Y₃MnAu₅: Three Distinctive <i>d</i>‑Metal Functions in an Intergrown Cluster Phase

The new Y3MnAu5 intermetallic phase is obtained from the arc-melted elements in virtually quantitative yields after annealing at 1000 °C for ∼3 d. Its remarkable structure [rhombohedral, R3̅, Z = 6; a = 8.489(1) Å, c = 18.144(2) Å] consists of a 2:1 cubic-close-packed intergrowth between edge-shared Mn-centered Au rhombohedra (Mn@Au8) with gold-centered antiprismatic (Au@Y6) clusters via a common gold network. Magnetic susceptibility (χ) data for Y3MnAu5 were fitted by a Curie–Weiss law. The Curie constant indicates a large effective moment corresponding to nearly localized Mn spins S = 5/2, and the Weiss temperature demonstrates the dominance of ferromagnetic (FM) interactions. An antiferromagnetic (AFM) transition at TN = 75 K and a possible spin reorientation transition at 65 K were observed. Analysis of the χ data for T < TN suggests a planar noncollinear helical AFM structure that arises from competing AFM interactions between FM-aligned layers of spins in the ab-plane with a turn angle of 69° between the spins along the helix c-axis. A magnetic field-induced spin flop transition is observed below TN. Spin-polarized LMTO-LSDA calculations indicate an ∼2 eV splitting of the Mn 3d states and a metallic ground state, and their COHP analyses demonstrate that ∼81% of the total Hamilton populations originate from heteroatomic polar Y–Au and Mn–Au bonding. The Mn 3d, Y 4d, and Au 5d characteristics are remarkably diverse: localized and magnetically polarized for Mn; reducing and cationic for Y; and relativistically strongly bonded and oxidizing for Au, bonding of the latter two being broadly delocalized.