Abstract
Alkali metal thallides have been known since the report of E. Zintl on NaTl in 1932. Subsequently, binary and ternary thallides of alkali metals have been characterized. At an alkali metal proportion of approximately 33% (A:Tl~1:2, A = alkali metal), three different unique type structures are reported: K49Tl108, Rb17Tl41 and A15Tl27 (A = Rb, Cs). Whereas Rb17Tl41 and K49Tl108 feature a three-dimensional sublattice of Tl atoms, the A15Tl27 structure type includes isolated Tl11 clusters as well as two-dimensional Tl-layers. This unique arrangement is only known so far when the heavier alkali metals Rb and Cs are included. In our contribution, we present single-crystal X-ray structure analyses of new ternary and quaternary compounds of the A15Tl27 type structure, which include different amounts of potassium. The crystal structures allow for the discussion of the favored alkali metal for each of the four Wyckoff positions and clearly demonstrate alkali metal dependent site preferences. Thereby, the compound Cs2.27K12.73Tl27 unambiguously proves the possibility of a potassium-rich A15Tl27 phase, even though a small amount of cesium appears to be needed for the stabilization of the latter structure type. Furthermore, we also present two compounds that show an embedding of Tl instead of alkali metal into the two-dimensional substructure, being equivalent to the formal oxidation of the latter. Cs14.53Tl28.4 represents the binary compound with the so far largest proportion of incorporated Tl in the structure type A15Tl27.
Highlights
Alkali metal thallides represent a very interesting class of materials in terms of structural chemistry as they involve versatile thallium substructures depending on the amount of alkali metal involved, which is equitable to the valence electron concentration [1]
All compounds crystallize in the A15Tl27 (A = Cs, Rb)-type structure [14]. These alkali metal thallides naturally possess very high absorption coefficients (MoKα, μ > 70 mm−1), small single crystals were selected for the X-ray diffraction experiments, but the data sets still suffered from severe absorption effects, which could be reduced by applying absorption correction
It can be stated that substitution of the larger alkali metals in the A15Tl27 type structure by potassium is possible to a certain extent
Summary
Alkali metal thallides represent a very interesting class of materials in terms of structural chemistry as they involve versatile thallium substructures depending on the amount of alkali metal involved, which is equitable to the valence electron concentration [1]. Discrete Tl117— clusters are present, which are known from binary A8Tl11 (A=K, Rb, Cs) phases crystallizing in the K8In11-type structure [7,15]. In these compounds, an extra electron is present A8[Tr117—][e—] being responsible for pauli paramagnetism and metallic behavior. In the here investigated A15Tl27 compounds, beside the Tl11 clusters, an additional subunit of thallium atoms is present, two-dimensional Tl168— layers, which include large pores, in which alkali metal atoms reside [14]. The compounds Cs14.53Tl28.4 and Cs8.21Rb6.76Tl27.09 give the first evidence, that replacement of the alkali metal in the pores of the two-dimensional layer in Cs15Tl27 by thallium is possible
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