Crystal structure of Mg_0.39(2)NiSn_1.61(2) and Mg_2.61(2)Ni4Sn_3.39(2) featuring mg/ sn atom networks with different connections of ni at (Mg/Sn)₈ coordination polyhedra

The intermetallic compounds Mg_0.39(2)NiSn_1.61(2) and Mg_2.61(2)Ni4Sn_3.39(2) are prepared by the reaction of the elements in welded tantalum ampoules in a resistance furnace. The crystal structures of the compounds were investigated by single-crystal X-ray diffraction. Mg_0.39(2)NiSn_1.61(2) crystallizes in the PdSn₂ structure type with the space group I4₁/acd [a = 6.1941(4) Å, c = 23.498(2) Å, wR₂ = 0.078 (all data), 303 F² values, 19 variable parameters]. The main building block in the crystal structure of Mg_0.39(2)NiSn^1.61(2) are Ni₂@(Mg/Sn)₁₂ polyhedra, which consist of face-sharing Ni-centered square antiprisms. These polyhedra share vertices to form a three-di-mensional network of Mg and Sn atoms. The mixing of small amounts of magnesium in the binary Ni-Sn phase stabilizes the non-existing "NiSn₂" in the PdSn₂ structure type in form of NiSn₂-_xMg_x with x = 0.39. Mg_2.61(2)Ni4Sn_3.39(2) crystallizes in an own structure type with space group P 3¯ m1 [a = 4.3230(9) Å, c = 10.490(3) Å, wR₂ = 0.048 (all data), 216 F² values, 21 variable parameters]. The crystal structure of Mg_2.61(2)Ni₄Sn_3.39(2) is described as an intermediate of a Heusler and a Half-Heusler type structure what is demonstrated by group-subgroup relationships.