Crystal structures and hardness of novel compounds: Hexagonal Mo(Cu x Al 1x ) 6 Al 4 , MoCu 2 Al 8x and orthorhombic {Mo,W,Re}Ni 2x Al 8þx Atta U. Khan a , A. Grytsiv a , P. Rogl a, * , G. Giester b a Institute of Physical Chemistry, University of Vienna, Währingerstrasse 42, A-1090 Wien, Austria b Institute of Mineralogy and Crystallography, University of Vienna, Althanstrasse14, A-1090 Wien, Austria article info Article history: Received 17 October 2011 Received in revised form 2 December 2011 Accepted 5 December 2011 Available online 29 December 2011 Keywords: A. Aluminides, miscellaneous A. Intermetallics, miscellaneous B. Crystallography B. Phase diagrams abstract The crystal structures of a series of compounds have been solved from X-ray single crystal diffrac- tometry: Mo(Cu x Al 1x ) 6 Al 4 (x ¼ 0.416), MoNi 2x Al 8þx (x ¼ 0.165), WNi 2xy , y Al 8þxz , z ,(x ¼ 0.162, y ¼ 0.015, z ¼ 0.010) and ReNi 2 Al 8x (x ¼ 0.033). Mo(Cu x Al 1x ) 6 Al 4 crystallizes with hexagonal symmetry and lattice parameters, a ¼ 0.50030(1) and c ¼ 0.76279(3) nm; space group P6/mmm, No. 191, as an unfilled structure variant of the MgFe 6 Ge 6 -type with a vacant 2c site. This structure is made up of alternate blocks of (unfilled) CaCu 5 and Zr 4 Al 3 in a ratio of 1:1. A superstructure of this compound (a ¼ a 0 O3, c ¼ 2c 0 ) was solved with a Rietveld refinement of X-ray powder diffraction data as a fully ordered structure with formula MoCu 2 Al 8x . It adopts space group P6, No. 168 with lattice parameters a ¼ 0.86769(1), c ¼ 1.52149(2) nm. A Bärnighausen tree was derived reflecting the close relation among these two structure types. The compounds {Mo,W,Re}Ni 2x Al 8þx crystallize in a novel structure type (ReNi 2 Al 8x etype; space group Pbam, No. 55). Whereas ReNi 2 Al 8x is a fully ordered structure with some defects in the two 4g sites occupied by Al, Ni þ Al atoms randomly share one crystallographic site in isotypic {Mo,W}Ni 2x Al 8þx , which causes splitting of three neighboring Al-sites. Lattice parameters and residual values of the refinements were: a ¼ 1.00320(2), b ¼ 1.51258(3), c ¼ 0.83890(2) nm; R F2 ¼ 0.017 for the Re-compound, a ¼ 1.00664(2), b ¼ 1.53108(2), c ¼ 0.85205(2) nm, R F2 ¼ 0.035 for the Mo-compound and a ¼ 1.00683(2), b ¼ 1.53236(3), c ¼ 0.85232(2) nm; R F2 ¼ 0.024 for the W-compound. As the ReNi 2 Al 8x -type structure is another superstructure of the hexagonal Mo (Cu x Al 1x ) 6 Al 4 type, a Bärnighausen tree was derived, which documents the close relation among these two structure types. Phase equilibria in the Al-rich corner of WeNieAl at 930  C is also reported showing the presence of a WNi 2 Al 8 phase. Precise atom positions have been derived from X-ray single crystal data for binary Ni 2 Al 3 confirming the space group P 3m1 (No. 164). Atom positions and lattice parameters (a ¼ 0.40533(2), c ¼ 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}Ni 2 Al 8x revealing a hardness of H V ¼ 965  25 MPa for the Re-containing material, which is higher by about 100 MPa in comparison to the Mo and W containing phases. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Molybdenum has become an important constituent for Ni-based superalloys because it can improve the creep properties [1]. Besides this, binary Al(Mo)-based alloys with less than 30% Mo show melting temperatures higher than 1500  C but still are light in weight [2]. Low-temperature brittleness, however, has so far restricted utilization of molybdenum aluminides and requests further improvement of mechanical properties. In this respect, group-VIII elements as alloy constituents prove very interesting and among them particularly nickel. Successful alloy design of such high-strength materials is based on a profound knowledge of phase equilibria and crystal structures. For the MoeNieAl system, two recent assessments [3,4] have summarized all information existing in the literature confirming two ternary compounds in the Al-rich part of the phase diagram: the so-called N-phase and the X-phase. The N-phase exists in a small homogeneity region, Mo(Ni x Al 1x ) 3 with 0.53  x  2 in as cast samples [5] and was identified as TiAl 3 -type by Markiv et al. [6]. The crystal structure of the X-phase (earlier denoted as Mo 5 Ni 18 Al 77 [6]), however, remained unknown, although a detailed TEM (Transmission Elec- tron Microscopy) study [5] revealed orthorhombic symmetry and * Corresponding author. Tel.: þ43 1 4277 52456; fax: þ43 1 4277 9524. E-mail address: peter.franz.rogl@univie.ac.at (P. Rogl). Contents lists available at SciVerse ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.12.003 Intermetallics 23 (2012) 187e198