Ca 2 NiSn 2 – A Polymorphic Intermetallic Phase: Atomic and Electronic Structure as well as a Topological Description of the Phase Transition by a Sigmatropic‐Type Rearrangement of Ni and Sn Atoms

Abstract The synthesis and structural characterization of two polymorphic modifications of the intermetallic compound Ca 2 NiSn 2 are described. The monoclinic modification mC ‐Ca 2 NiSn 2 is obtained by arc melting and rapid cooling and forms two‐dimensional Ni–Sn slabs that contain four‐ and six‐membered rings. These nets are interconnected by Sn 2 dumbbells and thus 12‐membered rings form between the slabs. The cavities of the resulting three‐dimensional network are filled with Ca atoms. At annealing temperatures between 800 and 870 °C, an orthorhombic modification oP ‐Ca 2 NiSn 2 forms. The two‐dimensional Ni–Sn substructure contains Ni–Sn nets of four‐ and six‐membered rings. Sn 2 dumbbells coordinate to these nets end‐on to form additional five‐membered Ni–Sn rings. The resulting slabs are separated by Ca atoms. Temperature‐dependent synthesis and differential thermal analysis experiments indicate that the monoclinic modification mC ‐Ca 2 NiSn 2 is a metastable modification. Two approaches to the topological description of the phase transition are presented: 1) a conversion by a simple shift of ribbons of atomic layers and 2) a least‐motion process describing the topology of the phase transformation by an electrocyclic mechanism through Ni–Sn bond scission and formation analogous to sigmatropic shifts known for organic molecules. The electronic structures and chemical bonding are discussed in terms of DFT calculations, including density of states, band structures and a topological analysis of the electron localization function, which revealed covalent Sn2–Sn2 bonds with partial π bonding of the Sn 2 dumbbells in mC ‐Ca 2 NiSn 2 and a short Sn2–Sn3 contact with dominant lone‐pair character at the terminal Sn2 atom in oP ‐Ca 2 NiSn 2 .