The local atomic structure and chemical bonding in sodium tin phases

To understand the electrochemically-derived Na-Sn we have reinvestigated the formation of Na-Sn alloys to identify all the phases which form when x greater than or equal to 1 (Na sub(x)Sn) and characterized the local bonding around the Sn atoms with X-ray diffraction, super(119)Sn Mossbauer 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) Na sub(7)Sn sub(3) is a new thermodynamically-stable phase with a rhombohedral structure and R3&cmb.macr; mspace group; (ii) orthorhombic Na sub(9)Sn sub(4) (Cmcm) has relatively slow formation kinetics suggesting why it does not form at room temperature during the electrochemical reaction; (iii) orthorhombic 'Na sub(14.78)Sn sub(4)' (Pnma), better described as Na sub(16-x)Sn sub(4 ), is Na-richer than cubic Na sub(15)Sn sub(4) (I4&cmb.macr; 3d). Characterization of electrochemically prepared Na-Sn alloys indicate that, with the exception of Na sub(7)Sn sub(3) and Na sub(15)Sn sub(4), 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.