic properties of the cubic, tetragonal and hexagonal phases of Mn3Z (Z=Ga, Sn and Ge) Heusler compounds using first-principles density-functional theory. We propose that the cubic phase plays an important role as an intermediate state in the phase transition from the hexagonal to the tetragonal phases. Consequently, Mn3Ga and Mn3Ge behave differently from Mn3Sn, because the relative energies of the cubic and hexagonal phases are different. This result agrees with experimental observations for these three compounds. The weak ferromagnetism of the hexagonal phase and the perpendicular magnetocrystalline anisotropy of the tetragonal phase obtained in our calculations are also consistent with experiment.

Magnetism in Nb1-yFe2+y: Composition and magnetic field dependence

We present a systematic study of transport and thermodynamic properties of the Laves phase system

Planar Fe6 Cluster Units in the Crystal Structure of RE15Fe8C25 (RE=Y, Dy, Ho, Er)

{\text{Nb}}_{1\ensuremath{-}y}{\text{Fe}}_{2+y}

Preparation, phase stability and structure of the C36 Laves phase Nb1-xCo2+x

. Our measurements confirm that Fe-rich samples, as well as those rich in Nb (for

Towards realistic quasiperiodic structures: modelling, synthesis and structure of (Ga,Zn)(175-delta)Mg97+delta - a large 3/2-2/1-2/1 Fibonacci approximant

|y|\ensuremath{\ge}0.02

On the crystal structure of γ-AgMg4

), show bulk ferromagnetism at low temperature. For stoichiometric

Increasing Curie temperature in tetragonal Mn2RhSn Heusler compound through substitution of Rh by Co and Mn by Rh

{\text{NbFe}}_{2}