Abstract
To understand these electrochemically-derived materials we have reinvestigated the formation
of Na-Sn alloys to identify all the phases which form when x≥1 (NaxSn) and characterized the
local bonding around the Sn atoms with X-ray diffraction, 119Sn Mössbauer spectroscopy, and
X-ray absorption spectroscopies. The results from the well-defined crystallographic materials
were compared to the spectroscopic measurements of the local Sn structures in the
electrochemically prepared materials. The reinvestigation of the Na-Sn compounds yields a
number of new results: (i) Na7Sn3 is a new thermodynamically-stable phase with a
rhombohedral structure and R-3m space group; (ii) orthorhombic Na9Sn4 (Cmcm) has
relatively slow formation kinetics suggesting why it does not form at room temperature during
the electrochemical reaction; (iii) orthorhombic ‘Na14.78Sn4’ (Pnma), better described as
Na16-xSn4, is Na-richer than cubic Na15Sn4 (I-43d). Characterization of electrochemically
prepared Na-Sn alloys indicate that, at the exception of Na7Sn3 and Na15Sn4, different crystal
structures than similar Na-Sn compositions prepared via classic solid state reactions are
formed. These phases are composed of disordered structures characteristic of kinetic-driven
solid-state amorphization reactions. In these structures, Sn coordinates in asymmetric
environments, which differ significantly from the environments present in Na-Sn model
compounds.
