AuNi3Si6 and Au2Ni4Si7: Two New Structure Types Related to the CaF2-Type Structure.

The two compounds AuNi3Si6 and Au2Ni4Si7 were prepared from the elements by arc melting and subsequent annealing at 500 °C. They were investigated by X-ray diffraction (powder and single-crystal methods) and by scanning-electron microscopy which was used for the independent determination of the phase compositions. Both compounds show only minor homogeneity ranges. Single-crystal XRD was used for structure determination. Both title compounds crystallize in new structure types; AuNi3Si6 (Pearson symbol hR30, space group R $\bar{3}$ m) and Au2Ni4Si7 (Pearson symbol mC26, space group C2/ m). The new crystal structures are related to that of NiSi2 (CaF2-type); AuNi3Si6 may be derived from this structure by a shear operation, while Au2Ni4Si7 may be regarded as intergrowth structure of the CaF2 and Hg2Pt-type. [ABSTRACT FROM AUTHOR]