Abstract
Alkali metal thallides go back to the investigative works of Eduard Zintl about base metals in negative oxidation states. In 1932, he described the crystal structure of NaTl as the first representative for this class of compounds. Since then, a bunch of versatile crystal structures has been reported for thallium as electronegative element in intermetallic solid state compounds. For combinations of thallium with alkali metals as electropositive counterparts, a broad range of different unique structure types has been observed. Interestingly, various thallium substructures at the same or very similar valence electron concentration (VEC) are obtained. This in return emphasizes that the role of the alkali metals on structure formation goes far beyond ancillary filling atoms, which are present only due to charge balancing reasons. In this review, the alkali metals are in focus and the local surroundings of the latter are discussed in terms of their crystallographic sites in the corresponding crystal structures.
Highlights
They completed their publication with the statement that, in case of a base metal, the formed structure type does not correlate with the average number of valence electrons per atom (VEC) according to the prevously introduced rules by Hume-Rothery [2], Westgren, and Phragmen [3]
Thallides very rich in alkali metal are only reported for lithium and sodium and in general, a high content of alkali metal increases the complexity of the compounds [70,71]
Lithium compounds crystallize in known intermetallic structure types, interestingly they are isotypic to lithium tetrelides, The surroundings of both potassium atoms is similar, the main discrepancy is due to the involvement of the Tl3 trimer instead of a sodium atom for K2
Summary
Dullenkopf reported on the crystal structure determination of sodium thallide NaTl [1] They completed their publication with the statement that, in case of a base metal, the formed structure type does not correlate with the average number of valence electrons per atom (VEC) according to the prevously introduced rules by Hume-Rothery [2], Westgren, and Phragmen [3]. This set a milestone in intermetallic chemistry and was the birth of polar intermetallic compounds, including metal-metal bonding (i.e., Zintl phases) [4,5,6,7,8,9] which perceive continually increasing interest in materials science [10]. Special attention will be drawn at the (extended) local environment of the alkali metals in the different alkali metal thallides based on the different crystallographic sites (Wyckoff positions) of the alkali metals in the respective published crystal structures
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