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