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Abstract:
Powder samples as well as red and transparent single crystals of the Zintl phase Cs7NaSi8 were synthesized and characterized by means of X-ray diffraction and differential thermal analysis. Cs7NaSi8 was found to be isotypic to the recently reported phase Rb7NaSi8. It crystallizes in the Rb7NaGe8 structure type forming trigonal pyramidal Si-4(4-) anions. Two unique environments of the cations are observed, a linear arrangement [Na(Si-4)(2)](7-) with short Na-Si distances of 3.0 angstrom and a Cs2 atom coordinated by six Si-4(4-) anions with long Cs-Si distances of 4.2 angstrom. The bonding situation was investigated by a combined application of Si-29, Na-23, and Cs-133 solid-state NMR spectroscopy and quantum mechanical calculations of the NMR coupling parameters. In addition the electronic density of states (DOS), the electron localizability indicator (ELI) and the atomic charges using the QTAIM approach were studied. Good agreement of the calculated and experimental values of the NMR coupling parameters was obtained. An anisotropic bonding situation of the silicon atoms is indicated by the chemical shift anisotropy being similar to Rb7NaSi8. Confirmation is given by the observation of one lone-pair-like feature for each silicon atom and two types of two-center Si-Si bonds using the ELI. Calculation and NMR spectroscopic determination of the Na-23 and Cs-133 electric field gradients prove anisotropies of the charge distribution around the cations. Due to the similar values for the Na atoms in M7NaSi8 (M = Rb, Cs) equal bonding situations can be concluded. The much larger anisotropy of the charge distribution of the Cs atoms can be addressed as the main difference to Rb7NaSi8.
