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c axis. Within each layer, the nearest neighbour As-As bond distances are 2.517 Å. Each arsenic has three near neighbours in the next layer at a distance of 3.120 Å. Each arsenic is therefore in an distorted octahedral site. Under vacuum at 616 °C, arsenic sublimes mainly in the form of As₄ molecules with a smaller proportion of As₂. Rapid condensation of the vapour onto a cold surface (<200 K), yields the metastable molecular allotrope known as yellow arsenic (As_{4) which slowly converts to the alpha form if left at room temperature. Condensation of arsenic vapour onto a heated surface generates amorphous black arsenic. The crystalline form of black arsenic can also be isolated, and the amorphous form can be annealed to return to the metallic gray arsenic form. Yellow arsenic can also be returned to the gray allotrope in a facile manner through application of light or by returning the molecule to room temperature.}

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