Four zirconium iodide cluster phases centered by boron, aluminum, or silicon

Abstract

Well-faceted crystals of Zr/sub 6/I/sub 12/B, MZr/sub 6/I/sub 14/B (M = Cs or K), Cs/sub 0.7/Zr/sub 6/I/sub 14/Si are obtained in good yield from reactions of stoichiometric amounts of Zr, ZrI/sub 4/, CsI, or KI when appropriate and B, Si, or AlI/sub 3/ at 850/sup 0/C in sealed tantalum containers after 2 weeks. The compounds were established to be isostructural with Zr/sub 6/I/sub 12/C (space group R/sup 3/) or (Cs)Zr/sub 6/I/sub 14/C (Cmca), and one crystal structure for each of the four examples (M = Cs) was refined with use of single-crystal X-ray techniques. All of these compounds contain Zr/sub 6/I/sub 12/-type clusters with an interstitial B, Al, or Si (Z) atom in the center of the cluster at full occupancy. Changes in Z cause substantial changes in the host lattice, most directly in the size of the cluster where anti d(Zr-Zr) ranges between 3.580 A (Si) and 3.195 A (Zr/sub 6/I/sub 12/C). The observed anti d(Zr-Z) values are tenths of an angstrom less than those consistently observed in ZrZ/sub x/ phases, especially for aluminum. Extended-Hueckel calculations illustrate the important role of the interstitial in providing both additional electrons and strong Zr-Z bonding to the cluster. Increasing Hi/sub i/ valuesmore » for the interstitial valence orbitals in the order C, Si, B, Al reduce Zr-Z bonding and charge transfer to the interstitial atom, especially for aluminum 3p orbitals. The bonding in Zr/sub 2/Al shows some comparable properties.« less