High-Pressure Raman Study of Fe(IO3)3: Soft-Mode Behavior Driven by Coordination Changes of Iodine Atoms

Abstract

We report high-pressure Raman spectroscopy studies of Fe(IO3)3 up to nearly 21 GPa that have been interpreted with the help of density-functional theory calculations, that include the calculation of phonon dispersion curves and elastic constants at different pressures. Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence of the changes induced by pressure in the coordination sphere of iodine, which gradually change from a three-fold to almost six-fold coordination under compression. The coordination change is favored by the decrease of the stereoactivity of the iodine lone electron pair, so that likely real six-fold coordination is attained after a first-order phase transition previously reported to occur above 21 GPa. The strong non-linear behavior found in Raman-active modes as well as in theoretically calculated elastic constants has been discovered to be related to the occurrence of two previously unreported isostructural phase transitions at 1.5-2.0 GPa and 5.7-6.0 GPa as shown by dynamic instabilities close to the Brillouin zone center.