Crystal Chemistry and Electronic Structure of Magnesium based Mackay Icosahedron Type Approximants
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Crystal Chemistry and Electronic Structure of Magnesium based Mackay Icosahedron Type Approximants
Julien Pierre Amelie Makongo, Yuri Moguilnikov, Christian Kudla, Daniel Grüner, Melanie Schäpers1 and Guido Kreiner Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany 1 Fortis GmbH, Stockumer Str. 28, 58453 Witten, Germany
ABSTRACT
In order to obtain reliable experimental information on the long-range order, the role of the valence electron concentration and the type of disorder for Mackay icosahedron type quasicrystals we have studied binary and ternary Mg based approximant phases. An overview is presented on the crystal structures, homogeneity ranges, physical properties, and electronic structures of the following intermetallic phases: Mg6Pd, Mg57Pd13, Mg56.4Pd13.5, Mg306Pd77, Mg3Pd, Mg5Pd2, Mg2Pd, Ga30-xMg32+xPd21-y, Ga15-x-yMg38+yPd13+x, GaMg3Pd2, Ag17Mg54, Ag7Mg26, and AgMg4.
INTRODUCTION The Mackay icosahedron (MI) is a fundamental structural unit of many icosahedral quasicrystals and approximants. The multi shell structure of the Mackay icosahedron is composed of 55 atoms with one or two transition metals and one main group metal as components. In the past decade many Al based quasicrystals and their approximants have been investigated. These alloys are easily accessible by arc or high frequency induction melting of the elements. However, binary and ternary aluminides show a distinct tendency to complex positional disorder phenomena, e.g., atomic split positions. These structural phenomena hamper the analysis of even simple crystal structures and worse of complex alloy phases like quasicrystals and approximants. To avoid the structural complexity of the aluminides we have focused our investigations on Mg based quasicrystals and approximants with one transition metal and one or two main group metals as components. A drawback is the high vapor pressure of Mg which raises difficulties in the preparation of Mg-rich alloys. Here, we report on the synthesis of Mackay icosahedron approximants with Pd and Mg using an ampoule technique. To study effects due to the number of valence electrons the elements Ga and Ag have been chosen to substitute Mg and Pd, respectively. Chemical and structure analysis, measurements of electrical resistivity and magnetic susceptibility, and quantum mechanical calculations have been performed to study the long-range order, the role of the valence electron concentration and the type of disorder for approximants and related phases.
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EXPERIMENTAL DETAILS
Alloys of the present investigation were prepared from high-purity elements (Mg, chips, 99.99%, Pd, foil, 99.99%, Ag, foil, 99.995%, Ga, pellets, 99.999%; Chempur) and were obtained in the following manner: Mixtures of the elements were loaded in Ta ampoules and sealed by arc-welding under dry argon. After melting the samples using a high frequency induction furnace the tantalum ampoules were encapsulated in evacuated silica tubes, annealed, and quenched into water. The no
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