Synthesis, Structure, and Bonding of Eu₃Bi(Sn,Bi)₄. A Rare Inverse-Cr₅B₃-Type Structure with a New Tin/Bismuth Network

The ternary phase Eu₃Bi(Sn_1-xBi_x)₄ (∼0 < x < ∼0.15) has been synthesized by solid-state methods at high temperature. The crystal structure of the limiting Eu₃Bi(Sn_3.39Bi_0.61(3)) has been determined by single-crystal X-ray analysis to be isopointal with an inverse-Cr₅B₃-type structure [space group I4/mcm, Z = 4, a = 8.826(1) Å, c = 12.564(3) Å, and V = 978.6(3) ų]. The structure contains slabs of three-bonded Sn/Bi atoms as puckered eight- and four-membered rings interlinked at all vertices, and these are separated by planar layers of individual Eu and Bi atoms. In the normal (stuffed) Cr₅B₃-type analogue Eu₅Sn₃H_x, these two units are replaced by a more highly puckered network of Eu cations around isolated Sn atoms and planar layers of isolated Eu atoms and Sn dimers, respectively. Band structures of limiting models of the phase calculated by TB-LMTO-ASA methods show a metallic character and indicate that the mixed Sn/Bi occupancy in the slabs in this structure for x > 0 probably originates with the electronic advantages of the pseudogap that would occur at the electron count of the ideal Zintl phase Eu₃Bi(Sn₃Bi). The stability of a competing phase reduces this limit to Eu₃Bi(Sn_3.4Bi_0.6).