Eu₅In₂Sb₆, Eu₅In_2-xZn_xSb₆: Rare earth zintl phases with narrow band gaps

The new Zintl phase Eu₅In₂Sb₆ was obtained from a direct element combination reaction in a sealed graphite tube at 870°C and its structure was determined. It crystallizes in the orthorhombic space group Pbam(No. 55), with a unit cell of a = 12.510(3) Å, b = 14.584(3) Å, c = 4.6243(9) Å, and Z = 2. Eu₅In₂Sb₆ shows the Ca₅Ga₂As₆-type structure and has a one-dimensional structure with infinite anionic double chains [In₂Sb₆]^10- separated by Eu²⁺ ions. Each single chain is made of corner sharing InSb₄ tetrahedra. Two such tetrahedral chains are bridged by a Sb₂ group to form double chains. The compound satisfies the classical Zintl concept and is a narrow band gap semiconductor. The substitution of Zn for In atoms in Eu₅In₂Sb₆ resulted in an increase in hole concentration and the material is more metallic. Polycrystalline ingots of Eu₅In₂Sb₆ and Eu₅In_2-xZn_x,Sb₆ showed electrical conductivity of ∼36 S cm^-1, ∼146 S cm^-1 and Seebeck coefficient of ∼76 μV K^-1and ∼51 μV K^-1 at room temperature, respectively.