On the structure and stability of BaAl4-type ordered derivatives in the Sr-Au-Sn system for the 600 ° C section

The well-known BaAl4-type structure consists of three ordered ternary derivatives, i.e., BaNiSn3-(I4mm), ThCr2Si2-(I4/mmm), and CaBe2Ge2-type (P4/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 SrAuxSn4-xsolid 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 CaBe2Ge2-type SrAuxSn4-xsolid solution were established to be α = 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 SrAu2Sn2, no Au/Sn mixing was found, but for x < 2 compositions, Au/Sn mixings were only located at the Wyckoff 2α (3/4 1/4 0) site and for x > 2 compositions, at the 2b (3/4 1/4 1/2) site. Differential scanning calorimetry (DSC) analyses indicated that no phase transition occurred for the CaBe2Ge2-type SrAuxSn4-xphase up to 950 ° C. Attempts to synthesize the ThCr2Si2-and BaNiSn3-type SrAuxSn4-xphases under the same reaction conditions were unsuccessful, and the BaNiSn3-type phase could not be attained even at a pressure of 3 GPa. The instability of a BaNiSn3-type "SrAuSn3" was investigated by both DSC measurements and first principles electronic structure calculations