SHORT-RANGE SN ORDERING AND CRYSTAL-STRUCTURE OF LI4.4SN PREPARED BY AMBIENT-TEMPERATURE ELECTROCHEMICAL METHODS

The unique powder diffraction pattern of Li4.4Sn (or Li22Sn5), prepare d by the electrochemical reaction of Li with Sn at room temperature, d isplays both sharp diffraction peaks and intense broad oscillations. T his pattern is explained here by an underlying BCC lattice, of cube ed ge a, on which all the Li and Sn atoms are placed, with the tin atoms positioned in groups of randomly oriented tetrahedra of Sn, having edg es of length root 2a. Calculations of the diffraction patterns of mode l structures show that the sharp peaks arise from the underlying latti ce and the broad intense oscillations from the tin tetrahedra. The cry stal structure of Li,,Sn, prepared at elevated temperatures has the sa me underlying BCC lattice but has the tin tetrahedra arranged in a reg ular fashion, leading to a diffraction pattern showing only sharp peak s. Recent total energy calculations of the binary phases in the Li-Sn system, used to predict the voltage versus composition of Li/LixSn ele ctrochemical cells, assumed that the ordered bulk crystalline phases f ormed sequentially as the cell discharged (Courtney et al., unpublishe d results). Although experiment (Courtney and Dahn, J. Electrochem. Se c. 144 (1997) 2045) showed this to be true for Li2Sn5 and LiSn, the ot her bulk phases were not directly observed experimentally. Instead the pattern which we now recognize as characteristic of an underlying BCC lattice and randomly positioned groups of tin tetrahedra was observed over a wide range of 2.5 <x<4.4 in LixSn. Thus, our work also serves as a warning to theorists, that ordered equilibrium phases are not alw ays observed during room temperature electrochemical reactions. (C) 19 98 Elsevier Science B.V. All rights reserved.