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 ≥ 1 (Na_xSn) 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 R3m 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 'Na_14.78Sn₄' (Pnma), better described as Na_16-xSn₄, is Na-richer than cubic Na₁₅Sn₄ (I43d). Characterization of electrochemically prepared Na-Sn alloys indicate that, with 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.