First-principles studies of solid molecular halogens under pressure: metallization pressures, Raman-active Ag modes and scaling relations

We conducted theoretical studies of the properties of solid halogens (iodine, bromine and chlorine) under pressure, employing the full potential linear muffin–tin orbital method with the local density approximation. From the electronic dispersion curves, we estimated the metallization pressures as 10, 35 and 100 GPa for iodine, bromine and chlorine, respectively. We also examined the pressure dependence of frequencies of Raman-active Ag modes using the frozen-phonon method, and found that there was good agreement with experimental data. We found frequency softening in iodine and bromine, and consider this as not only a precursor of molecular dissociation, but also a consequence of metallization. We examined the scaling rules, and found that these agreed with the band-theoretical results.