Crystal structure of novel compounds in the systems Zr–Cu–Al, Mo–Pd–Al and partial phase equilibria in the Mo–Pd–Al system

Abstract

The crystal structures of three Al-rich compounds have been solved from X-ray single crystal diffractometry: τ₁-MoPd_2−xAl₈₊x (x = 0.067); τ₇-Zr(Cu_1−xAl_x)₁₂ (x = 0.514) and τ₉-ZrCu_1−xAl₄ (x = 0.144). τ₁-MoPd_2−xAl₈₊x adopts a unique structure type (space groupPbcm; lattice parameters a = 0.78153(2), b = 1.02643(3) and c = 0.86098(2) nm), which can be conceived as a superstructure of the Mo(Cu_xAl_1−x)₆Al₄ type. Whereas Mo-atoms occupy the 4d site, Pd(2) occupies the 4c site, Al and Pd(1) atoms randomly share the 4d position and the rest of the positions are fully occupied by Al. A Bärnighausen tree documents the crystallographic group-subgroup relation between the structure types of Mo(Cu_xAl_1−x)₆Al₄ and τ₁. τ₇-Zr(Cu_1−xAl_x)₁₂ (x = 0.514) has been confirmed to crystallize with the ThMn₁₂ type (space groupI4/mmm; lattice parameters a = 0.85243(2) and c = 0.50862(3) nm). In total, 4 crystallographic sites were defined, out of which, Zr occupies site 2a, the 8f site is fully occupied by Cu, the 8i site is entirely occupied by Al, but the 8j site turned out to comprise a random mixture of Cu and Al atoms. The compound τ₉-ZrCu_1−xAl₄ (x = 0.144) crystallizes in a unique structure type (space groupP4/nmm; lattice parameters a = 0.40275(3) and c = 1.17688(4) nm) which exhibits full atom order but a vacancy (14.4%) on the 2c site, shared with Cu atoms. τ₉-ZrCu_1−xAl₄ is a superstructure of Cu with an arrangement of three unit cells of Cu in the direction of the c-axis. A Bärnighausen tree documents this relationship. The ZrCu_1−xAl₄ type (n = 3) is part of a series of structures which follow this building principle: Cu (n = 1), TiAl₃ (n = 2), τ₅-TiNi_2−xAl₅ (n = 4), HfGa₂ (n = 6) and Cu₃Pd (n = 7). A partial isothermal section for the Al-rich part of the Mo–Pd–Al system at 860 °C has been established with two ternary compounds τ₁-MoPd_2−xAl₈₊x and τ₂ (unknown structure). The Vickers hardness (H_v) for τ₁ was found to be 842 ± 40 MPa.