Decagonal Al Ni Co Quasicrystals Research Articles

Diffusion in Decagonal and Icosahedral Quasicrystals and a Related Hexagonal Phase

We present investigations of gallium diffusion in decagonal AlNiCo quasicrystals in both principal directions and of gallium diffusion in icosahedral AlPdMn quasicrystals. Gallium was used to approximate aluminium self-diffusion, which cannot be measured using conventional tracer techniques. The depth-profiles of the stable isotopes were analysed by SIMS profiling. We compare our results in decagonal AlNiCo with experimental data on self-diffusion of cobalt and nickel and with MD-simulations of Al self-diffusion from literature. Our experiments and the MD simulations show that the diffusion parallel to the decagonal axis is faster than perpendicular to it. In addition we carried out diffusion studies of the major component zinc in icosahedral ZnMgY, icosahedral ZnMgHo (fci-phases) and a related hexagonal ZnMgY composition (Z-phase) along both principal directions. We report the Arrhenius parameters and compare the diffusion behaviour of the various materials.

NMR methods for detection of slow atomic motions in quasicrystals

We present NMR techniques for the detection of slow, low-activation-energy, diffusive atomic motion in quasicrystals, within the frequency window from several 10 kHz down to the milli-Hz region. The techniques involve two-dimensional exchange NMR experiments, the diffusion-induced decay of the Hahn spin echo in an inhomogeneous electric field gradient and the Carr–Purcell echo-train experiment. These techniques were applied to the icosahedral AlPdRe and decagonal AlNiCo quasicrystals and to the ‘vacancy-ordered’ periodic AlCuNi phase. In all compounds we detected slow, low-activation-energy, diffusive atomic jumps that persist down to low and very low temperatures. Identical motions were detected also in the periodic AlCuNi alloy, suggesting that the observed atomic dynamics are not quasicrystal-specific.

Vacancy-mediated diffusion in quasicrystalline alloys

In this paper we summarize recent diffusion studies from our laboratory. Diffusion of the following elements has been studied in icosahedral quasicrystals: Mn, Fe, Ga, In and Zn in AlPdMn and Zn in ZnMgRE (RE=rare earth metal). The diffusivities follow Arrhenius-like temperature dependencies. In addition, for Mn and Zn in icosahedral AlPdMn we have studied the dependence of the diffusivities on hydrostatic pressure. Signs and values of the activation volumes around 0.7 atomic volumes are strongly in favour of a vacancy-type diffusion mechanism. Diffusion of Co and Ni has been studied in both principal directions of decagonal AlNiCo quasicrystals. The small diffusion anisotropy is a strong argument in favour of vacancy-mediated diffusion. Significant contributions of phason flips to diffusion should result in faster diffusion within the layers perpendicular to the decagonal axis, which is not observed.

Ab initio structure solutions of quasicrystals by a density modification method

A unique and powerful density modification method for solving multi-dimensional crystal structures has been developed. The method is an extension of the Low Density Elimination (LDE) method1,2,3. The LDE method was made applicable to multi-dimensional crystal structures and applied to several quasicrystals including an unknown structure. Tests on the X-ray single crystal data of icosahedral AlPdMn4 and synthetic data of decagonal AlNiCo quasicrystals suggests that the multi-dimensional LDE method can easily solve quasicrystal structures starting from completely random phases. The procedure has also been applied to an unknown structure of icosahedral ZnMgHo quasicrystal. Multi-solution strategy was employed and 28 resultant maps clearly showed the positions and shapes of occupation domains. We were convinced by analyzing the map that the 28 results were the solutions.

Diffusion of 57Co in decagonal Al–Ni–Co-quasicrystals

Self-diffusion of 57Co in decagonal Al–Ni–Co single quasicrystals was studied for both principal directions as a function of the temperature by means of the radiotracer method. In combination with serial sectioning, diffusivities were measured over a large temperature range from 732 to 1273K. Below about 1120K diffusion is described by an Arrhenius behaviour along both the periodic and the quasiperiodic directions. The magnitude of the activation enthalpies and pre-exponential factors support a vacancy-type diffusion mechanism. A small anisotropy of diffusion which is temperature-dependent but never exceeds 30% was detected. Above 1120K, a kink in the Arrhenius curve is observed. We attribute this behaviour to a phase transition from the decagonal basic Co-rich phase to an ordered modification.