Studies of IBr, ICl, and i₂Cl₆ crystal properties by means of the mössbauer effect in¹²⁹I. II. Lattice dynamics

Lattice-dynamics information on polycrystalline I₂, IBr, ICl, and I₂Cl₆ molecular crystals was obtained by means of the Mössbauer effect in the 27.8-keV line of ¹²⁹I. In the case of I₂ and IBr, which are isomorphous, evidence was found that the mean-square displacement is larger in the direction of the molecular axis, and from considerations regarding the molecular arrangements, it is concluded that the larger 〈x²〉 is in the of plane. Similar evidence was found also for ICI. No anisotropy in 〈x²〉 was detected in the case of I₂Cl₆. From the temperature dependence of the recoilless fraction and from its absolute value, the Debye temperature was calculated and found to be 58°±2°K for both I₂ and IBr, suggesting a similar density of phonon states, in accordance with far-infrared results obtained with the halogens at 77 °K. The 〈x ²〉 found for I₂Cl₆ is compared with that calculated from Raman data for ICl₄^-, and it is concluded that the I-Cl intramolecular vibrations are not excited by the recoilless mechanism, which explains the relatively large effect observed.