M₂Ba₂Sn₆ (M =Yb, Ca): Metallic Zintl Phases with a Novel Tin Chain Substructure

The compounds M2Ba2Sn6 (M = Yb, Ca) have been synthesized by solid-state reactions in welded Ta tubes at high temperature. Their structures were determined by single-crystal X-ray diffraction studies to be orthorhombic; space group Cmca (No. 64); Z = 8; a = 15.871(3), 15.912 (3) Å; b = 9.387(2), 9.497(2) Å; c = 17.212(3), 17.184(3) Å; and V = 2564.3(9), 2597.0(9) Å3, respectively. These contain infinite tin chains along a⃗ constructed from butterflylike 3-bonded Sn tetramers interconnected by pairs of 2-bonded Sn. The chains are further interconnected into corrugated layers by somewhat longer Sn−Sn bonds along c⃗. The compounds with the chains alone would be Zintl phases, but the interchain bonding makes them formally one-electron rich per formula unit. The electronic structures calculated by extended Hückel and TB-LMTO-ASA methods indicate that these compounds are metallic but with a deep pseudogap at the Fermi level. States that bind the extra electrons lie just below EF and involve important Yb(Ca)−Sn contributions. The origin of metallic Zintl phases is briefly discussed.