Zigzag chains of alternating atoms in A₂AuBi (A = Na, K) and K₂AuSb. synthesis, structure, and bonding

The polar intermetallic phases A₂uBi (A = Na (I), K (II)), and K₂AuSb (III) were obtained by direct reaction of the elements at elevated temperatures in niobium ampoules. The three isotypic compounds extend the prominent A₂MPn (Ai = transition metal; Pn = group 15) structure family. Single-crystal X-ray diffraction experiments show that these phases (I / II / III) crystallize in the orthorhombic space group Cmcm (No. 63), with a = 9.447(2) / 10.632(2) / 10.450(2) A; b = 7.700(2) / 7.918(2) / 7.857(2) Å; c = 5.849(1) / 6.597(1) / 6.501(1) Å; V = 425.4(2) / 555.4(2) / 533.8(2) A³, respectively, and Z = 4. The simple structures, which contain zigzag chains of linear two-bonded gold atoms and acute angles (64 to 76°) at two-bonded antimony or bismuth, can readily be rationalized with the Zintl-Klemm concept. TB-LMTO calculations show distinct band gaps at Ef, and analyses of DOS and COHP curves show optimized AU-PM bonding, appreciable Au-Au interactions across the chains, and covalent interactions between A and Pn atoms. The observed structural preference over an isoelectronic NaTl type structure, the cation influences, and site coloring were investigated with VASP and EHTB calculations.