A single-crystal XRD and TEM study of "ScB17C0.25"

In auxiliary metal fluxes (Si, Sn) at temperatures around 1650 degreesC the crystal growth of "ScB17C0.25" was successfully performed. While in the Si flux agglomerates of whiskerlike crystals are formed, the change to Sn sig nificantly improved the quality of the columnar crystals with respect to si ze and surface smoothness. Based on single-crystal X-ray data collected on a four-circle diffractometer using MoK alpha radiation and high-resolution transmission electron microscopy (HRTEM) the structure was solved by direct methods and refined to an reliability value R-1 of 0.055 for 661 F-0 > 4 s igma and 64 variables. The novel crystal structure belongs to the hexagonal system (space group P6/mmrn), with lattice constants a, b=14.5501(15) Angs trom and c=8.4543(16) Angstrom and is formed by a framework based on B-12 i cosahedra which are radially connected to unusual "tubular" boron-based str uctural units. The Sc atoms reside on interstitial sites. In addition to th e boron based clusters interstitial B atoms are found in the structure. The main structural features, which are governed by the arrangement of the bor on clusters and the Sc atoms in the unit cell, could be unambiguously resol ved by HRTEM, and simulated images match observed ones perfectly. (C) 2000 Academic Press.