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Crystal Structure of ϵ‐Ag 7+x Mg 26–x – A Binary Alloy Phase of the Mackay Cluster Type
Author(s) -
Kreiner Guido,
Spiekermann Stefan
Publication year - 2001
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/1521-3749(200111)627:11<2460::aid-zaac2460>3.0.co;2-l
Subject(s) - crystallography , crystal structure , alloy , cluster (spacecraft) , cubic crystal system , lattice (music) , solid solution , annealing (glass) , type (biology) , phase (matter) , materials science , atom (system on chip) , lattice constant , topology (electrical circuits) , chemistry , physics , metallurgy , combinatorics , diffraction , ecology , mathematics , organic chemistry , biology , computer science , acoustics , embedded system , programming language , optics
The binary alloy phase ϵ‐Ag 7+x Mg 26–x with x ≈ 1 and small amounts of the β′‐AgMg phase crystallize by annealing of Ag–Mg alloys with starting compositions between 24–28 At‐% Ag at 390 to 420 °C. A model structure for the ϵ‐phase consisting of a fcc packing of Mackay clusters was derived from the known structure of the ϵ′‐Ag 17 Mg 54 phase. Crystals of the ϵ‐phase were obtained by direct melting of the elements and annealing. The examination of a single crystal yielded a face‐centered cubic unit cell, space group Fm3 with a = 1761.2(5) pm. The refinement was started with the parameters of the model: wR2(all) = 0.0925 for 1093 symmetrically independent reflections. A refinement of the occupancy parameters indicated a partial replacement of silver for magnesium at two metal atom sites, resulting in the final composition ϵ‐Ag 7+x Mg 26–x with x = 0.96(2). There are 264 atoms in the unit cell and the calculated density is 3.568 gcm –3 . The topology of the model was confirmed. Mackay icosahedra are located at the lattice points of a face‐centered cubic lattice. Differences between model and refined structure and their effects on the powder patterns are discussed. The new binary structure type of ϵ‐Ag 7+x Mg 26–x can be described in terms of the I3‐cluster concept.