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

Abstract

Gd(1.33)Pt(3)Al(8) 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(1.33)Pt(3)Al(7)Si. Both compounds grow as rodlike crystals with hexagonal cross section. The structures were refined in the rhombohedral space group R(-)3m, with cell parameters a = 4.3359(6) A and c = 38.702(8) A for the ternary and a = 4.3280(8) A and c = 38.62(1) A for the quaternary compound. The structure is comprised of stuffed arsenic-like PtAl(2) layers and disordered Gd/Al layers. Analysis of the hk0 zone reflections indicate the presence of an a = radical 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 Gd(4)Pt(9)Al(24) 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(0.67)Pt(2)Al(5) and its quaternary analogue Gd(0.67)Pt(2)Al(4)Si form in the hexagonal system P6(3)/mmc with cell parameters a = 4.2907(3) A and c = 16.388(2) A for the ternary and a = 4.2485(6) A and c = 16.156(3) A for the quaternary compound.

Ternary compounds Ce0.67Pt2Al5 and Ce1.33Pt3Al8

“…In Ref. [3], the disordering of the layer in the structures of Gd 1.33 Pt 3 (Al,Si) 8 , Gd 0.67 Pt 2 (Al,Si) 5 , Gd 1.33 Pt 3 Al 8 , and Gd 0.67 Pt 2 Al 5 is related to the duration of the homogenizing annealing.…”

“…These compounds have a pseudohexagonal structure, which has much in common with that of R 2−x Pt 4 Ga 8+y . Besides, the intermetallic compounds of the Gd 0.67 Pt 2 (Al,Si) 5 , Gd 0.67 Pt 2 Al 5 , Gd 1.33 Pt 3 (Al,Si) 8 , and Gd 1.33 Pt 3 Al 8 compositions were obtained by the reaction of the elemental components in an aluminum flux at 1000 • C [3]. The crystal structures of the former two compounds belong to the Ce 0.67 Pt 2 Ga 5 structural type, and those of the latter two compounds belong to the R 4 Pt 9 Al 24 structural type.…”

“…According to Ref. [3], Gd 0.67 Pt 2 (Al,Si) 5 and Gd 0.67 Pt 2 Al 5 crystallize in space group P6 3 /mmc, whereas the symmetry of Ce 0.67 Pt 2 Ga 5 is described in Ref. [1] by the noncentrosymmetric space group P31c.…”

“…There is also a difference in opinion as to the crystal system of Gd 1.33 Pt 3 (Al,Si) 8 , and Gd 1.33 Pt 3 Al 8 (trigonal [3]) and R 4 Pt 9 Al 24 (monoclinic for R = Er and triclinic for other elements [2]). Although the symmetry and space group of the structures differ [1][2][3], in reality there are only two structural models described in Ref. [3]: R 0.67 Pt 2 Al 5 (I) (a = 4.291, c = 16.388, P6 3 /mmc, Z = 2) and R 1.33 Pt 3 Al 8 (II) (a = 4.336, c = 38.702, R3m, Z = 3).…”

“…Although the symmetry and space group of the structures differ [1][2][3], in reality there are only two structural models described in Ref. [3]: R 0.67 Pt 2 Al 5 (I) (a = 4.291, c = 16.388, P6 3 /mmc, Z = 2) and R 1.33 Pt 3 Al 8 (II) (a = 4.336, c = 38.702, R3m, Z = 3).…”