First-principles calculations of the structural and dynamic properties, and the equation of state of crystalline iodine oxides I<inf>2</inf>O<inf>4</inf>, I<inf>2</inf>O<inf>5</inf>, and I<inf>2</inf>O<inf>6</inf>

The structural and dynamical correlations, and the equation of state of crystalline I2O4, I2O5, and I 2O6 are investigated by first-principles calculations based on the density functional theory (DFT). The lattice dynamics results reveal distinctive features in the phonon density of states among the three crystals. The frequencies of the stretch modes in I2O4 and I2O5 are clearly separated from those of the other (e.g., bending) modes by a gap, with all stretch modes above the gap. In contrast, the gap in I2O6 separates the highest-frequency stretch modes with other stretch modes, and there is no gap between the stretch and the other modes in I2O6. The motion of iodine atoms is involved in all vibrational modes in I2O5, but only in low-frequency lattice modes in I2O6. In I2O4, iodine atoms are involved in modes with frequency below 700 cm-1. Van der Waals correction within our DFT calculations is found to reduce the overestimation of the equilibrium volume, with its effect on structure similar to the pressure effect. Namely, both effects significantly decrease the inter-molecular distances, while slightly increasing the bond lengths within the molecules. This causes the frequencies of some vibrational modes to decrease with pressure, resulting in negative modes Grneisen parameters for those modes. Thermodynamic properties, derived from the equation of state, of crystalline I2O4, I2O5, and I2O 6 are discussed within the quasi-harmonic approximation. © 2011 American Institute of Physics.