The monoclinic superstructure of the M₂Pt₆Al₁₅ series (M=Ca, Sc, Y, La, Lu)

The five ternary intermetallic compounds M₂Pt₆Al₁₅ (M=Ca, Sc, Y, La, Lu) were prepared from the elements by arc-melting. The crystal structure was determined via single crystal X-ray diffraction. The title compounds crystallize in a superstructure of the RE_0.67Pt₂Al₅ type structure (P6₃/mmc) in the monoclinic crystal system with space group P12₁/m1 (Sc₂Pt₆Al₁₅: a=734.19(2), b=1628.96(10), c=734.19(2) pm, β=119.999(3) ; wR=0.0356, 3034 F² values, 68 variables). The superstructure can be derived by the superspace formalism using (3+2)D or (3+1)D interpretations of the diffraction data. The structural relation to the subcell structure is discussed on the basis of a group-subgroup scheme. In the crystal structure strongly bonded [Pt₂Al₄]^δ- slabs are alternatingly stacked with ordered layers containing M atoms and Al₃ triangles.