THE ARSENIDES OF TUNGSTEN AND MOLYBDENUM: WAs2, W2As3, MoAs2, Mo2As3, AND Mo5As4

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Abstract
Binary alloys in the systems tungsten–arsenic and molybdenum–arsenic have been annealed at temperatures between 600 and 1 000 °C and, after quenching, examined by X-ray diffraction techniques. The only intermediate phases observed are WAs2, W2As3, MoAs2, Mo2As3, and Mo5As4. No solid solubility was detected in the elements or the compounds. The diarsenides are monoclinic, C2, and have the NbAs2 structure. WAs2, a = 9.085 ± 0.002 Å, b = 3.318 ± 0.002 Å, c = 7.690 ± 0.002 Å, β = 119° 31′ ± 2′, MoAs2, a = 9.07 Å, b = 3.30 Å, c = 7.73 Å, β = 119° 20′. The X2As3 compounds are monoclinic, condensed diffraction symbol C*/*; W2As3, a = 13.346 ± 0.002 Å, b = 3.278 ± 0.002 Å, c = 9.599 ± 0.002 Å, β = 124° 48′ ± 2′; Mo2As3, a = 13.339 ± 0.002 Å, b = 3.240 ± 0.002 Å, c = 9.628 ± 0.002 Å, β = 124° 31′ ± 2′. Mo5As4 is tetragonal, I4/m, with the Ti5Te4 structure, a = 9.595 ± 0.001 Å, c = 3.284 ± 0.002 Å. Data for MoAs2 and Mo5As4 confirm or amplify previous reports. In contrast to other reports, MoAs and W2As were not observed. W4As5 and Mo4As5, which have been mentioned briefly elsewhere, are here identified as W2As3 and Mo2As3.

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