Fundamental Cluster and Hydrogen Sites in Ti-Zr-Ni Quasicrystals
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E.H. Majzoub, R.G. Hennig, K.F. Kelton, P.C. Gibbons, and S.T. Misture1 Department of Physics, Washington University, Saint Louis, MO 63130 (USA) 1 New York State College of Ceramics, Alfred, New York, USA ABSTRACT
A Rietveld re nement con rmed by ab initio calculations [1] has shown that the structure of the related W-phase [2] 1/1 approximant consists of Bergman clusters connected by \glue" sites. To investigate if the quasicrystal structure contains the same cluster we construct two constrained icosahedral glass models using the Bergman or Mackay cluster, respectively. A comparison with neutron di raction data yields better agreement if the Bergman cluster is used, suggesting that this is a frequently occurring element in the quasicrystalline structure. Since absorbed hydrogen is known to locate preferentially in tetrahedral interstitial sites in many metals, the tetrahedral sites in the constrained glass were lled with hydrogen. The calculated powder neutron di raction spectra are similar to the experimental data. It was not possible, however, to di erentiate between possible tetrahedral sites where the hydrogen atoms sit. Hydrogen site energies from ab initio calculations indicate preferences that are consistent with llings of non-Ni bearing tetrahedral sites. INTRODUCTION
In this paper a constrained icosahedral glass is used to model the structure of the stable Ti-based quasicrystal in Ti41:5 Zr41:5 Ni17 [3]. A related 1/1 cubic crystal approximant forms in Ti50 Zr35 Ni15. A Rietveld re nement of this approximant shows that this phase contains twoshell Bergman icosahedral clusters in a bcc packing, face-to-face along the h111i directions of the unit cell. Inter-cluster atoms in overlapping and distorted partial third shells form a \glue" between the clusters. The results from the glass model of the i-phase are compared with those found from a canonical cell tiling (CCT) model of the i-phase in Ti41:5 Zr41:5 Ni17 . The CCT decoration using the same Bergman cluster, and incorporating the \glue" atoms [1, 4], ts the experimental neutron powder di raction data for the quasicrystal with a residual, R (de ned below), of 6.5%. This will be contrasted with results from the glass model. We investigate the cluster structure in Ti-Zr-Ni for two reasons. First, models of quasicrystal structure based on packings of icosahedral clusters [5, 6] may lead to the identi cation of the fundamental clusters, Bergman or Mackay; the two prominent competing cluster decorations thought to exist in quasicrystals. Preliminary work in Ti-Zr-Ni alloys [7] showed that icosahedral packings of the Bergman cluster, for example, produced neutron di raction spectra that were similar to the experimental data. However, cluster packings that ignore the \glue" atoms between clusters can have density de cits near 50% [7, 8] and caution must be exercised when interpreting the modeling results. Second, for metastable quasicrystals such as icosahedral Ti-TM-Si-O (TM=Cr,Mn) [9, 10] which are highly disordered, a CCT structural re nement, such as made
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