Gd_1.33Pt₃(Al,Si)₈ and Gd_0.67Pt₂(Al,Si)₅: Two structures containing a disordered Gd/Al layer grown in liquid aluminum

Gd_1.33Pt₃Al₈ 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 Gd_1.33Pt₃Al₇Si. 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) Á 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. Gd_0.67Pt₂Al₅ and its quaternary analogue Gd_0.67Pt₂Al₄Si 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.