Chemical Bonding and Pseudogap in Zn- and Cd-based Compounds with Complex Hexagonal Structures
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Chemical Bonding and Pseudogap in Zn- and Cd-based Compounds with Complex Hexagonal Structures Y.Ishii,1,2 K.Nozawa2,1 and T.Fujiwara3 1 Department of Physics, Chuo University, Kasuga, Tokyo 112-8551, Japan 2 SORST, Japan Science and Technology Agency, (SORST-JST), Kawaguchi, Saitama 332-0012, Japan 3 Department of Applied Physics, University of Tokyo, Hongo, Tokyo 113-8654, Japan ABSTRACT Electronic structures of hexagonal Zn-Mg-Y and Cd58Y13 compounds are studied by first-principles calculations. Both of the systems show deep pseudogap in the electronic density of states near the Fermi level and considered to be stabilized electronically. To illustrate bonding nature of electronic wavefunctions, the crystal orbital Hamilton population (COHP) is calculated for neighboring pairs of atoms in the unit cell. It is found that the bonding nature is changed from bonding to anti-bonding almost exactly at the Fermi level for Zn-Zn and Cd-Cd bonds. On the contrary, for Zn/Cd-Y bonds, both of the states below and above the pseudogap behave as bonding ones. Possible effects of the p-d hybridization are discussed. INTRODUCTION A pseudogap in the electronic density of states (DOS) is a universal feature of quasicrystals (QC) and approximant crystals and it is considered to be responsible for stability of such complex structured alloys [1]. It is believed that origin of the pseudogap is primarily due to the Hume-Rothery (HR) mechanism. However, this point may be still controversial for a family of QC without transition elements. Recently we have shown [2] that the HR mechanism does not play a principal role in the pseudogap formation for Cd- and Zn-based alloys, which are approximant phases of newly discovered QC [3,4], but the hybridization of the d states just above the Fermi level with a wide sp band is essential. The above conclusion on the hybridization mechanism has been derived from the first-principles electronic structure calculations for the cubic Cd- and Zn-based compounds. In this paper, we shall study the electronic structures of other family of Zn- and Cd-based compounds; hexagonal Zn-Mg-Y and Cd58Y13. Although these compounds may not be interpreted as approximant crystals, they are found in composition range very close to those of QC and involve many atomic sites with icosahedral coordination. So we expect these systems share common features in the electronic structures with the icosahedral phases. MODELS AND CALCULATION METHOD Crystal structure of hexagonal Zn-Mg-Y was studied by Takakura et al. [5]. A hexagonal unit cell with a=14.579 Å and c=8.687Å contains 92 atoms with space group P63/mmc. This hexagonal phase is one in a series of unique hexagonal crystals, including hexagonal Laves Zn2Mg, with almost the same periodicity along the c-axis. There is one crystallographic site occupied fractionally by either Mg or Y. We assume here that this site is occupied only by
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Mg but have checked that substitution of Y does not cause any essential change in the electronic structures. For Cd58Y13, the sp
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