Growth, Structure, Optical and Ultrasonic Investigations of Icosahedral Zn-Mg-Y Quasicrystals
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Growth, Structure, Optical and Ultrasonic Investigations of Icosahedral Zn-Mg-Y Quasicrystals R. Sterzel, G. Bruls, A. Kounis1, G. Miehe1, K. Saitoh1, H. Fuess1, V. Karpus2, G.-J. Babonas2/5 ]D2 and W. Assmus Physikalisches Institut, J.W.Goethe University Frankfurt, Robert-Mayer-Str. 2-4, D-60325 Frankfurt, Germany 1 Materials Science Dept, TH-Darmstadt, Petersen-Str. 23, D-64287 Darmstadt, Germany 2 Semiconductor Physics Institute, Goštauto 11, LT-2600 Vilnius, Lithuania ABSTRACT It is shown how the history of the growth of an icosahedral Zn-Mg-Y single grain can be determined by measuring the yttrium distribution. The growth mechanism and the stabilization of the icosahedral Zn-Mg-Y, RE (RE = rare earth: Ho, Er, Dy, Gd, Tb) quasicrystals are discussed with respect to structural investigations on related crystalline phases. We also show results of optical and ultrasonic investigations on icosahedral Zn-Mg-Y single crystals. They fit well to the discussed growth and stabilization mechanism. INTRODUCTION Face centered icosahedral (fci) Zn-Mg-Y, RE quasicrystals are thermodynamic stable and contain in contrast to the most other systems with stable icosahedral phases no aluminum. The structural perfection of these quasicrystals is as high as in the best Al-Pd-Mn samples [1]. The high structural quality of fci Zn-Mg-Y, RE grown by the liquid encapsulated top seeded solution growth (LETSSG) method [2] from the primary solidification area [3] can also be seen different experiments (e.g. low amount of diffuse scattering). The crystals grow by slow cooling of a melt with a composition within the primary solidification area [3]. Therefore the knowledge of its position is absolutely essential for the growth of large single crystals. When the crystal growth starts the stoichiometry of the melt shifts away from the stoichiometry of the growing crystal and the melt will become more and more yttrium deficient. The best starting composition starts the longest way within the primary solidification area and was found to be Zn46Mg51Y3, while the composition of the growing crystal is Zn60Mg29Y11 at the start. This incongruent solidification behavior makes a low growth rate necessary. The yttrium has to reach the crystallization front by diffusion through the melt. Diffusion needs time to take place and so does the growth of crystals in an incongruent solidifying system. CRYSTAL GROWTH The shifting composition of the melt during growth intersects the primary solidification area. In the LETSSG-method a water-cooled tungsten tip on the top of the melt initiates crystal growth. An eutectic mixture of LiCl and KCl is used as a liquid encapsulation to prevent Mg and Zn evaporation. The temperature of the furnace is then reduced with a rate of 0.6 K/h with respect to the incongruent solidification behavior. For details see reference 2.
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Figure 1. Binary cut through the ternary phase diagram. The liquidus line measured by means of differential thermal analysis, the solidus line by analyzing a bridgman grown sample. A bridgman cru
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