STRUCTURE OF SOLID SOLUTIONS RCo 2-x Li x Si 2 (R = Gd, Dy)

Анотація

A search for quaternary alumogermanides was carried out in the system R–Ni–Al–Ge. Ten alloys  of  nominal  composition  R ₃ NiAl 3 Ge ₂   (R  =  Y,  Sm,  Gd,  Tb,  Dy,  Ho,  Er,  Tm,  Yb,  Lu)  were synthesized  from  high-purity  metals  by  arc-melting  and  annealed  at  600°C  for  1800  h.  Phase  and structural analyses were performed based on X-ray powder diffraction data. The structural parameters were refined by the Rietveld method. All of the alloys appeared to be single-phase. The existence of two known (with Y and Er)mand eight new (with Sm, Gd, Tb, Dy, Ho, Tm, Yb, and Lu) quaternary R ₃ NiAl ₃ Ge ₂  compounds wasmestablished. The compounds are isotypic and their crystal structures belong to the type Y ₃ NiAl ₃ Ge ₂  (Pearson symbol hP9, space group P-62m). As expected, the cell parameters decrease with decreasingnradius of the rare-earth metal, from Sm to Lu, with the exception of the Yb-containing compound due to the valence of Yb (Yb II ).

The hexagonal structure type Y ₃ NiAl ₃ Ge ₂  (Z = 1) is a quaternary variant of the binary type Fe ₂ P (Z = 3) and the ternary types β ₁ -K ₂ UF ₆ , Lu 3 CoGa ₅ , Zr ₃ Cu ₄ Si ₂ , and ZrNiAl. It belongs to a family of structures with trigonal prismatic coordination of the small atoms (Ni and Ge). The Ni- and Ge- centered  trigonal  prisms  form  infinite  columns  via  common  triangular  faces.  The  columns  of  Gecentered  prisms  (GeY ₆ )  share  prism  edges  and  form  a  three-dimensional  framework  with  six-membered rings in the (001) plane, whereas the columns of Ni-centered prisms (NiAl ₆ ) are isolated and located in the channels of the framework.

 Key  words:  quaternary  alumogermanide,  X-ray  powder  diffraction,  crystal  structure, isotypic compounds.