ARSENIDES OF THE TRANSITION METALS: VII. THE PALLADIUM–ARSENIC SYSTEM

Abstract

In the Pd–As system the existence of seven intermediate phases, PdAs₂, α-Pd₂As, β-Pd₂As, Pd₅As₂, Pd₂.₆₅As, Pd₃As, and Pd₅As has been established by X-ray diffraction and thermal analysis. Single crystals of β-Pd₂As and PdAs₂ have been grown by slow cooling from a temperature above the liquidus and of Pd₃As and Pd₅As by halide vapor transport. PdAs₂ is cubic, C2 (pyrite) structure, space group [Formula: see text] a = 5.983 ± 0.001 Å. α-Pd₂As (stable below 455 °C) is monoclinic, a = 9.24 ± 0.01 Å, b = 8.47 ± 0.01 Å, c = 10.45 ± 0.01 Å β = 94°. β-Pd₂As (stable above 455 °C) has the revised C22 (Fe₂P) structure, space group [Formula: see text] a = 6.650 ± 0.003 Å, c = 3.583 ± 0.003 Å, c/a = 0.5388. Pd₅As₂ displays a form of polytypism and shows trigonal as well as hexagonal symmetry with different c parameters. Pd_2.65As decomposes peritectically and its structure is unknown. Pd₃As has tetragonal D0_e (Fe₃P) structure, space group [Formula: see text] redetermined lattice constants are a = 9.974 ± 0.002 Å, c = 4.822 ± 0.002 Å, c/a = 0.4834. Pd₅As is C-face centered monoclinic, a = 5.514 ± 0.01 Å, b = 7.725 ± 0.01 Å, c = 8.427 ± 0.01 Å, and β = 99°± 5′.