Gd1.33Pt3(Al,Si)8 and Gd0.67Pt2(Al,Si)5:  Two Structures Containing a Disordered Gd/Al Layer Grown in Liquid Aluminum

Gd1.33Pt3Al8 was synthesized by the combination of Gd and Pt in excess liquid aluminum. Addition of silicon resulted in the incorporation of a small amount of this element into the material to form the isostructural Gd1.33Pt3Al7Si. Both compounds grow as rodlike crystals with hexagonal cross section. The structures were refined in the rhombohedral space group R3̄m, with cell parameters a = 4.3359(6) Å and c = 38.702(8) Å for the ternary and a = 4.3280(8) Å and c = 38.62(1) Å for the quaternary compound. The structure is comprised of stuffed arsenic-like PtAl2 layers and disordered Gd/Al layers. Analysis of the hk0 zone reflections indicate the presence of an a* = √3a supercell, but the structure is not ordered along c, as revealed by the highly diffuse reflections in the 0kl zone photos. Therefore, the compounds are disordered variants of the Gd4Pt9Al24 type. Magnetic susceptibility studies reveal antiferromagnetic transitions at 15 K for the ternary and 7 K for the quaternary compound. Variation of the reactant ratio produces a different structure comprised of the same structural blocks, including the disordered Gd/Al layer. Gd0.67Pt2Al5 and its quaternary analogue Gd0.67Pt2Al4Si form in the hexagonal system P63 /mmc with cell parameters a = 4.2907(3) Å and c = 16.388(2) Å for the ternary and a = 4.2485(6) Å and c = 16.156(3) Å for the quaternary compound.