Lithium Mobility in the Stannides Li₂CuSn₂ and Li₂AgSn₂

Abstract

The stannides Li 2 CuSn 2 and Li 2 AgSn 2 were synthesized by induction-melting (or in a muffle furnace) of the elements in sealed niobium ampoules. The new phases were characterized by powder Xray diffraction. The structures of both stannides were investigated by X-ray diffraction on single crystals: Li 2 AuSn 2 type, I4 1 /amd, a = 442.6(1), c = 1940.9(8) pm, wR2 = 0.0742, 310 F 2 values for Li 2 CuSn 2 and a = 456.33(9) c = 2018.2(6) pm, wR2 = 0.0626, 339 F 2 values for Li 2 AgSn 2 with 10 variables for each refinement. The transition metal (T) atoms have tetrahedral tin coordination. The TSn 4 tetrahedra are condensed via common corners forming layers that are further con-* Prof. Dr. R. Pöttgen

The Cu-Li-Sn Phase Diagram: Isopleths, Liquidus Projection and Reaction Scheme

“…Since there are no such equilibrium phase diagrams for the Cu-Li-Sn system available in literature, it is the first comprehensive description of phase relations for all compositions and temperatures up to 1200°C and at atmospheric pressure. It is in consistence with four ternary isothermal sections at temperatures between 300 and 600°C which have been recently published by the authors [6] and considers recent findings of new ternary intermetallic compounds [3–5, 18] and the binary subsystems [8–10, 19]. There is strong experimental evidence for a stable liquid miscibility gap in the ternary system, which is discussed in more detail below.…”

Electronic Structure, Chemical Bonding and Electrochemical Characterization of Li2CuSn2 and Li2AgSn2

“…The purity of the polycrystalline Li 2 CuSn 2 and Li 2 AgSn 2 samples was verified through Guinier powder patterns (image plate system Fujifilm, BAS-1800), using CuKα 1 radiation and α-quartz (a = 491.30, c = 540.46 pm) as an internal standard. The experimental patterns were compared to calculated ones [19], using the crystallographic data of the previous structure refinements [12].…”

“…Before we start discussing the chemical bonding peculiarities and the electrochemical behavior of these stannides, we briefly recall their crystal chemistry, exemplarily for Li 2 CuSn 2 [12]. A view of the crystal structure approximately along the crystallographic b axis is presented in Fig.…”

“…Although the structure consists of a simple basic building unit, the bonding pattern is not that simple. The temperature-dependent solid-state NMR spectra indicate almost fully ionized Li + in all three Li 2 T Sn 2 (T = Cu, Ag, Au) stannides [10,12]. According to the Zintl-Klemm concept, a zig-zag chain of tin atoms is expected for a Sn 2− species, similar to CaSn [38] with a 290 pm Sn-Sn distance.…”

“…The highest lithium mobility has been observed for the channel-like polyanions in Li 2 T Sn 2 phases (T = Cu, Ag, Au) [10 -12]. Temperature-dependent 7 Li NMR spectroscopic studies showed activation energies in the range of 0.29 to 0.47 eV [12]. Parallel electrochemical characterization of Li 2 AuSn 2 by GITT and PITT techniques yielded a chemical diffusion coefficient of 1.5 × 10 −6 cm 2 s −1 [11].…”