SrPt₃In₂- an orthorhombically distorted coloring variant of SrIn₅

The new intermetallic phase SrPt₃In₂ was synthesized by induction-melting of the elements in a sealed tantalum ampoule followed by long-term annealing for crystal growth. The SrPt₃In₂ structure was refined from single crystal X-ray diffraction data: Imma, a = 1674.7(6), b = 921.2(4), c = 971.2(4) pm, wR₂ = 0.0551, 1192 F² values and 55 variables. Electronic structure calculations indicate strong covalent Pt-In bonding and a substantial charge transfer from the strontium atoms to the three-dimensional [Pt₃In₂]^δ- polyanionic network. The strontium atoms fill larger cavities within the network and the bonding of strontium to the polyanion is of the electrostatic type. The Bader charge calculations classify SrPt₃In₂ as a ternary platinide. The close relationship between the SrPt₃In₂ structure and the aristotype CaCu₅ is discussed on the basis of a group-subgroup scheme in the Bärnighausen formalism along with other CaCu₅ coloring variants and superstructures.