Abstract
Mg3−xGa1+xIr (x = 0.05) was synthesized by direct reaction of the elements in welded tantalum containers at 1200 °C and subsequent annealing at 500 °C for 30 days. Its crystal structure represents a new prototype and was determined by single-crystal technique as follows: space group P63/mcm, Pearson symbol hP90, Z = 18, a = 14.4970(3) Å, c = 8.8638(3) Å. The composition and atomic arrangement in Mg3GaIr do not follow the 8–N rule due to the lack of valence electrons. Based on chemical bonding analysis in positional space, it was shown that the title compound has a polycationic–polyanionic organization. In comparison with other known intermetallic substances with this kind of bonding pattern, both the polyanion and the polyanion are remarkably complex. Mg3−xGa1+xIr is an example of how the general organization of intermetallic substances (e.g., formation of polyanions and polycations) can be understood by extending the principles of 8–N compounds to electron-deficient materials with multi-atomic bonding.
Highlights
During their scientific history, intermetallic compounds have challenged chemists because their compositions and crystal structures cannot be rationalized or understood using usual valence rules
Refinement of the occupancies of the magnesium positions resulted in values of 0.99(1), 1.00(2), 1.05(2) and 1.13(3) for Mg1, Mg2, Mg3 and Mg4, respectively, indicating the presence of stronger scatterers in the Mg3 and Mg4 positions
The final atomic coordinates and isotropic displacement parameters are listed in Table 2, while the interatomic distances are listed in Table 3; the anisotropic displacements parameters can be obtained from the database (CSD deposition No 2129909) or from the corresponding author
Summary
Intermetallic compounds have challenged chemists because their compositions and crystal structures cannot be rationalized or understood using usual valence rules. The main difference between the “classical” inorganic and intermetallic compounds is the low number of electrons in the last shell per atom (ELSA) in the latter [2] This hinders the application of bonding concepts based on the 8–N rule to understand the composition and structure of intermetallic compounds. It was suggested to extend the 8–N concept by taking into account the participation of d orbitals (penultimate shell) in bonding events (18–N rule). This situation involves the concept of multi-atomic bonding
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