On the Allotropy of Arsenic

The system As? Hg was investigated using low temperature X-ray diffraction (200 and 120 K). The samples ranging in composition from 1 to 60 at.-% Hg were alternatively slowly cooled and /or quenched from 700°C. The first series crystallizes as a solid solution of Hg in rhombohedral α-As. Quenched samples possess orthorhombic structure, identical with the mineral “arsenolamprit” (?-As) and are isomorphous with Bridgmans high pressure or mercury doped black phosphorus. The same form is obtained in the course of the disproportionation reaction 5 As₂O₃ ? As₄ + 3 As₂O₅. The orthorhombic modification absorbs an appreciable amount of helium, but it decreases as the mercury content becomes bigger. It is concluded that besides the formation of the solid solution As? Hg some foreign atoms may fill the voids which are present in ?-As.