On the Structure and Stability of BaAl 4 ‐Type Ordered Derivatives in the Sr–Au–Sn System for the 600 °C Section

The well‐known BaAl 4 ‐type structure consists of three ordered ternary derivatives, i.e., BaNiSn 3 ‐ ( I 4 mm ), ThCr 2 Si 2 ‐ ( I 4/ mmm ), and CaBe 2 Ge 2 ‐type ( P 4/ nmm ). Few systems, such as Ba‐Au‐Sn, have been confirmed to manifest all three types as a function of valence electron count. In this work, the SrAu x Sn 4– x solid solution at the 600 °C section was studied thoroughly using both single crystal and powder X‐ray diffraction. The crystal structures and phase width for the CaBe 2 Ge 2 ‐type SrAu x Sn 4– x solid solution were established to be a = 4.6528(2)–4.6233(3) Å and c = 11.3753(4)–11.2945(10) Å for x ≈ 1.65(1)–2.19(1). In the structure of SrAu 2 Sn 2 , no Au/Sn mixing was found, but for x < 2 compositions, Au/Sn mixings were only located at the Wyckoff 2 a (¾ ¼ 0) site and for x > 2 compositions, at the 2 b (¾ ¼ ½) site. Differential scanning calorimetry (DSC) analyses indicated that no phase transition occurred for the CaBe 2 Ge 2 ‐type SrAu x Sn 4– x phase up to 950 °C. Attempts to synthesize the ThCr 2 Si 2 ‐ and BaNiSn 3 ‐type SrAu x Sn 4– x phases under the same reaction conditions were unsuccessful, and the BaNiSn 3 ‐type phase could not be attained even at a pressure of 3 GPa. The instability of a BaNiSn 3 ‐type “SrAuSn 3 ” was investigated by both DSC measurements and first principles electronic structure calculations.