A single-crystal XRD and TEM study of 'ScB₁₇C_0.25'

In auxiliary metal fluxes (Si, Sn) at temperatures around 1650°C the crystal growth of 'ScB₁₇C_0.25' was successfully performed. While in the Si flux agglomerates of whisker-like crystals are formed, the change to Sn significantly improved the quality of the columnar crystals with respect to size and surface smoothness. Based on single-crystal X-ray data collected on a four-circle diffractometer using MoKα radiation and high-resolution transmission electron microscopy (HRTEM) the structure was solved by direct methods and refined to an reliability value R₁ of 0.055 for 661 F₀ > 4σ and 64 variables. The novel crystal structure belongs to the hexagonal system (space group P6/mmm), with lattice constants a, b = 14.5501(15) Å and c = 8.4543(16) Å and is formed by a framework based on B₁₂ icosahedra which are radially connected to unusual 'tubular', boron-based structural units. The Sc atoms reside on interstitial sites. In addition to the boron based clusters interstitial B atoms are found in the structure. The main structural features, which are governed by the arrangement of the boron clusters and the Sc atoms in the unit cell, could be unambiguously resolved by HRTEM, and simulated images match observed ones perfectly. (C) 2000 Academic Press.