Zhiyun Wu

Electronic structure, physical properties and ionic mobility of LiAg2Sn

The stannide LiAg2Sn was synthesized from the elements by reaction in a sealed tantalum tube in a resistance furnace. LiAg2Sn crystallizes with a ternary ordered version of the cubic BiF3 structure, space group Fmm: a = 659.2(2) pm, wR2 = 0.0450, 69 F2 values, 5 variables. The silver and tin atoms form an antifluorite structure of composition Ag2Sn (285 pm Ag–Sn) in which the lithium atoms fill octahedral voids. Electronic structure calculations reveal weak Ag–Ag and strong Ag–Sn bonding within the Ag2Sn substructure. LiAg2Sn is weakly Pauli paramagnetic and a good metallic conductor. Nevertheless, the modestly small 7Li Knight shift is consistent with a nearly complete state of lithium ionization. The high local symmetry at the tin site is reflected by the absence of a nuclear electric quadrupolar splitting in the 119Sn Mossbauer spectra and a small chemical shift anisotropy evident from 119Sn solid state NMR. Static 7Li solid state NMR spectra reveals motional narrowing effects above 300 K, consistent with lithium atomic mobility on the kHz timescale.

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

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.

Structure Refinement of AuSn2

Well-shaped single crystals of binary AuSn2 were obtained as a side product during the synthesis of LiAu3Sn4. The structure of AuSn2 has been studied by X-ray diffractometer data: Pbca, Z = 8, a = 689.8(1), b = 701.1(1), c = 1177.3(2) pm, wR2 = 0.0533, 1234 F2 values, and 29 variables. The gold atoms show a distorted octahedral coordination by tin at Au-Sn distances ranging from 272 to 283 pm. The structure can be considered as an intergrowth of pyrite and marcasite related slabs. Consequently one observes Sn1-Sn2 dumb-bells with a Sn-Sn distance of 289 pm, while all other Sn-Sn distances are larger than 322 pm.

The Stannide LiRh3Sn5 – Synthesis, Structure, and Chemical Bonding

The lithium rhodium stannide LiRh3Sn5 was synthesized from the elements in a sealed tantalum tube and investigated via X-ray powder and single crystal diffraction: Pbcm, a = 538.9(1), b = 976.6(3), c = 1278.5(3) pm, wR2 = 0.0383, 1454 F2 values, and 44 variables. Refinement of the occupancy parameters revealed a lithium content of 92(6)%. LiRh3Sn5 crystallizes with a new structure type. The structure is built up from a complex three-dimensional [Rh3Sn5] network, in which the lithium atoms fill channels in the b direction. The [Rh3Sn5] network is governed by Rh-Rh (274 - 295 pm), Rh-Sn (262 - 287 pm), and Sn-Sn (289 - 376 pm) interactions. The lithium atoms have CN 13 (4 Rh+9 Sn). Electronic band structure calculations and the COHP bond analysis reveal strong Rh−Sn bonds and also significant Rh−Rh bonding within the Rh3Sn5 network, which is additionally stabilized by weak but frequent Sn−Sn interactions.