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 SrAu_xSn_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 CaBe₂Ge₂-type SrAu_xSn_4-x solid solution were established to be a = 4.6528(2)-4.6233(3) Aa and c = 11.3753(4)-11.2945(10) Aa 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 2a (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 SrAu_xSn_4-x phase up to 950 C. Attempts to synthesize the ThCr₂Si₂- and BaNiSn₃-type SrAu_xSn_4-x phases 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. (Copyright copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)