Structure of the icosahedral Ti-Zr-Ni quasicrystal

The atomic structure of the icosahedral Ti-Zr-Ni quasicrystal is determined by invoking similarities to periodic crystalline phases, diffraction data and the results from ab initio calculations. The structure is modeled by decorations of the canonical cell tiling geometry. The initial decoration model is based on the structure of the Frank-Kasper phase W-TiZrNi, the 1/1 approximant structure of the quasicrystal. The decoration model is optimized using a new method of structural analysis combining a least-squares refinement of diffraction data with results from ab initio calculations. The resulting structural model of icosahedral Ti-Zr-Ni is interpreted as a simple decoration rule and structural details are discussed.Comment: 12 pages, 8 figure

Borohydrides: from sheet to framework topologies

The five novel compounds ALiM(BH4)4 (A = K or Rb; M = Mg or Mn) and K3Li2Mg2(BH4)9 crystallizing in the space groups Aba2 and P2/c, respectively, represent the first two-dimensional topologies amongst homoleptic borohydrides. The crystal structures have been solved, refined and characterized by synchrotron X-ray powder diffraction, neutron powder diffraction and solid-state DFT calculations. Minimal energies of ordered models corroborate crystal symmetries retrieved from diffraction data. The layered Li–Mg substructure forms negatively charged uninodal 4-connected networks. It is shown that this connectivity cannot generate the long sought-after, bimetallic Li–Mg borohydrides without countercations when assuming preferred coordination polyhedra as found in Mg(BH4)2 and LiBH4. The general properties of the trimetallic compound series are analogous with the anhydrous aluminosilicates. Additionally, a relationship with zeolites is suggested, which are built from three-dimensional Al–Si–O networks with a negative charge on them. The ternary metal borohydride systems are of interest due to their potential as novel hydridic frameworks and will allow exploration of the structural chemistry of light-metal systems otherwise subject to eutectic melting

Automatic software correction of residual aberrations in reconstructed HRTEM exit waves of crystalline samples

We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a β-Si(3)N(4) wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and lattice fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves

Potassium zinc borohydrides containing triangular [Zn(BH4)3]-- and tetrahedral [Zn(BH4)xCl4-x]2-- anions

Three novel potassium-zinc borohydrides/chlorides are described. KZn(BH4)3 and K2Zn(BH4)xCl4-x form in ball-milled KBH4:ZnCl2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K3Zn(BH4)xCl5-x forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH4)3 contains an anionic complex [Zn(BH4)3]− with D3 (32) symmetry, located inside a rhombohedron K8. KZn(BH4)3 contains 8.1 wt % of hydrogen and decomposes at 385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn2(BH4)5 (M = Li, Na) and NaZn(BH4)3. The decomposition temperature is much lower than for KBH4. Monoclinic K2Zn(BH4)xCl4-x contains a tetrahedral complex anion [Zn(BH4)xCl4-x]2- located inside an Edshammar polyhedron (pentacapped trigonal prism) K11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K2ZnCl4 (structure type K2SO4). K2Zn(BH4)xCl4-x releases BH4 starting from 395 K, forming Zn and KBH4. As the reaction proceeds and x decreases, the monoclinic distortion of K2Zn(BH4)xCl4-x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K2ZnCl4. Tetragonal K3Zn(BH4)xCl5-x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs3CoCl5 structure type possibly with the space group P42/ncm. K3Zn(BH4)xCl5-x decomposes nearly at the same temperature as KZn(BH4)3, i.e., at 400 K, with the formation of K2Zn(BH4)xCl4-x and KBH4, indicating that the compound is an adduct of the two latter compounds