COMPOUNDS OF THE TYPE M₅X₂: Pd₅As₂, Ni₅Si₂, AND Ni₅P₂

Abstract

Space group, lattice parameters, and powder data indicate that Pd 5 As 2 Ni 5 Si 2 , and Ni 5 P 2 are structurally related. Single crystal studies have established that Pd 5 As 2 exhibits polytypism. Pd 5 As 2 has a = 7.31 ± 0.01 Å but c-values of 13.7, 10.34, 27.48, and 96.2 Å were observed on different crystals corresponding to a "single layer" repeat distance of 3.43 Å along the c-axis. The last three cells have trigonal symmetry, space group [Formula: see text], [Formula: see text], P3m1 [Formula: see text], or P321 [Formula: see text] and the first cell has hexagonal symmetry, space group P6 3 22[Formula: see text]. The Guinier focussing camera pattern of polycrystalline Pd 5 As 2 can be indexed on a hexagonal cell with a = 7.320 ± 0.002, c = 96 Å. Single crystal and focussing camera studies of Ni 5 Si 2 showed it to be of trigonal symmetry with a = 6.68 ± 0.02, c = 12.28 ± 0.03 Å, and space group P321, P3m1, or [Formula: see text]. A Guinier pattern of Ni 5 P 2 can be indexed with hexagonal parameters a = 13.220 ± 0.002 Å, c = 24.632 ± 0.002 Å; a subcell with a = 6.610 and c = 12.316 Å can be used to index the strong lines. Comparison of the powder data for Pd 5 As 2 , Ni 5 Si 2 , and Ni 5 P 2 with previously published data for Ni 5 As 2 and Mn 5 Ge 2 shows that the compounds are structurally related.