Eu7Ga6Sb8: A Zintl phase with Ga-Ga bonds and polymeric gallium antimonide chains

The Zintl phase Eu₇Ga₆Sb₈ was obtained from a direct element combination reaction at 900°C. It crystallizes in the orthorhombic space group Pbca (No. 61) with a=15.6470(17) Å, b=17.2876(19) Å, c=17.9200(19) Å, and Z=8. In Eu₇Ga₆Sb₈, the anionic framework forms infinite chains of [Ga₆Sb₈]¹⁴⁻ which are arranged side by side to make a sheet-like arrangement but without linking. The sheets of chains are separated by Eu²⁺ atoms and also within the sheet, Eu²⁺ atoms fill the spaces between two chains. The chain is made up of homoatomic tetramers (Ga₄)⁶⁺ and dimers (Ga₂)⁴⁺ connected by Sb atoms. The compound is a narrow band-gap semiconductor with E_g∼0.6 eV and satisfies the classical Zintl concept. Extended Hückel band structure calculations confirm that the material is a semiconductor and suggest that the structure is stabilized by strong Ga-Ga covalent bonding interactions. Magnetic susceptibility measurements for Eu₇Ga₆Sb₈ show that the Eu atoms are divalent and the compound has an antiferromagnetic transition at 9 K.