Arsenides of the Transition Metals: Vii. The Palladium–arsenic System

Abstract

In the Pd-As system the existence of seven intermediate phases, PdAsz, a-Pd2.4s, 0-Pc12.1s, PdjAs2, Pd~.ejAs, PdaAs, and P d d s has been established by X-ray diffraction and thermal analysis. Single crystals of P-PdzAs and PdAsz have been grown by slow cooling from a temperature above the liquidus and of PdaAs and PdjAs by halide vapor transport. Pd-Ass is cubic, C2 (pyrite) structure, space group Pa3(Tg), a = 5.983f 0.001 A. a-PdzAs (stable below 455" C) is monoclinic, a = 9.24h0.01 A, b = 8.47h0.01 A, c = 10.45~k0.01 a, /3 = 94". p-PdzAs (stable above 455" C) has the revised C22 (Fe2P) structure, space group ~6 2~ (Djh), a = 6.6501 0.003 A, c = 3.58310.003 A, c/a = 0.5388. PdbAsz displays a form of polytypism and shows trigonal as well as hexagonal symmetry with different c parameters.decomposes peritectically a_nd its structure is unknown. PdaAs has tetragonal DO, (Fe3P) structure, space group 14(S2); redetermined lattice constants are a = 9.974Zt0.002 A, c = 4.822Zt 0.002 A, / a = 0.4834. PdjAs is C-face centered monoclinic, a = 5..jl4f 0.01 A, b = 7.725f 0.01 A, c = 8.4274~ 0.01 A, and P = 99"h.j'.The literature on the Pd-As system is very scanty. To our knowledge there are no reports on the formation and structure of palladium arsenides except an early study of PdAsz by Thomassen (I), short reports on PdsAs by Schubert (2), Heyding and Calvert (3), and a recent note on PdzA%s (high temperature phase) by Schubert (4). In order to establish the existence and composition of the intermediate phases and to characterize them, an investigation of this system by X-ray diffraction and thermal analysis has been undertaken in this laboratory. EXPERIMENTALThe palladium sponge was supplied by Engelhard Industries and had a purity of 99.9970 as determined spectrographically. I t was used without further purification. Arsenic (99.999% purity) was obtained from Cominco. I t was fractionally sublimed in vacuo a t 450' C to remove oxide contami~lant not recorded in the spectrographic analysis. Calculated quantities (5 to 10 g total) of palladium and arsenic were sealed in quartz ampoules a t 1 0 -h m Hg and heated a t 700' C for 24 hours. The reaction between palladium and arsenic being highly exothermic, all specimens were found to have melted after the initial reaction. The products were removed, crushed, and resealed in quartz ampoules for a series of annealing periods ranging from 2 months a t 300' C to 7 days a t 850' C. After each annealing period the specimens were quenched in water and Debye-Scherrer patterns obtained in 11.46 cm Norelco cameras using Ni-filtered Cu radiation. Powder photographs were also obtained on a Guinier-deWolff focussing camera with As203 as an internal standard. The intensities of the powder diffraction lines were obtained by comparison with a logarithmic interval density scale. Single crystal photographs were taken on a precession camera using zirconirlmfiltered rnolybdenum radiation. Densities were determined pycnometrically using water except in the cases of 0-PdzAs and PdbAs whe...

The crystal structure of Pd8As3, a synthetic analogue of stillwaterite

“…The stillwaterite ideal formula is established as Pd 8 As 3 (Cabri et al, 1975) and corresponds to the formula of the synthetic phase Pd 2.65 As reported earlier by Saini et al (1964). The mineral X-ray diffraction pattern for stillwaterite is very similar to those of the synthetic Pd 2.65 As, however there are differences in the weak intensity reflections which are not resolved in the stillwaterite XRD data.…”

Rhodarsenide, (Rh,Pd)2As, a new mineral

“…The X-ray pattern obtained is presented in Table 3. With the exception of some reflections the data are different from those of palladoarsenide (monoclinic) (Begizov et al, 1974) and of synthetic Pd2As (Saini et al, 1964;Biilz & Schubert, 1969). The latter has been reported as monoclinic (a-form), hexagonal (~-form) and orthorhombic phases.…”

New Data on Vincentite

“…End members in the above system occurring in nature are limited to "guanglinite", Pd 3 As (Chao & Cabri 1976), which has been discredited as probably the equivalent of isomertiite (Cabri 1980), and keithconnite, Pd 3-x Te (Cabri et al 1979). The X-ray pattern of vincentite, however, does not show any similarity with either "guanglinite" or keithconnite, nor with synthetic Pd 3 As (Saini et al 1964); nor is the pattern of vincentite similar to the X-ray pattern of other minerals in the system Pd-As-Sb, such as arsenopalladinite, Pd 8 (As,Sb) 3 (Cabri et al 1977), mertieite I, Pd 11 (Sb,As) 4 and mertieite II, Pd 8 (Sb,As) 3 (Desborough et al 1973, Cabri et al 1975, isomertieite, Pd 11 Sb 2 As 2 (Clark et al 1974), stibiopalladinite, Pd 5 Sb 2 (Cabri & Chen 1976) and stillwaterite, Pd 8 As 3 (Cabri et al 1975).…”