R. Sterzel

A new well-ordered simple icosahedral quasicrystalline phase in the Zn-Mg-Er system

A well-ordered simple icosahedral quasicrystalline phase has been found in the Zn-Mg-Er system. The structure of a splat-cooled and annealed sample was investigated by electron and X-ray powder diffraction. A quasilattice constant of 0.513nm was determined by the Elser method.

Preparation of Zn-Mg-rare earth quasicrystals and related crystalline phases

Abstract We present our results in growing large single quasicrystals of the icosahedral phase in the Zn–Mg–Ho system with the top-seeded solution growth method. We also show new results of investigations on the Zn–Mg–Y phase diagram focussing on the sector with low Y content, to obtain some information about the solidification behavior of the decagonal phase. In addition, the R-phase, a novel cubic approximant for the icosahedral phase in the Zn–Mg–Er system, is presented.

Growth of simple icosahedral single quasicrystals in the Zn-Mg-Ho system

Single crystals of the simple icosahedral phase Zn 76 Mg 14 Ho 10 have been grown from the melt. The composition of the melt was selected by differential thermal analysis in combination with scanning electron microscopy and wavelength-dispersive X-ray analysis. The quasicrystals have been characterized by single-crystal and powder X-ray diffraction. The quasilattice constant of the primitive six-dimensional hypercubic lattice is a = 0.5143(2) nm. There is no indication of atomic disorder. Holmium on a primitive icosahedral lattice presents a promising probe for possible quasiperiodic long-range magnetic order.

Medium range real atomic structure of face-centred icosahedral Ho9Mg26Zn65

Abstract A complementary approach to solve quasi crystalline atomic structures in 3-dimensional (3D) real space is presented: The atomic pair distribution function (PDF) of face centred icosahedral Ho9Mg26Zn65 [a(6D) = 2 × 5.18(3) Å] has been obtained from in-house powder X-ray diffraction data (MoKα1). For the first time, full profile PDF refinements of a quasicrystal were performed: Starting with rational approximant models, derived from 1/1- and 2/1-Al—Mg—Zn, its local and medium range structure was refined (r < 27 Å; R = 12.9%) using the PDF data. 85% of all atoms show Frank-Kasper (FK) type coordinations. The basic structural unit is the 3-shell, 104-atom Bergman cluster (d ≈ 15 Å) comprising a void at its center. The clusters are interconnected sharing common edges and hexagonal faces of the 3rd shells. The remaining space is filled by some glue atoms (9% of all atoms), yielding an almost tetrahedrally close packed structure. All Ho atoms are surrounded by 16 neighbours (FK-polyhedron “P”). Most of them (89%) are situated in the 2nd shell and form a Ho8 cube (edge length 5.4 Å); they are completed by 12 Mg atoms to a pentagon dodecahedron. Cubes in neighbouring clusters are tilted with respect to each other; their superposition generates diffraction symmetry 2/m3̅5̅. The remaining Ho atoms act as glue atoms. As a result and as can be expected for real matter, local atomic coordinations in quasicrystals are similar when compared to common crystalline intermetallic compounds. From our results, the long range quasiperiodic structure of icosahedral Mg—Zn—RE (RE = Y and some rare earths) is anticipated to be a canonical cell tiling (CCT, after Henely) decorated with Bergman clusters.

A cubic approximant in the Zn-Mg-Er alloy

A cubic approximant for the icosahedral phase is found in the Zn-Mg-Er system. The preparation of the Zn-Mg-Er ternary phase (the so-called R phase) through quenching and annealing is described in detail. The R phase was found by means of scanning electron microscopy in combination with wavelength-dispersive X-ray analysis. The composition of the R phase varies around Zn63Mg23Er14 in different samples depending on the initial composition. The structure has been studied by a combination of high-resolution transmission electron microscopy (HRTEM) and X-ray powder diffraction. The R phase is cubic with a 0 = 2.02 nm and crystallizes in the space group F 43m. The close relationship of this phase to the icosahedral phase in the Zn-Mg-Y, RE system (RE = rare earth) is demonstrated through electron diffraction features and HRTEM images.

Space-group determination of cubic and its superlattice-ordered rhombohedral phases of Zn63Mg23Er14 using the convergent-beam electron diffraction method

Abstract The space group of a cubic phase of Zn63Mg23Er14 has been determined to be F4̅3m by convergent-beam electron diffraction (CBED). This space group is in agreement with that assumed by X-ray structure analysis (Sterzel et al., Philos. Mag. Lett. 80 (2000) 239-247). A rhombohedral phase, which is considered as a superlattice ordered phase of the cubic phase, was found. The space group of the rhombohedral phase has been determined to be R3m by the CBED method.

Atomic dynamics of icosahedral Zn63Mg26.3Y10.7 studied by neutron inelastic scattering

Abstract The dynamic structure factor of ZnMgY has been determined using neutron inelastic scattering techniques. From the time-of-flight spectra also the generalized vibrational density of states (GVDOS) was determined, which was used to calculate the temperature dependence of some of the thermodynamic properties like the vibrational heat capacity. The GVDOS of icosahedral ZnMgY consists of two main bands. The main dominant band at lower energies is composed of three subbands. This is characteristically different from the GVDOS of the Mackay type of icosahedral alloys investigated so far. The more pronounced separation of the subbands seems to be characteristic for the GVDOS of Frank–Kasper type of icosahedral quasicrystals.

Preparation of decagonal Zn-Mg-Y by melt spinning

It is shown, for the first time, how pure decagonal Zn-Mg-Y can be prepared by melt spinning and annealing. Study of the phase formation was performed by differential scanning calorimetry and the sample was further investigated by X-ray and electron diffraction. The lattice constants of the decagonal phase are a q =4.592Å and c =5.198Å calculated from a physical space model, or a q =4.485Å and c =5.193Å from a hyperspace model.

A comparative study of the atomic dynamics of icosahedral ZnMgHo and ZnMgEr by neutron inelastic scattering

Abstract The atomic dynamics of the Frank–Kasper type of FI icosahedral alloys ZnMgHo and ZnMgEr have been determined at 200 K using thermal neutron energy loss scattering. From the sum of all double differential scattering cross-sections d 2 σ ( d Ω d E) weighted with sin( θ ) the generalised vibrational density of states (GVDOS) was calculated. The calculated GVDOS consists of two bands, a dominant one centered around 20 meV, which is composed of several subbands, and a rather weak one centered around 50 meV. Both alloys show very much the same atomic dynamics with small differences near 15, 22 and 38 meV.

Structure of a Zn?Mg?Er cubic phase and its relation to icosahedral phases

The crystal structure of a cubic phase in the system Zn-Mg-Er has been solved by a combination of high-resolution electron microscopy and X-ray powder diffraction. This phase is considered to be related to that of the quasicrystalline phase. The structure consists of 448 atoms in the unit cell with lattice constant of a 0 = 20.20Å and the space group is F43m. Important structural elements in the cubic structure are interpenetrating icosahedral units around Zn and Mg atoms and Frank-Kasper polyhedra around the Mg atoms. No giant icosahedral atomic cluster, such as the 136-atom Bergman cluster, was found in the stucture.