Gold-tin ordering in SrAu2Sn2

Samples of the solid solutions SrAuxSn4-x (1.7 ≤ x ≤ 2.2) were obtained by high-frequency melting of the elements in sealed niobium ampoules. Powder and single-crystal X-ray data confirmed the CaBe2Ge2-type structure, space group P4/nmm. The structures of SrAu1.76Sn2.24, SrAu2Sn2, SrAu2.16Sn1.84 (crystal A), SrAu2.16Sn1.84 (crystal B), and SrAu2.22Sn1.78 were refined from single-crystal diffractometer data. Only the SrAu2Sn2 crystal shows complete Au-Sn ordering while all other crystals show substantial mixed occupancies on the four crystallographically independent sites of the polyanionic networks in which the strontium atoms fill cages of coordination number 16. Temperature-dependent susceptibility measurements have revealed diamagnetism for SrAu2Sn2. 119Sn Moessbauer spectroscopic data of a bulk SrAu2Sn2 sample have resolved the tetrahedral and square-pyramidal tin sites but point to substantial Au-Sn disorder.