On the divalent character of the Eu atoms in the ternary Zintl phases Eu5In2Pn6 and Eu3MAs3 (Pn = As–Bi; M = Al, Ga)

Abstract

Five Zintl phases in the ternary system Eu–M–Pn (M = Al, In; Pn = As, Sb, Bi) were prepared from the elements in tantalum containers. Eu₅In₂As₆ and Eu₅In₂Sb₆ crystallize in the orthorhombic Ca₅Ga₂As₆ type structure (Pbam, oP26), while Eu₅In₂Bi₆ is isostructural to orthorhombic Ca₅Al₂Bi₆ (Pbam, oP26). Eu₃AlAs₃ adopts a monoclinic structure type (P2₁/c, mP28), which is isopointal to Rb₃TlO₃, and Eu₃GaAs₃ (Cmce, oS56), finally, crystallizes in the orthorhombic Ba₃AlSb₃ type structure. All structures have been refined from single crystal X-ray diffraction experiments and can be considered to be Zintl phases with a valence precise sum formula according to (Eu²⁺)₅(In³⁺)₂(Pn³⁻)₄(Pn²⁻)₂ and (Eu²⁺)₃(M³⁺)(As³⁻)₃. They all feature [MPn₄] tetrahedra, which are connected in different ways. While in the Ca₅Ga₂As₆ and Ca₅Al₂Bi₆ type representatives double strands via Pn–Pn bonds are formed, in Eu₃AlAs₃ and Eu₃GaAs₃, [M₂As₆]⁶⁻ tetrahedral dimers exist. The divalent europium atoms are located in between the chains, providing electroneutrality. The magnetic properties of four compounds have been investigated and complex (antiferro)magnetic ordering has been observed at T_N = 16.1(1) (Eu₅In₂As₆), 17.8(1) K (Eu₅In₂Sb₆), 10.0(1) K (Eu₃AlAs₃) and 10.7(1) K (Eu₃GaAs₃). The effective magnetic moment and ¹⁵¹Eu Mössbauer spectroscopic investigations unambiguously proved the divalent character of the Eu atoms. The spectra recorded below the magnetic ordering showed a (full) hyperfine field splitting. Additionally, ¹²¹Sb Mössbauer spectroscopic studies have been conducted on the antimonide Eu₅In₂Sb₆. Finally, computational studies of Eu₃AlAs₃ and Eu₅In₂Sb₆ indicate semiconducting behavior for the arsenide with a bandgap of ca. 1 eV, while an increased metallicity, manifested in a pseudo gap for the antimonide, is visible at the Fermi level.