Theoretical calculations of phase transitions and optical properties of solid iodine under high pressures

The structural stability and optical properties of solid iodine under pressure have been studied using the ab initio pseudopotential plane-wave method. The dependence of lattice parameters on pressure indicates that the first structural phase transition from phase I to phase V occurs at about 20 GPa. From the pressure dependence of our elastic constants for solid iodine in phase I, it is found that the first structural transformation from molecular phase I to the intermediate phase V occurs at about 20 GPa due to the softening of the elastic constant C₄₄, which is very close to the transition pressure of 20 GPa obtained by geometry optimizations and 23.2 GPa obtained by experimental measurements. The optimized structure for phase V is a face-centered orthorhombic (fco) phase with equal interatomic distances d₁ = d₂ = d₃, but this fco structure is mechanically unstable, with shear elastic stiffness coefficient C₄₄<0. To understand the modulated phase V, we use a periodic crystal structure to mimic the incommensurate phase V and obtain some quantitative information. In our calculation, the modulated phase is thermodynamically and mechanically stable. It is believed that phase V is not a monatomic phase but an intermediate state between a molecular and a monatomic state.