Compression mechanisms in quasimolecular XI3 (X = As, Sb, Bi) solids

We explore the structural, vibrational, and electronic properties of a prot otypical family of quasimolecular layered solid of the type XI3 where (X=As ,Sb,Bi) under compression. We use a combination of angle-dispersive powder x-ray diffraction and Raman spectroscopy to study the structural and vibrat ional response to pressure. We also perform first-principles density functi onal pseudopotential calculations using both the local density approximatio n and gradient-corrected techniques for the description of electron exchang e and correlation to further examine the electronic properties under pressu re. We find that an unusual nonmonotonic variation of the symmetric X-I str etch frequency can be unambiguously attributed to the formation of intermol ecular bonds and that compression results in a sequence of transitions from hexagonal molecular to hexagonal layered to monoclinic. The pressure depen dence of the ambient pressure hexagonal structure is given as a full struct ural determination of the high-pressure phase. The structural and vibration al response (including the complex pressure dependence of the bond-stretch frequency) is well accounted for by quantum mechanical simulation. We furth er find that gradient corrections are necessary for an appropriate descript ion of equilibrium structure, bonding, vibrational properties, and compress ion mechanisms and that the local density approximation appears to fail bad ly. [S0163-1829(98)07445-1].