Compressibility and vibrational modes in solid As4O6

The phonon spectrum of solid As4O6 (arsenolite, Fd3m, Z = 8) ambient and hi gh pressures is studied with Raman and infrared spectroscopies and analyzed in relation to the vibrational spectrum of the adamantane As4O6 molecule i n the vapor state. The behavior of arsenolite upon compression can be inter preted as a result of an enhancement of intermolecular interactions between two equivalent As4O6 molecules at T-d symmetry sites of the "primitive" un it cell, The enforced interactions between the molecules through intermolec ular O ... O repulsions lead to a large increase of correlation field split tings of the internal vibrations due to the predominant bending and bridge stretching deformations. Eventually, at about 6 GPa they lift degeneracies of the internal modes and lower the symmetry of the crystal lattice. The ob served phase transition in solid As4O6 is reversible without any hysteresis . (C) 1999 Academic Press.