Ca₂NiSn₂ - 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

The synthesis and structural characterization of two polymorphic modifications of the intermetallic compound Ca₂NiSn₂ are described. The monoclinic modification mC-Ca₂NiSn₂ 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 Sn₂ 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-Ca₂NiSn₂ forms. The two-dimensional Ni-Sn substructure contains Ni-Sn nets of four- and six-membered rings. Sn₂ 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-Ca₂NiSn₂ 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 Sn₂ dumbbells in mC-Ca₂NiSn₂ and a short Sn2-Sn3 contact with dominant lone-pair character at the terminal Sn2 atom in oP-Ca₂NiSn₂. The two- and three-dimensional Ni-Sn networks of two new polymorph modifications of Ca₂NiSn₂ exhibit structural similarities. In addition to the syntheses and structural characterizations, two topological descriptions of the phase transition are presented: a conversion by a simple shift of ribbons of atomic layers and a least-motion process through Ni-Sn bond scission and formation.