Isothermal section of the La-Mg-Sn system at 500 °C and crystal structure of the new ternary stannide LaMgSn2

The 500 °C isothermal section of the La–Mg–Sn ternary system was established by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and powder X-ray diffraction (XRPD) on more than fifty alloys synthesized by induction melting and annealed. A few boundary binary compounds were found to dissolve the third element, forming ternary solid solutions, among which La(MgxSn1−x)3 (0≤x≤0.14, cP4-AuCu3) is the most extended. The existence and crystal structure of three ternary compounds were confirmed, and their homogeneity ranges at 500 °C determined: La3Mg4−xSn2+x (0.12≤x≤0.40, hP9-Zr3Cu4Si2), LaMg3−xSn2 (0.33≤x≤0.78, hP34-LaMg3−xGe2) and LaMgSn (oP12-TiNiSi). The crystal structures of two novel ternary stoichiometric compounds, found during this work, were determined from powder X-ray diffraction pattern analysis and refined using the Rietveld method: La6Mg23Sn (Fm3¯m, cF120-Zr6Zn23Si, a =1.47107(1) nm) and LaMgSn2 (I4¯2m, tI32-LaMgSn2, a =0.83325(3) nm, c =1.24385(5) nm). The latter is the first representative of a new structure type and can be presented as composed by a complex Mg-Sn network hosting the bigger La atoms.