Evidence for molecular dissociation in bromine near 80 GPa

Synchrotron x-ray diffraction studies using a diamond-anvil cell and an imaging plate reveal that solid bromine begins to undergo a molecular-to-monatomic phase transition near 80 GPa. The structure of the high-pressure phase is found to be body-centered orthorhombic ($D_{2h}^{25}$-Immm), as previously observed in iodine above 21 GPa. These results on bromine plus the previous data on iodine and iodine monobromide show that the molecular dissociation takes place when the scaled unit-cell volume as ṽ=v/8$r_s^3$ in the molecular phase ($r_s$ is the intramolecular bond length in the solid) reaches v${̃}_c$=1.29. We also observe universal behavior of the scaled lattice constants versus ṽ in all solid halogens.