Synthesis and Structure of Isolated Silicon Clusters of Nine Atoms

We have discovered and structurally characterized the first “naked” silicon clusters larger than four atoms, Si9, in the compound Rb12Si17. Silicon is of unparalleled importance for many electronic applications, and the research associated with it spans over many different areas of science: chemistry, physics, surface science, materials processing, etc. In addition to the extensively studied silicon clusters in the gas phase,1 much interest has been focused lately on silicon nanoparticles and porous silicon due to their valuable optical properties.2 Recently, the first synthesis of such Si and Ge nanoparticles in liquid medium was reported.3 This approach utilizes a metathesis reaction (comproportionation) between the negatively charged silicon or germanium in the solids KSi or NaGe, respectively, and the corresponding positively charged ions in the tetrachlorides. Perhaps a better approach to the synthesis of such nanoparticles is to use premade silicon clusters in the solid state, extract them intact into a solution, and then use a metathesis reaction. Unfortunately, the only known such species, Si4 tetrahedra in A4Si4 (A ) alkali metal)4 and some lithium-stabilized planar silicon formations,5 cannot be extracted, apparently due to the large charge per atom ratio.6 The same is true for the tetrahedra of the other tetrels (tetrel ) a group 14 element), Ge4, Sn4, and Pb4. On the other hand, nine-atom clusters of germanium, tin, and lead have been identified in solutions made by dissolving the corresponding alkali-metal tetrelides in ethylenediamine or liquid ammonia.7 Such clusters with different shapes and charges have also been crystallized from the corresponding solutions and have been structurally characterized.6 It should be pointed out that in all reports the tetrelide precursors for such solutions have been labeled either “alloys” or “melts”, i.e., substances with no particular structure and without defined cluster formations. Hence, the current understanding on the formations of the clusters in solution is that they do not exist in the precursors but are rather assembled somehow during the process of dissolution.6 However, our recent studies of the systems alkali-metal-tetrel show that clusters of Ge9 and Pb9 exist in the compounds Cs4Ge9 and K4Pb9, respectively.8 Similarly, recent Raman studies of these and other alkali-metal-tetrel systems also suggest the existence of such clusters.9 The same clusters most likely existed in the precursors for the solution studies. The new compound, Rb12Si17, contains isolated nine-atom silicon clusters, Si9, and is a potential candidate for a precursor for silicon clusters in solution. The compound is made at 900 °C (kept for 1 h at that temperature and then slowly cooled with a rate of 5°/h) by direct synthesis from the pure elements sealed in niobium containers and jacketed in evacuated ampules of fused-silica. The same approach also yields the isostructural K12Sn17 (the phase which most likely “produces” the Sn9 in solution) and (KxRb1-x)12Si17. Furthermore, phases with the same or very similar structures and stoichiometries seem to exist in the systems Cs-Si, Cs-Sn, RbSn, Cs-Pb, and Rb-Pb, according to their X-ray diffraction powder patterns.