Compression mechanisms in quasimolecularXI3 (X=As,S

We explore the structural, vibrational, and electronic properties of a prototypical family of quasimolecular layered solid of the type $X{\mathrm{I}}_{3}$ where $(X=\mathrm{A}\mathrm{s},\mathrm{S}\mathrm{b},\mathrm{B}\mathrm{i})$ under compression. We use a combination of angle- dispersive powder x-ray diffraction and Raman spectroscopy to study the structural and vibrational response to pressure. We also perform first-principles density functional pseudopotential calculations using both the local density approximation and gradient-corrected techniques for the description of electron exchange and correlation to further examine the electronic properties under pressure. We find that an unusual nonmonotonic variation of the symmetric $X\ensuremath{-}\mathrm{I}$ stretch frequency can be unambiguously attributed to the formation of intermolecular bonds and that compression results in a sequence of transitions from hexagonal molecular to hexagonal layered to monoclinic. The pressure dependence of the ambient pressure hexagonal structure is given as a full structural determination of the high-pressure phase. The structural and vibrational response (including the complex pressure dependence of the bond-stretch frequency) is well accounted for by quantum mechanical simulation. We further find that gradient corrections are necessary for an appropriate description of equilibrium structure, bonding, vibrational properties, and compression mechanisms and that the local density approximation appears to fail badly.