The stannides LiAuSn and LiAu3Sn4: synthesis, structure and chemical bonding

Abstract

The stannides LiAuSn and LiAu3Sn4 were synthesized from the elements by reaction in sealed tantalum tubes in a resistance furnace. LiAuSn crystallizes with a ZrBeSi type structure, space group P63/mmc: a = 467.08(8) pm, c = 603.7(1) pm, wR2 = 0.0403, 87 F2 values, 7 variables. The gold and tin atoms form a planar network of condensed Au3Sn3 hexagons (270 pm Au–Sn) which are rotated by 60° around the c axis in every other layer. The lithium atoms are located between these layers. LiAuSn may be classified as a lithium intercalated heterographite according to Li+[AuSn]−. LiAu3Sn4 adopts a polar structure, space group P63mc, which has been refined for an inversion twin: a = 448.31(6) pm, c = 2055.7(4) pm, wR2 = 0.0814, BASF = 0.13(2), 699 F2 values, 25 variables. This new structure type may be considered as an intergrowth of slightly distorted NiAs and CaAl2Si2 related slabs of compositions AuSn and LiAu2Sn2. Short (285 pm) Au–Au distances within the AuSn slab are indicative for significant Au–Au bonding. The lithium atoms fill tetrahedral voids formed by the gold atoms within the LiAu2Sn2 slab. Chemical bonding analysis by TB-LMTO-ASA calculations confirm the covalently bonded [AuSn]− polyanion in LiAuSn. Each network atom participates in three two-electron two-center Au–Sn σ-bonds, whereas π-bonding is absent. The crystal orbital Hamilton populations (COHP) analysis of LiAu3Sn4 reveals almost equal Au–Sn bonding energies for the three Au atoms despite their different tin environments. Au–Au bonding in LiAu3Sn4 is strong and shows the same electronic features as in binary AuSn with NiAs structure.