A new phase in solid state arsenic [9]

It has generally been considered that in the solid state, arsenic exists as four allotropie modifications-yellow (As(I)), amorphous (As(II)), orthorhombic (As(III)), and rhombohedral (As(IV)). Free energy content is lowest for the rhombohedral form. The other forms are metastable, the yellow form being the least stable, existing only at low temperatures and in the dark¹. It is the yellow form which has been the least studied. Since the effect of temperature, as well as that of irradiation with light or X rays, is to induce amorphisation in yellow arsenic, no X-ray structural data are yet available, and only indirect observations have been available. From these a cubic form has been tentatively assigned to yellow arsenic crystals. Spectroscopic data suggest molecular bonding in the crystal, with the As₄ molecule as the structural unit-a molecule previously observed in the vapour state².