Synthesis and crystal structures of Ln2Al3Si2 and Ln2AlSi2 (Ln: Y, Tb-Lu)

Eight new ternary aluminium silicides were prepared by heating mixtures of the elements and investigated by means of single-crystal X-ray methods. Tb2Al3Si2 (a = 10.197(2), b = 4.045(1), c = 6.614(2) A, β = 101.11(2) ) and Dy2Al3Si2 (a = 10.144(6), b = 4.028(3), c = 6.580(6) A, β = 101.04(6) ) crystallize in the Y2Al3Si2 type structure, which contains wavy layers of Al and Si atoms linked together by additional Al atoms and linear Si-Al-Si bonds. Through this there are channels along [010], which are filled by Tb and Dy atoms respectively. The silicides Ln2AlSi2 with Ln = Y (a = 8.663(2), b = 5.748(1), c = 4.050(1) A), Ho (a = 8.578(2), b = 5.732(1), c = 4.022(1) A), Er (a = 8.529(2), b = 5.719(2), c = 4.011(1) A), Tm (a = 8.454(5), b = 5.737(2), c = 3.984(2) A) and Lu (a = 8.416(2), b = 5.662(2), c = 4.001(1) A) crystallize in the W2CoB2 type structure (Immm; Z = 2), whereas the structure of Yb2AlSi2 (a = 6.765(2), c = 4.226(1) A; P4/mbm; Z = 2) corresponds to a ternary variant of the U3Si2 type structure. In all compounds the Si atoms are coordinated by trigonal prisms of metal atoms, which are connected by common faces so that Si2 pairs (dSi-Si: 2.37-2.42 A) are formed. (orig.)