W. Assmus

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.

Elastic properties of NiTi

The authors present measurements of the elastic constants on a single crystal of the shape-memory alloy NiTi in a wide temperature range. Step-like anomalies and hysteresis are detected at the austenitic-martensitic phase transition which provide evidence for the strain-order parameter coupling in the pre-martensitic phases. The results are discussed in the framework of existing Landau theory models.

Grüneisen parameter of cerium compounds with unstable 4f shells

We give experimental results of magnetic susceptibility, thermal expansion and elastic constant measurements for CeSn3 and CePd3. We interpret these results with a phenomenological scaling function for the free energy and we include a brief discussion of CeAl3. The coupling between the electronic system and the lattice can be described by an electronic Grüneisen parameter Ωg associated with the unstable 4f system which turns out to become a system specific constant far below the fluctuation temperatureT0. The coupling parameter T0Ωg is an order of magnitude larger than the coupling constant for stable valentRE systems extracted from crystal field effects. A generalization to include magnetic field dependent effects is also given.

Determination of the primary solidification area of the icosahedral phase in the ternary phase diagram of Zn-Mg-Y

We present the results of our research on the ternary phase diagram of the Zn-Mg-Y system. We focus on the liquidus surface which is relevant for the preparation of the icosahedral phase. Two sections of the phase diagram and the primary crystallization area of the icosahedral phase have been determined.

Magnetic and elastic properties of CeAg

Abstract Magnetic and elastic measurements in single crystals of CeAg and Ce 0.09 La 0.91 Ag are presented. The magnetic susceptibility of Ce 0.09 La 0.91 Ag can be quantitatively interpreted with taking Γ 8 as the ground state of the crystal field split J = 5 2 state of the Ce 3+ -ion. In CeAg we confirm a ferromagnetic phase transition at T c = 5.3 K and a structural transition at 15 K. This latter one is clearly observed with the elastic constant measurements where c 11 - c 12 exhibits appreciable softening. This structural transition is probably due to a phonon softening at the zone boundary. The magnetic properties of CeAg, especially the small magnetic moment in the ordered region, are discussed.

Crystal growth of HTSC materials

This paper deals with the growth of single crystals of HTSC materials. We describe the main techniques and innovations of the last few years. The materials reviewed here are from the Y-Ba-Cu-O system, the Bi-Sr-Ca-Cu-O system and the Tl-Ba-Ca-Cu-O system.

Phononic filter effect of rattling phonons in the thermoelectric clathrate Ba8Ge40+xNi6-x.

One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone, which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime, which contradicts the usual assumption of strong anharmonic phonon-phonon scattering processes. Within the framework of ab initio density-functional theory calculations we interpret these hybridizations as a series of anticrossings which act as a low-pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low-pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba8Ge40Ni6 and compare it to those of pure Ge and a Ge-46 empty-cage model system.

Magnetocaloric effect and magnetic cooling near a field-induced quantum-critical point

The presence of a quantum-critical point (QCP) can significantly affect the thermodynamic properties of a material at finite temperatures T. This is reflected, e.g., in the entropy landscape S(T,r) in the vicinity of a QCP, yielding particularly strong variations for varying the tuning parameter r such as pressure or magnetic field B. Here we report on the determination of the critical enhancement of ∂S/∂B near a B-induced QCP via absolute measurements of the magnetocaloric effect (MCE), (∂T/∂B)S and demonstrate that the accumulation of entropy around the QCP can be used for efficient low-temperature magnetic cooling. Our proof of principle is based on measurements and theoretical calculations of the MCE and the cooling performance for a Cu2+-containing coordination polymer, which is a very good realization of a spin-½ antiferromagnetic Heisenberg chain—one of the simplest quantum-critical systems.

Growth of large single grains of the icosahedral quasicrystal ZnMgY

In this paper we report on the growth of icoshedral (i-)ZnMgY single quasicrystals by a liquid encapsulated (LE) top seeded solution growth (TSSG) method. This method meets both main requirements for the growth of i-ZnMgY: to avoid the peritectic growth reaction and to deal with the high vapour pressure of the component Zn. We obtain high quality single grains of i-ZnMgY with dimensions of a few mm in each direction.

Resistivity studies under hydrostatic pressure on a low-resistance variant of the quasi-two-dimensional organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br: Search for intrinsic scattering contributions

Resistivity measurements have been performed on a low (LR)- and high (HR)-resistance variant of the kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br superconductor. While the HR sample was synthesized following the standard procedure, the LR crystal is a result of a somewhat modified synthesis route. According to their residual resistivities and residual resistivity ratios, the LR crystal is of distinctly superior quality. He-gas pressure was used to study the effect of hydrostatic pressure on the different transport regimes for both variants. The main results of these comparative investigations are (i) a significant part of the inelastic-scattering contribution, which causes the anomalous rho(T) maximum in standard HR crystals around 90 K, is sample dependent, i.e. extrinsic in nature, (ii) the abrupt change in rho(T) at T* approx. 40 K from a strongly temperature-dependent behavior at T > T* to an only weakly T-dependent rho(T) at T < T* is unaffected by this scattering contribution and thus marks an independent property, most likely a second-order phase transition, (iii) both variants reveal a rho(T) proportional to AT^2 dependence at low temperatures, i.e. for T_c < T < T_0, although with strongly sample-dependent coefficients A and upper bounds for the T^2 behavior measured by T_0. The latter result is inconsistent with the T^2 dependence originating from coherent Fermi-liquid excitations.