First-Order Isostructural Phase Transition Induced by High Pressure in Fe(IO₃)₃

Abstract

The high-pressure (HP) behavior of Fe(IO3)3 was studied up to 35 GPa using powder X-ray diffraction, infrared micro-spectroscopy, and ab initio density-functional theory calculations. Fe(IO3)3 shows a pressure-induced structural phase transition at 15–22 GPa. Powder X-ray diffraction was employed to obtain the structure of the HP phase. This phase can be described by the same space group (P63) as the low-pressure phase but with a substantial different c/a ratio. This conclusion is supported by our computational simulations. The discovered phase transition involves a large volume collapse and a change in the coordination polyhedron of iodine, being a first-order transition. It also produces substantial changes in the infrared and Raman vibrational spectra. The pressure dependences of infrared and Raman phonon frequencies and unit-cell parameters have been obtained. A mode assignment is proposed for phonons based upon ab initio calculations. The bulk modulus of the two phases was obtained by fitting a Birch–Murnaghan equation of state to synchrotron X-ray powder diffraction data resulting in B0 = 55(2) GPa for the low-pressure phase and B0 = 73(9) GPa for the HP phase. Calculations gave B0 = 36(1) GPa and B0 = 48(3) GPa for the same phases, respectively. The results are compared with other iodates, in particular LiIO3, for which we have also performed density-functional theory calculations. A possible mechanism driving the observed phase transition will be discussed.