Crystal structures and hardness of novel compounds: Hexagonal Mo(CuxAl1−x)6Al4, MoCu2Al8−x and orthorhombic {Mo,W,Re}Ni2−xAl8+x

Abstract

The crystal structures of a series of compounds have been solved from X-ray single crystal diffractometry: Mo(CuxAl1−x)6Al4 (x = 0.416), MoNi2−xAl8+x (x = 0.165), WNi2−x−y□yAl8+x−z□z, (x = 0.162, y = 0.015, z = 0.010) and ReNi2Al8−x (x = 0.033). Mo(CuxAl1−x)6Al4 crystallizes with hexagonal symmetry and lattice parameters, a = 0.50030(1) and c = 0.76279(3) nm; space group P6/mmm, 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 = a0√3, c = 2c0) 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 Bärnighausen 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; RF2 = 0.017 for the Re-compound, a = 1.00664(2), b = 1.53108(2), c = 0.85205(2) nm, RF2 = 0.035 for the Mo-compound and a = 1.00683(2), b = 1.53236(3), c = 0.85232(2) nm; RF2 = 0.024 for the W-compound. As the ReNi2Al8−x-type structure is another superstructure of the hexagonal Mo(CuxAl1−x)6Al4 type, a Bärnighausen 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 P3¯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.