Pressure-induced phase transition and increase of oxygen-iodine coordination in magnesium iodate

Abstract

The structural and vibrational behavior of $\mathrm{Mg}{({\mathrm{IO}}_{3})}_{2}$ under compression has been investigated via a combination of high-pressure (HP) synchrotron x-ray diffraction (XRD), Raman scattering, and infrared spectroscopy experiments as well as first-principles ab initio calculations. In this paper, we reveal that $\mathrm{Mg}{({\mathrm{IO}}_{3})}_{2}$ undergoes a pressure-induced phase transition between 7.5 and 9.7 GPa at ambient temperature from a monoclinic (space group $P{2}_{1}$) to a trigonal (space group P3) structure. $\mathrm{Mg}{({\mathrm{IO}}_{3})}_{2}$ also exhibits the gradual formation of additional bonds between iodine and oxygen atoms in neighboring ${\mathrm{IO}}_{3}^{\ensuremath{-}}$ units with increasing pressure, thereby increasing the oxygen-iodine coordination from 3 to 6. The bond formation under compression is a consequence of the existence of lone electron pairs on the iodine cation. To accommodate the additional bonds, the I--O bonds within the original ${[{\mathrm{IO}}_{3}]}^{\text{--}}$ trigonal pyramids increase in length under increasing compression. The appearance of additional Raman modes at 7.7 GPa and infrared modes at 9.6 GPa supports the phase transition observed in XRD experiments. Interestingly, the lengthening of I--O bonds causes a softening of several Raman modes under compression. We provide the crystal structure of the HP phase, the pressure-volume equations of state for both low- and HP phases, and the symmetry assignment of the Raman- and infrared-active modes of both phases.