Where are the Atoms in Quasicrystals? Experimental and Theoretical Studies of Ternary and Quaternary Approximants
- PDF / 1,047,610 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 71 Downloads / 199 Views
Where are the Atoms in Quasicrystals? Experimental and Theoretical Studies of Ternary and Quaternary Approximants Gordon J. Miller, Chi-Shen Lee and Wen-Jiuan Tsai Department of Chemistry and Ames Laboratory, Iowa State University, Ames, IA 50011, USA ABSTRACT Problems concerning complete structural characterization of quasicrystals involve locating the atomic positions as well as determining the distribution of elements at each site. Quasicrystalline approximants provide models for potential building units of quasicrystals, but a clear determination of the elemental decorations in such approximants also remains incomplete. We report experimental and theoretical studies of new, quaternary Bergman phases in the Li-MgZn-Al system and a new, quasicrystalline approximant Li10Mg6Zn31Al3 (A16M34-type). A theoretical model using averaged Mulliken populations provides a means to track the segregation of elements (and vacancies) onto different sites as a function of valence electron concentration. As the Li content decreases, vacancies begin to occur at a specific site in the Bergman structure. The new approximant demonstrates how truncated tetrahedra can play an important role in forming clusters with possible fivefold symmetry in quasicrystalline structures.
INTRODUCTION For all the efforts over the past 1.5 decades to structurally characterize quasicrystalline phases [1], locating and identifying the atoms in quasicrystals still remains an active and vital segment of quasicrystal research. To answer the question, “where are the atoms in a structure,” two more specific questions must be solved: (1) what is the framework of atoms in threedimensional space, i.e., finding atomic coordinates for a structure; and (2) how do the elements decorate the structural framework of atomic positions? For crystalline structures, diffraction is the typical method used to answer both of these questions. Quasicrystalline approximants have provided models for atomic positions for quasicrystals, which have been used for significant and thorough theoretical treatments of the electronic structure of quasicrystals [2-4]. Furthermore, X-ray and neutron diffraction has been successfully applied to solve the structures of various approximants for icosahedral quasicrystals [5]. One prototypical approximant is the cubic Mg32(Zn,Al)49 structure that was first identified by Bergman and Pauling in 1956 [6], called the “Bergman phase.” Although the structure type has been observed for other ternary, intermetallic phases, significant details of the structure (i.e., “where are the atoms?”) and the solid state chemistry of the Mg-Zn-Al phase remain in question. In this paper, we report results of our efforts to study the chemistry, structure and bonding of the Bergman phases in ternary Li-Zn-Al and Mg-Zn-Al systems as well as in the quaternary Li-MgZn-Al system [7,8], which we hope will shed some new light on the atomic decoration of icosahedral quasicrystals from the main-group element systems.
K3.1.1
The Role of Aluminum We became interested in the Ber
Data Loading...
