Crystal structures and hardness of novel compounds: Hexagonal Mo(CuxAl1-x)(6)Al-4, MoCu2Al8-x and orthorhombic {Mo,W,Re}Ni2-xAl8+x
- Author(s)
- Andrij Grytsiv, Peter Franz Rogl, Gerald Giester
- Abstract
The crystal structures of a series of compounds have been solved from X-ray single crystal diffractometry: Mo(CuxAl1-x)(6)Al-4 (x = 0.416), MoNi2-xAl8+delta (x = 0.165), WNi2-x-y square Al-y(8+x-z)square(z). (x = 0.162, y = 0.015, z = 0.010) and ReNi2Al8-x (x = 0.033). Mo(CuxAl1-x)(6)Al-4 crystallizes with hexagonal symmetry and lattice parameters, a = 0.50030(1) and c = 0.76279(3) nm; space group P6/rnmm, No. 191, as an unfilled structure variant of the MgFe6Ge6-type with a vacant 2c site. This structure is made up of alternate blocks of (unfilled) CaCu5 and Zr4Al3 in a ratio of 1:1. A superstructure of this compound (a = a(0)root 3, c = 2c(0)) was solved with a Rietveld refinement of X-ray powder diffraction data as a fully ordered structure with formula MoCu2Al8-x,. It adopts space group P6, No. 168 with lattice parameters a = 0.86769(1), c = 1.52149(2) nm. A Barnighausen tree was derived reflecting the close relation among these two structure types. The compounds {Mo,W,Re)Ni2-xAl8+x crystallize in a novel structure type (ReNi2Al8-x-type; space group Pbam, No. 55). Whereas ReNi2Al8-x, is a fully ordered structure with some defects in the two 4g sites occupied by Al, Ni + Al atoms randomly share one crystallographic site in isotypic (Mo,W)Ni2-xAl8+x, which causes splitting of three neighboring Al-sites. Lattice parameters and residual values of the refinements were: a = 1.00320(2), b = 1.51258(3), c = 0.83890(2) nm; R-F2 = 0.017 for the Re-compound, a = 1.00664(2), b = 1.53108(2), c = 0.85205(2) nm, R-F2 = 0.035 for the Mo-compound and a = 1.00683(2), b = 1.53236(3), c = 0.85232(2) nm; R-F2 = 0.024 for the W-compound. As the ReNi2Al8-x-type structure is another superstructure of the hexagonal Mo (Cu5Al1-x)(6)Al-4 type, a Barnighausen tree was derived, which documents the close relation among these two structure types. Phase equilibria in the Al-rich corner of W-Ni-Al at 930 C is also reported showing the presence of a WNi2Al8 phase. Precise atom positions have been derived from X-ray single crystal data for binary Ni2Al3 confirming the space group P (3) over bar m1 (No. 164). Atom positions and lattice parameters (a = 0.40533(2), c = 0.49038(3) nm) are very close to the parameters hitherto reported in the literature from X-ray powder diffraction data. Vicker's hardness was measured for all isotypic compounds (Mo,W,Re)Ni2Al8-x revealing a hardness of Hv = 965 +/- 25 MPa for the Re-containing material, which is higher by about 100 MPa in comparison to the Mo and W containing phases.
- Organisation(s)
- Department of Physical Chemistry, Department of Mineralogy and Crystallography
- Journal
- Intermetallics
- Volume
- 23
- Pages
- 187-198
- No. of pages
- 12
- ISSN
- 0966-9795
- DOI
- https://doi.org/10.1016/j.intermet.2011.12.003
- Publication date
- 2012
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 104003 Inorganic chemistry, 105113 Crystallography, 104011 Materials chemistry
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/fe75fbbb-f211-45f1-a7e5-cb9081455049