Phase relations and crystal structures in the ternary systems Sr-{Ag, Au}-{Si, Ge}

Phase equilibria in the isothermal sections of the ternary systems Sr-{Ag, Au}-{Si, Ge} were determined in the (Si) or (Ge) rich part up to 33.3 at % Sr after annealing at 700 C (Sr-{Ag, Au}-Ge) or 800 C (Sr-{Ag, Au}-Si). Tentative liquidus projections were constructed for the Si, Ge-rich part of all the four phase diagrams. These systems are characterized by a series of ternary compounds, exhibiting in some cases extended homogeneity regions at a constant Sr content. Compounds in the Si, Ge-rich region essentially form along two sections: at 33.3 at % Sr (AlB₂-family) and at 20 at % Sr (BaAl₄-family of structure types). The crystal structures of the novel equilibrium phases were derived by X-ray single crystal diffraction: Sr₂Ag_1+xSi_3-x-y□_y [Sr₂LiSi₃ type; x = 0.17, y = 0.43: a = 8.4488(2), b = 14.6376(2), c = 18.4018(2) Aa], SrAg_2-xSi_2+x [ThCr₂Si₂ type; x = 0.1: a = 4.37664(6), c = 10.4517(2) Aa], Sr(Au_1-y□_y)(Si_1-xAu_x)₃ [BaNiSn₃ type; x = 0.59, y = 0.64: a = 4.4594(2), c = 10.1013(6) Aa] and SrAu_5-x□_xSi₂ [BaAu₅Si₂ type; x = 0.7: a = 8.7557(3), b = 6.9945(2), c = 9.8873(3) Aa]. The crystal structures of Sr(Ag_xGe_1-x)₂ [AlB₂ type; x = 0.25: a = 4.3431(2), c = 4.5938(1) Aa], SrAg_2-xGe_2+x [ThCr₂Si₂ type; x = 0.2: a = 4.4476(2), c = 10.822(1) Aa] and Sr(Au_xGe_1-x)₂ [AlB₂ type; x = 0.25: a = 4.3263(2), c = 4.5852(7) Aa] were evaluated by X-ray powder Rietveld analysis. Composition dependent polymorphism was observed among the BaAl₄-type derivative structures for SrAu_2-xGe_2+x: CaBe₂Ge₂ type [x = 0.45: a = 4.4796(2), c = 10.6315(5) Aa], BaCu₂Sb₂ type [x = 0.35: a = 4.4866(2), c = 31.808(1) Aa] and ThCr₂Si₂ type [x = 0.2: a = 4.5214(4), c = 10.336(1) Aa]. Similarly, Sr(Au_xSi_1-x)₂ exhibits the Ca₂AgSi₃ structure type [x = 0.25: a = 8.3407(2), b = 14.4465(2) and c = 9.2664(3) Aa] and the AlB₂ type [x = 0.375: a = 4.2241(2), c = 4.5799(3) Aa]. The structural chemistry of the BaAl₄ derivative structures within the sections SrT_2-xX_2+x is discussed. (Copyright copyright 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)